US3746954A

US3746954A - Adjustable voltage thyristor-controlled hoist control for a dc motor

Info

Publication number: US3746954A
Application number: US00181515A
Authority: US
Inventors: A Myles; F Erb
Original assignee: SQARE D CO
Current assignee: SQARE D CO; SQARE D CO US
Priority date: 1971-09-17
Filing date: 1971-09-17
Publication date: 1973-07-17
Anticipated expiration: 1990-07-17
Also published as: DE2244763A1; AU461170B2; FR2154033A5; GB1409601A; CA969601A; IT975005B; ZA726328B; GB1409602A; JPS5334285B2; AU4656372A; JPS4876016A; DE2244763B2

Abstract

A direct current series motor is powered by adjustable voltage from an alternating current source. The motor is series connected during hoisting and powered by a single AC-DC converter, and shunt connected during lowering with a second AC-DC converter supplying the field. An isolation resistor permits dynamic lowering and provides emergency dynamic braking even with the independently energized armature and field. A teaser field resistor which prevents overspeeding when the motor is hoisting a light load is automatically disconnected while the motor is hoisting a sufficiently heavy load and is reconnected if the load becomes too small. During lowering, a dynamic braking resistor may be removed from across the armature without any material change in motor speed.

Description

United States Patent 1 Myles et al.

[ July 17, 1973 ADJUSTABLE VOLTAGE Primary Examiner-Bernard A. Gilheany THYRISTOR-CONTROLLED HOIST Assistant Examiner-W. E. Duncanson, Jr. CONTROL FOR A D MOTOR Attorney-Harold J. Rathbun et al.

[75] Inventors: Asa H. Myles, Solon; Fred El'b,

, Northfield, both of Ohio i ABSTRACT d b d bl meet current series motor is powere y a justa e [73] Asslgnee: Square D Company Park Rldgei voltage from an alternating current source. The motor {22] Filed: Sept. 17, 1971 is series connected during hoisting and powered by a single AC-DC converter, and shunt connected during [21 1 Appl' 181515 lowering with a second AC-DC converter supplying the field. An isolation resistor permits dynamic lowering 52 vs. Cl. 318/247, 318/258 and Provides emergency dynamic braking even with 51 Int. Cl. 1102 5/06 the independently energized armature and field- A [58] Field of Search 318/258, 261, 269, teaser field resistor which Prevents everspeedins when 318/274, 341, 342, 344, 375, 247, 248 the motor is hoisting a light load is automatically disconnected while the motor is hoisting a sufficiently [56] References Ci d heavy load and is reconnected if the load becomes too UNITED STATES PATENTS small. During lowering, a dynamic braking resistor may be removed from across the armature without any ma- 3,590,352 6/1971 Ries et al. 318/2 58 X 3,636,424 1/1972 Reed 318/341 x change speed 21 Claims, 9 Drawing Figures FIRING CIRCUIT HOISTEL WE 75w AC c 44 47 y g% convenes I 760 as: 3 b

7 2 We 3? red CONVERTER u a I 1:: H:1-A(5 7 4.9 W I f J as 9/ Ac 0c 7 {52 FIRING ONVERTER s2 I cmculT 8 MODULE Pa tentd July 17, 1973 4 Sheets-Sheet l mm mmEw zoo m6 mum-Puma Patent ed July 17, 1973 4 Sheets-Shet 5 Patented' July 17, 1973 4 Sheets-Sheet 4 i S/C OUT P UT VOLTAGE I] I S/C OUTPUT VOLTAGE s/c OUTPUIT VOLTAGE I l I S/C OUTPUT VOLTAGE S/C OUTPUT VOLTAGE ADJUSTABLE VOLTAGE THYRISTOR-CONTROLLED I-IOIST CONTROL FOR A DC MOTOR This invention relates to hoist control systems for DC motors, and more particularly to a hoist control system for operating a direct current series motor from a single-phase or three-phase alternating current source.

It is an object of this invention to provide an improved hoist control for a DC series motor.

It is a further object to provide an improved adjustable voltage speed control for operating a DC series motor from an AC source.

A still further object is to provide a hoist control in which, during hoisting operation, the armature and field of a DC series motor are connected as a series motor, and in which, during lowering operation, the armature and field are shunt connected and generally independently powered.

Another object is to provide an improved adjustable voltage hoist control system for a DC series motor in which dynamic braking, dynamic lowering, and emergency dynamic braking are provided for the motor during lowering operation.

Another object is to provide a DC hoist control system in which improved automatic control for selective application and removal of a teaser field resistor during hoisting operation of the motor is provided.

Another object is to provide a DC hoist control system in which automatic removal of the dynamic braking resistor during lowering operation of the motor is accomplished without any material change in motor speed.

Other objects and advantages of the invention will be apparent from the following description wherein reference is made to the drawings, in which:

FIG. 1 is a schematic wiring diagram, partly in block form, of a hoist control system in accordance with this invention;

FIG. 2 is a schematic wiring diagram of a reference control circuit for use in the hoist control system of FIG. 1;

FIG. 3 is a schematic wiring diagram of a load sensing module for use in the hoist control system of FIG. 1;

FIGS. 4-9 are graphs illustrating relationships between output voltage of a signal converter of the hoist control system of FIG. 1 and various amplifier voltages appearing in the reference control circuit of FIG. 2.

A preferred embodiment of the thyristor hoist control system of the present invention is illustrated in FIG. 1 wherein a direct current motor comprising an armature 11A and a series-wound field 11F is powered by a three-phase alternating current source Ll-L2-L3 and used to selectively hoist and lower a load 14. An

overhoist limit switch 15 of the power circuit type is preferably arranged to be operated by the load 14, as is well known in the art. The limit switch 15 has two sets of normally open contacts 15a and 15b and two sets of normally closed

contacts

15c and 15d. A springapplied electromagnetically-released friction brake 16 having an operating winding 16w is preferably provided for armature 11A.

A conductor 17 extends between a junction 19 and a junction 20 and forms a series connection comprising the junction 19, normally open contacts 21a of a first electromagnetic hoisting contactor 21, the limit switch contacts 15c, the armature 11A, the limit switch contacts 15d, an isolation resistor 22, the motor field 11F,

the operating winding 16w of the brake l6, normally open contacts 24a of a main contactor 24, a winding 25w of an overload relay 25, a resistor serving as a first current-indicating means or current measuring shunt 26, and the junction 20. The circuit is preferably grounded at the junction 20. M

A conductor 27 forms a series connection from the junction 19 through normally open contacts 28a of an electromagnetic lowering contactor 28, the limit switch contacts 15b, a limit switch resistor 29 to the side of the field .llF adjacent the operating winding 16w of the brake 16. The conductor 27, between the contacts 280 and the contacts 15b, is connected by a winding 3w of a limit switch relay 30 to the conductor 17 between the contacts 21a and the contacts 15c, by the series combination of a dynamic braking resistor 31 and normally closed contacts 32a of an electromagnetic dynamic braking contactor 32 to one side of the armature 11A adjacent the limit switch contacts 15c, and by a conductor 34 to the other side of the armature 11A adjacent the limit switch' contacts 15d.

A conductor 35 is connected from the one side of the armature 11A through a pair of normally closed contacts 36a of an electromagnetic teaser field contactor 36, a teaser field resistor 37, and a pair of normally open contacts 39a of a second electromagnetic hoisting contactor 39 to the side of the field 11F adjacent the isolation resistor 22. A conductor 40 connects a pair of normally closed contacts 41a of an electromagnetic dynamic lowering contactor 41 between the one side of the armature 11A and the other side of the field 11F. A conductor 42 connects the normally open limit switch contacts 15a between the conductor 40, adjacent the armature 1 1A, and the conductor 17, between the limit switch contacts 15d and the isolation resistor 22. The conductor 42 is also connected to the conductor 35 between the teaser field resistor 37 and the hoisting contacts 39a.

A first controlled rectifier means, preferably in the form of an AC-DC converter 44, has its DC output connected by a conductor 45 through a winding 46w of an overload relay 46 to the junction 19 and by a conductor 47 to the junction 20. The AC-DC converter 44 may be either single phase or three phase in accordance with the requirements of the motor, and may be either a semi-converter of a full converter, the circuits of which are well known to those skilled in the art. A second controlled rectifier means, preferably in the form of an AC-DC converter 49, has one side of its DC output connected by a conductor 50 to one side of the field 11F adjacent the isolation resistor 22. The other side of the DC output of the AC-DC converter 49 is connected by a conductor 51 through a resistor serving as a second current measuring shunt 52 to the junction 20. The AC-DC converter 49 may also be either single phase or three phase, but this converter cannot include a free wheeling diode-if a limit switch is to be used. If desired,

the AC-DC converter 49 may comprise three thyristors, each connected to one leg of the secondary winding 53b. The neutral of the secondary 53b would then be an output terminal of the converter 49 for connection of the conductor 51.

The AC-

DC converters

44 and 49 are powered by the I secondary windings 53a and 53b, respectively, of a transformer 53. The transformer 53 has a primary winding 53p, which may be either delta or wyeconnected, connected to the source of alternating cur-- rent Ll-L2-L3. The secondary winding 53a is connected by conductors 54 to the AC-DC converter 44 and the secondary winding 53b is connected by conductors 55 to the AC-DC converter 49. Although a three-phase transformer 53 has been illustrated, it will be readily apparent to those skilled in the art that the motor control circuitry of the present invention may be utilized, when appropriate, with a single phase power supply and that the secondary windings 53a and 53b could be energized by separate primaries.

A pair of conductors 56 supply single phase alternating current from the source of alternating current Ll-L2 to a primary winding 57p of a transformer 57 having a secondary winding 57s. The secondary winding 57s of the transformer 57 supplies power for the control circuitry which will now be described in detail.

The motor is controlled by a multi-position, reversing master switch 58, which may be of the type described in U. S. Pat. No. 3,221,246, issued on Nov. 30, 1965, to Calvin B. Sanborn, Jr., and assigned to the assignee of the present invention. A pair of conductors 59 carry operating power from the secondary winding 57s to a rectifier 60 and a pair of conductors 61 carry operating power from the secondary winding 57s through normally open auxiliary contacts 24b of the main contactor 24 to the serially connected

primary windings

62p and 64p of a pair of

differential transformers

62 and 64, as is fully disclosed in the aforementioned patent.

The rectifier 60 changes alternating current from the secondary winding 57s of the transformer 57 to direct current and may be in the form of a standard diode bridge rectifier having its output fed through a positive conductor 65 and a negative conductor 66. The master switch 58 has normally closed contacts 67 and six normally open contacts 69 through 74. The closed or open condition of the

contacts

67 and 69 through 74 in the hoisting and lowering position is indicated by the presence or absence of substantially rectangular blocks aligned with the contacts. For example, the contacts 69 are closed in the hoisting range of the master switch 58 and open in the lowering range. The

contacts

67 and 69 through 74 control the energization of operating windings of various contactors and relays, power for which is obtained through the

conductors

65 and 66.

An operating winding 75w of an undervoltage relay 75 is energized through the contact 67 when the master switch is in an OFF position and is maintained in its energized state in other positions of the master switch through normally open contacts 75a of the relay 75, the contacts 75a being interposed in the conductor 65. Normally closed contacts 25a of the overload relay 25 and normally closed contacts 46a of the overload relay 46 are connected in series with the winding 75w of the undervoltage relay 75. The operation, interconnection, and function of the overload relays 25 and 46 and the undervoltage relay 75 are well known in the art.

A winding 41w of the dynamic lowering contactor 41 is energized through the contact 69, and normally closed auxiliary contacts 28b of the lowering contactor 28 in the hoisting range of the master switch 58. Connected in parallel with the winding 41w for operation through the

contacts

69 and 28b are a winding 21w of the first hoisting contactor 21, connected through normally open auxiliary contacts 41b of the dynamic lowering contactor 41, a winding 39w of the second hoisting contactor 39, connected through normally open auxiliary contacts 32b of the dynamic braking contactor 32, and a winding 36w of the teaser field contactor 36, connected through normally open contacts 76a of a first load sensing relay 76 (FIG. 3).

A winding 28w of a lowering contactor 28 is energized through the contact and normally closed auxiliary contacts 39b of the second hoisting contactor 39 whenever the master switch 58 is operating in its lowering range.

A winding 32w of the dynamic braking contactor 32 is energized through the contact 71 in the hoisting range of the master switch 58, through the contact 72 and normally open contacts 30a of the limit switch relay 30 in the high speed lowering range of the master switch 58, and through the contacts 73, a pair of normally open contacts 77a of a second load sensing relay 77 (FIG. 3) and the contacts 30a in the intermediate and high speed lowering ranges of the master switch 58.

A winding 24w of the main contactor 24 is energized through the contact 74 throughout the hoisting and lowering ranges of the master switch 58.

The

differential transformers

62 and 64 have

secondary windings

62s and 64s, respectively, connected to a signal converter 79 which converts its alternating current input to a direct current output preferably having a negative voltage output-during hoisting operation and a positive voltage output during lowering operation as is described in the above-mentioned Sanborn patent. It should be noted, however, that any source of adjustable voltage direct current adapted for output of one polarity during hoisting operation and another polarity during lowering operation may be used as a signal means. This direct current output is transmitted through a conductor 80 to a reference control circuit 81, which will be described in detail with reference to FIG. 2. A load sensing module 82 is electrically connected to the first shunt 26 by a conductor 83 and to the reference control circuit 81 by a conductor 84 and operates in a manner to be described in detail with reference to FIG. 3.

The reference control circuit 81 is connected by a conductor 85 to the second shunt 52 and provides a control output through a pair of

conductors

86 and 87 to a first firing circuit 88 which in turn controls operation of the first AC-DC converter 44. The reference control circuit 81 also provides a control output through

conductors

89 and 90 to a second firing circuit 91 which in turn controls operation of the second AC-DC converter 49.

The reference control circuit 81 is best described with reference to FIG. 2 and obtains a direct current input voltage through the conductor 80 from the signal converter 79 as is also shown in FIG. 1. Some of the other circuitry of FIG. 1 is also repeated in FIG. 2 for a clarity. The output of the reference control circuit 81 is transmitted through

conductors

86 and 87 to the first firing circuit 88 and through

conductors

89 and 90 to the second firing circuit 91.

A conductor 92 connects the conductor 80 through a diode 94 to an input resistor 95 of an inverting amplifier 96 having a feedback resistor 97. A conductor 99 connects the diode 94 to a hoisting permissive circuit 100. .The hoisting permissive circuit 100 produces a fixed negative output voltage in response to a variable negative input voltage transmitted through the conductor 99 in a manner well known to those skilled in the art. This output signal is applied to an input resistor 101 of the amplifier 96 and is applied to the first firing circuit 88 through the conductor 87. The conductor 84 transmits this output voltage also to the load sensing module 82 (see also FIG. 1). A minimum hoisting speed adjustment potentiometer 102 is connected between the input resistor 95 and the input resistor 101 and has a wiper 102w which is connected to ground. The output of the amplifier 96 is transmitted to the first firing circuit 88 through the conductor 86.

A conductor 104 connects the conductor 80 through a diode 105 to a lowering permissive circuit 106, an input resistor 107 of a non-inverting amplifier 108, an input resistor 109 of an inverting amplifier 110, and to a conductor 11 l which is connected to an input resistor 112 at the inverting input of an amplifier 113.

The lowering permissive circuit 106 responds to a positive variable input voltage to produce a fixed negative output voltage. This output voltage is transmitted to the first firing circuit 88 through the conductor 87 and to the second firing circuit 91 through the conductor 89. The output of the lowering permissive circuit 106 is also applied to an input resistor 114 at the inverting input of the amplifier 113.

The amplifier 108 has a feedback resistor 115 and has an input resistor 1 16 connected to a source of negative voltage 117. The output of the amplifier 108 is applied to an input resistor 119 of an inverting amplifier 120 having a feedback resistor 121. A second input resistor 122 of the amplifier 120 is connected to a source of positive voltage 124 and a third input resistor 125 is connected by a conductor 126 to the collector of a transistor 129. A conductor 130 transmits the output of the amplifier 120 through a diode 131 and a conductor 132 to an input resistor 133 of the amplifier 96.

The amplifier 110 has a feedback resistor 134. The input resistor 109 of the amplifier 110 is connected by a conductor 135 through a diode 136 to the emitter of a transistor 137 which is connected to ground through a resistor 139. The collector of transistor 137 is connected to a source of positive voltage 140, and the base of the transistor 137 is connected to the wiper of a potentiometer 141 one side of which is connected to a source of positive voltage 142. The other side of the potentiometer 141 is connected by a conductor 144 to a junction 145 joining a diode 146 and a diode 147. The diode 146 is connectedto a source of positive voltage 148 and, by a conductor 149, through normally closed contacts 30b of the limit switch relay30 to a source of negative voltage 150. The diode 147 is connected by a conductor 151 through normally open auxiliary contacts 32c of the dynamic braking contactor 32 and nor mally open auxiliary contacts 28c of the lowering contactor 28 to the source of negative voltage 150. The diode 147 is also connected to a source of positive voltage 152 and by a conductor 153 to the base of the transistor 129. The emitter of the transistor 129 is connected to ground and the'collector of the transistor 129 is connected through a resistor 156 to a source of negative voltage 157.

The output voltage of amplifier 110 is transmitted through a conductor 159, a diode 160, and the conductor 132 to the input resistor 133. As has been shown, the outputs of

amplifiers

120 and 110 are applied through

diodes

131 and 160, respectively, to the input resistor 133 of amplifier 96. i

The output voltage of the second shunt 52 is transmitted through the conductor 85 to an input resistor 161 of an inverting amplifier 162 having a feedback resistor 163. The output of amplifier 163 is transmitted to an input resistor 164 of the amplifier 113 which has a feedback resistor 165. The conductor 111, at the input resistor 112, is connected to ground through a potentiometer 166 having its wiper connected to the non-inverting input of amplifier 1-13 and to the collector of a transistor 167. The emitter of the transistor 167 is connected to ground and the base of the transistor 167 is connected to the junction 145.

The load sensing module 82 is best described with reference to FIG. 3 which also includes some of the circuitry shown in FIG. 1. Input to the load sensing module 82 is provided by the first shunt 26 through the conductor 83 (see also FIG. 1) and from the reference control circuit 81 through the conductor 84 (see also FIG. 2).

The conductor 83 feeds the input voltage from the first shunt 26 to an input resistor 168 of an inverting amplifier 169 having a feedback resistor 170. The output voltage of theamplifier 169 is transmitted to an input resistor 171 of an inverting amplifier 172 having a feedback resistor 174. A potentiometer 175 is connected by a conductor 176 between a source of positive voltage 177 and a grounding conductor 179 and has its wiper 175w connected to an input resistor 180 of the amplifier 172.

A conductor 181 connects the input terminal of the amplifier 172 to a combination of resistors comprising a resistor 182 which is connected in parallel with serially connected

resistors

184 and 185. A junction 186 between the resistor 184 and the resistor is connected to the collector of a transistor 187 the emitter of which is connected to a grounding conductor 189.

A conductor 190 connects the junction of the resistor 182 and the resistor 185 to a wiper 191w of a potentiometer 191. The potentiometer 191 is connected between the grounding conductor 189 and a diode 192. The diode 192 isconnected by a conductor 194 to the base of a transistor 195. The conductor 84, providing input from the reference control circuit 81, is connected to the conductor 194.

The output voltage of the amplifier 172 is transmitted through'a conductor 196 to an input resistor 197 of an inverting amplifier 199 having a feedback resistor 200.

The output of the amplifier 199 is transmitted by a conductor 201 to a voltage divider comprising

seriesconnected resistors

202, 204, and 205 and connected at its opposite end to the grounding conductor 179. A conductor 206 connects the collector of the transistor to the grounding conductor 179, and the emitter of the transistor 195 is connected to the voltage divider at a point between the

resistors

204 and 205.

The voltage divider, at a point between the

resistors

202 and 204, is connected by a conductor 207 through a diode 209 to an input resistor 210 of an integrating amplifier 211 and by a conductor 212 through a diode 214 to an input resistor 215 of the amplifier 211. The amplifier 211 has a feedback capacitor 216. A conductor 217 transmits the output of the amplifier 211 to the base of a transistor 219 which has its emitter connected to the grounding conductor 179 and its collector connected by a conductor 220 to a junction 221.

The junction 221 is connected by a conductor 222 through a capacitor 224 to an input resistor 225 of the amplifier 211, by a conductor 226 through a resistor 227 to a source of positive voltage 229, and by a con ductor 230 to the base of a transistor 231. The emitter of the transistor 231 is connected by a conductor 232 to a source of positive voltage 233 and the collector of the transistor 231 is connected by a conductor 234 through a diode 235 and a resistor 236 to a source of negative voltage 237. A conductor 238 is connected from the conductor 234 between the diode 235 and the resistor 236 through a diode 239 to the base of the transistor 187. A conductor 240 connects the collector of the transistor 231 to the base of a transistor 241 and, through a conductor 242, to the base of a transistor 243. The collector of the transistor 241 is connected through a winding 76w of the first load sensing relay 76 to a source of positive voltage 244 and the emitter of the transistor 242 is connected to a source of negative voltage 245. The emitter of the transistor 243 is connected to a source of negative voltage 246 and the collector of the transistor 241 is connected through a winding 77w of the second load sensing relay 77 to a source of positive voltage 247. A conductor 249 connects the collector of the transistor 241 through normally open auxiliary contacts 32d of the dynamic braking contactor 32 to a source of negative voltage 250.

Referring now to FIGS. 1-3, operation of the thyristor hoist control of the present invention will be described assuming that the motor, comprising the armature 11A and the field 11F, is being used to selectively hoist and lower a load 14 in response to operation of the master switch 58.

When the master switch 58 is in its OFF position, master switch contacts 67 are closed and master switch contacts 69-74 are open. Therefore, voltage is applied by the rectifier 60 to the winding 75w of the undervoltage relay 75 which closes its contacts 75a to form a holding circuit through a conductor 251 in a manner well known in the art.

When an operating handle of the master switch 58 is moved in a direction calling for hoisting operation of the motor, the normally closed master switch contacts 67 open and the normally open master switch contacts 69, 71 and 74 close. Energization of the winding 24w of the main contactor 24 through master switch contacts 74 closes the contacts 24a and 24b. The winding 32w of the dynamic braking contactor 32 is energized through the master switch contacts 71 and opens its normally closed contacts 32a and closes its normally

open contacts

32b, 32c (FIG. 2), and 32d (FIG. 3).

The energization of the winding 41w of the dynamic lowering contactor 41 through the master switch contacts 69 opens the contacts 410 and closes the contacts 41b. Closure of the contacts 4112 enables the winding 21w of the first hoisting contactor 21 to be energized and close its contacts 21a. Because the contacts 32b have been closed, the winding 39w of the second hoisting contactor 39 is also energized and the contacts 39a close. The contacts 3912 are opened to prevent operation of the lowering contactor 28. Until the winding 76w of the first load sensing relay 76 (FIG. 3) is energized, the teaser field contactor 36 will not operate.

In this manner, a hoisting circuit is provided for energizing the armature 11A and the field 11F as a seriesconnected motor through a circuit comprising the first AC-DC' converter 44, the conductor 45, the overload relay winding 46w, the junction 19, the conductor 17, the first hoisting contacts 21a, the limit switch contacts 15c, the armature 11A, the limit switch contacts 15d, the second hoisting contacts 39a, the field UP, the brake winding 16w, the main contacts 24a, the overload relay winding 25w, the first shunt 26, the junction 20, and the conductor 47. The teaser field resistor 37 is connected in parallel with the armature 11A through the still closed contacts 36a. I

During hoisting operation, the motor is energized solely by the first AC-DC converter 44; the second AC-DC converter 49 does not operate. Since, as is clear from the diagram of the master switch 58, hoisting operation is stepless, the speed of the motor is controlled by varying the output voltage of the first AC-DC converter 44.

When the operating handle of the master switch 58 is moved in a hoisting direction, a negative output voltage is produced in the signal converter 79 and transmitted to the reference control circuit 81 through the conductor 80.

Referring now to FIG. 2, the negative output voltage of the signal converter 79 is transmitted by the diode 94 and blocked by the diode 105 so that a negative voltage is applied only to the input resistor of amplifier 96 and to the hoisting permissive circuit 100. The hoisting permissive circuit 100, in response to the negative input voltage, transmits fixed negative output voltages to the input resistor 101 of amplifier 96, to the first firing circuit 88 through the conductor 87, and to the load sensing module 82 through the conductor 84 (see also FIG. 1). The voltage applied to the first firing circuit 88 by the hoisting permissive circuit acts as a permissive input which unclamps the first firing circuit 88 and permits it to respond to operating input voltage transmitted through the conductor 86. Since the lowering permissive circuit 106 is not activated during hoisting operation, the second firing circuit 91 is clamped by the lack of a permissive input so that the second AC-DC converter 49 remains inactive.

The amplifier 96 produces a positive output voltage which is proportional to the sum of the input voltages applied to the input resistors 95 and 101. This output voltage is transmitted through the conductor 86 to the first firing circuit 88 which, in a manner well known to those skilled in the art, produces a corresponding voltage output from the first AC-DC converter 44.

The constant negative biasing input voltage applied to amplifier 96 by the hoisting permissive circuit 100, through the input resistor 101, fixes the minimum output voltage of amplifier 96 and, accordingly, the minimum output voltage applied to the motor by the first AC-DC converter 44. Therefore, the input voltage to the resistor 101 determines the slowest speed at which the motor will operate and, also, the lightest load 14 which may be floated, i.e. held motionless by the motor with the brake disengaged. If it is desired to change the minimum available speed of the motor, or to float a lighter load 14, the wiper 102w of the minimum hoist adjusting potentiometer 102 may be adjusted to move the ground connection either closer to, or further from, the input resistor 101 to alter the biasing voltage applied to the input resistor 101. Moving the wiper 102w closer to the input resistor 101 lowers its input voltage and, accordingly, the minimum speed setting of the motor. Moving the wiper 102w away from the input resistor 101 increases its input voltage.

Although it is desirable to be able to adjust the minimum speed setting of the motor, this adjustment should not affect the motor speed obtained at the maximum speed setting of the master switch 58. Therefore, the input resistor 95 is connected to the other side of the potentiometer 102. When the wiper 102w is moved in a direction to increase the voltage through the input resistor 101, it causes a corresponding decrease in the voltage through the input resistor 95. Conversely, when the wiper 102w is moved in a direction to decrease the voltage through input resistor 101, it causes a corresponding increase in voltage through input resistor 95. In this manner, the voltage input through the resistor 95 is adjusted to compensate, at the maximum speed setting, for the change in input voltage at the resistor 101. At the minimum speed setting of the master switch 58, the voltage input through the resistor 95 is small so there is minimal compensation. However, when the output voltage of the signal converter 79 calls for full hoisting speed, the input voltage through the resistor 95 is of sufficient magnitude to fully compensate for the change in input voltage through the resistor 101 so that, regardless of the minimum hoisting speed setting, the maximum hoisting speed remains substantially the same.

The teaser field resistor 37, as is well known to those skilled in the art, is a relatively high resistance armature shunt which prevents a lightly loaded series motor from overspeeding. The teaser field resistor 37 should be removed from across the armature 1 1A to reduce heating of the field 11F whenever the load being hoisted is sufficient to, by itself, prevent overspeeding of the motor.

Application and removal of the teaser field resistor 37 is controlled by the load sensing module 82 (FIG. 3). When the motor is operated in the hoisting mode of operation, the magnitude of the motor current is dependent upon the torque required to hoist the load and thus upon the size of the load. Therefore, the load sensing module 82 can accurately determine the size of the load being hoisted by the motor by sensing motor current. Motor current is measured by the first shunt 26 the output of which is fed to the input resistor 168 of the amplifier. 169. This positive voltage signal is amplified and inverted by the amplifier 169 so that a negative voltage signal is transmitted by the amplifier 169 to the input resistor 171 of the amplifier 172.

A positive biasing voltage from the voltage source 177 is applied to the input resistor 180 of amplifier 172 through the potentiometer 175. The negative voltage from the reference control circuit 81 is transmitted through the conductor 84, the conductor 194, the diode 192 and across the potentiometer 191. The transistor 187 has its base connected through the conductor 238, the diode 239, the conductor 234 and the resistor 236 to a source of negative voltage 237, the transistor 231 being in a non-conducting condition. The resulting negative biasing of the transistor 187 causes it to conduct and ground the junction 186 so that, of the

resistors

182, 184 and 185, only the resistor 182 functions as an input resistor for the amplifier 172. The negative voltage across the potentiometer 191 is transmitted through the conductor 190 to the input resistor 182.

The negative voltage thus applied to the input resistor 182 is less than the positive voltage applied to the input resistor 180 so that a net positive biasing voltage is applied to amplifier 172. As long as the negative output voltage of amplifier 169 is less than this net positive biasing voltage, the amplifier 172 will produce a negative output voltage.

The amplifier 199 is a high gain switching inverting amplifier. If a positive voltage is applied to the input resistor 197, a negative output voltage of fixed magnitude is produced, regardless of the magnitude of the input voltage. Conversely, a negative input voltage produces a positive output voltage of fixed magnitude.

If the load being hoisted by the motor is too small to cause the input voltage through resistor 171 to exceed the net positive bias of the amplifier 172, the negative voltage to the input resistor 197 will cause the amplifier 199 to produce a positive voltage output signal. The negative voltage transmitted from the reference control circuit 81 through the conductor 84 is carried by the conductor 194 to the base of the transistor 195 which is thereby biased into conduction. This by-passes the resistor 205 so that the output voltage of the amplifier 199 is applied across a voltage divider comprising only the

resistors

202 and 204.

The amplifier 211 is an integrating amplifier and provides a time delay for the application of voltage to the base of the transistor 219. The duration of this time delay is dependent upon the magnitude of the input voltage to the amplifier and the resistance of the input resistor, the resistor 210 for a positive input voltage and the resistor 215 for the negative input voltage. The

resistors

210 and 215 are preferably chosen so that the input resistor 210 provides a time delay which is short in comparison with that provided by the input resistor 215.

Since the output voltage from amplifier 199 is positive, it is transmitted through the diode 209 to the input resistor 210. After a time delay, the amplifier 211 applies a negative output voltage to the base of the transistor 219. This biasing voltage will not enable the transistor 219 to turn on. Therefore, its collector, and accordingly the junction 221, is positively biased by the source of positive voltage 229 through the resistor 227.

The base of the transistor 231 is also positively biased so that it remains turned off with its collector maintained at negative voltage as previously described.

Transistors

241 and 243, their bases negatively biased, will not conduct so that neither the winding 76w of the first load sensing relay 76 nor the winding 77w of the second load sensing relay 77 is energized.

Since the contacts 76a (FIG. 1) of the first load sensing relay 76 remain open, the teaser field contactor winding 36w cannot be energized. Thus, when the motor is. hoisting a small load, the teaser field resistor 37 remains connected across the armature 11A.

If a load greater than a predetermined size is being hoisted by the motor, the current through the first shunt 26 is of a magnitude such that the negative output of the amplifier 169 (FIG. 3) exceeds the net positive biasing voltage input to amplifier 172 and amplifier 172 produces a positive output voltage. This positive output voltage, when applied to the input resistor 197 of amplifier 199, switches the output of amplifier 199 to its fixed negative voltage output. This is applied across the voltage divider, still comprising only the

resistors

202 and 204. The input of the integrating amplifier 211 is now tapped off the voltage divider through diode 214 to the input resistor 215.

After the time delay, which is of sufficient duration to prevent relay activation by current transients, the output of amplifier 211 becomes sufficiently positive to bias transistor 219 into conduction. The collector of transistor 219, and accordingly the junction 221, is now grounded thus causing the transistor 231 to conduct. The collector of the transistor 231 achieves a positive voltage and turns on

transistors

241 and 243. Conduction of transistor 243 activates the second load sensing relay 77 which has no effect upon the motor control circuit during hoisting operation. Conduction of transistor 241 activates the first load sensing relay 76. This causes the winding 36w (FIG. 1) of the teaser field contactor 36 to be energized resulting in opening of the contacts 36a removing the teaser field resistor 37 from across the armature 11A.

To provide positive, chatter-free operation of the first load sensing relay 76, a regenerative pulse feedback network comprising the resistor 227, the capacitor 224 and the input resistor 225 is utilized in conjunction with the integrating amplifier 211. The capacitor 224 is connected between the input resistor 225 and the junction 221. At the moment that the transistor 219 is biased into conduction so that the junction 221 attains ground potential, a negative voltage pulse is applied by the capacitor 224 to the input resistor 225. This provides additional input of short duration to enhance the output voltage of amplifier 21 1 and maintain the conducting condition of the transistor 219 until the unenhanced output of amplifier 211 has increased to its maximum value.

When the teaser field resistor 37 is removed from across the armature 11A, there is a decrease in motor current and, as a result thereof, in the voltage signal amplified and inverted by amplifier 169 and presented to the input resistor 171 of the amplifier 172. This decrease in negative voltage would cause the circuit to drop out the first load sensing relay 76 and result in relay chattering. Therefore, it is necessary to decrease the sensitivity of the circuitry after the relay 76 is activated to prevent reconnection of the teaser field resistor 37.

When the transistor 231 is turned on, its collector becomes positively biased, as has been previously shown. This positive voltage is not applied to the base of the transistor 187 because of the blocking action of the diode 239. This removes the biasing voltage to the transistor 187 which is thereby turned off. Now, the series combination of

resistors

184 and 185 is functionally connected in parallel with the resistor 182 and decreases the input resistance, and accordingly the gain, for the negative voltage provided by the potentiometer 191. The net positive bias of the amplifier 172 is thus further reduced and the sensitivity level of the circuit is lowered. Proper adjustment of the

potentiometers

175 and 191 will produce a change in sensitivity which provides proper compensation for the removal of the teaser fieldresistor 37 and permits the circuit to reconnect the teaser field resistor 37 in response to a decrease in current through the first shunt 26 to a level below that required for operation without the teaser field resistor 37.

If the load is decreased during operation of the motor, as when a load is released or a cable breaks, it is necessary to rapidly reconnect the teaser field resistor 37 across the armature 11A to prevent overspeeding of the motor. The decrease in current through the first shunt 26 corresponding to the decrease in motor load acts through

amplifiers

169 and 172, as previously indicated, to switch the output of amplifier 199 to full positive voltage. This voltage is transmitted by the diode 209 to the input resistor 210 which causes the amplifier 211 to have a time delay of very short duration (sufficiently short to provide rapid recycling after transients), after which the output of amplifier 211 decreases to a level which turns the transistor 219 and, in turn, the

transistors

231, 241 and 243 off. The winding 76w is deenergized and the contacts 76a open (FIG. 1) de-energizing the teaser field contactor winding 36w causing the contacts 36a to close the reconnect the teaser field resistor 37 across the armature 11A.

Motion of the operating handle of the master switch 58 (FIG. 1) in a direction toward the OFF position decreases the output voltage of the signal converter 79 and, correspondingly, of the reference control circuit 81 to reduce the direct current output of the AC-DC converter 44 and decrease the speed of the motor. When the operating handle reaches the OFF position, master switch contacts 69, 71 and 74 open to deenergize the contactors controlled thereby. The AC-DC converter 44 is turned off and operating power is, therefore, removed from the armature 1 1A and field 11F. No current now flows through the brake winding 16w and the brake 16 is thereby set and the motor stops.

When the operating handle of the master switch 58 (FIG. 1) is moved from the OFF position in a direction calling for lowering operation of the motor, the master switch contacts 67 open and the master switch contacts 70 and 74 close to energize the windings 28w and 24w, respectively. Energization of the main contactor winding 24w closes the contacts 24a and 24b. Energization of the lowering contactor winding 28w through the now closed hoisting contacts 39b close the contacts 28a and 280 (FIG. 2) and opens the contacts 28b to lock out the hoisting circuitry.

For lowering, the motor is shunt connected with the armature 11A being powered by the first AC-DC converter 44 through a circuit comprising the first AC-DC converter 44, the overload relay winding 46w, the conductor 45, the junction 19,'the conductor 27, the lowering contacts 28a, the conductor 34, the armature 11A, the conductor 40, the dynamic lowering contacts 41a, the brake winding 16w, the main contacts 24a, the overload relay winding 25w, the first shunt 26, the junction 20, and the conductor 47. The dynamic braking resistor 31 is connected by the dynamic braking contacts 32a across the armature 11A. The limit switch relay winding 30w is connected in parallel with the armature 11A, as during hoisting operation, to be energized by the first AC-DC converter 44 and close contacts 30a and open contacts 30b (see FIG. 2). The relay 30 becomes activated after the speed of the armature 11A has increased sufficiently to increase the counter emf and permit an adequate voltage to be applied across the winding 30w to energize it.

The field 11F is energized by the second AC-DC converter 49 through a circuit comprising the second AC-DC converter 49, the conductor 50, the field 11F, the brake winding 16w, the main contacts 24w, the overload relay winding 25w, the first shunt 26, the junction 20, the conductor 51 and the second shunt 52. The speed of the now shunt-connected motor is determined by the voltage applied to the armature 11A from the first AC-DC converter 44 and the voltage applied to the field 11F by the second AC-DC converter 49.

When the operating handle of the master switch 58 is moved in a lowering direction, a positive output voltage is produced by the signal converter 79 and transmitted to the reference control circuit 81 through the conductor 80. This positive voltage is transmitted by the diode (FIG. 2) and blocked by the diode 94 so that it is only applied through the conductor 104 to the lowering permissive circuit 106, the input resistor 107 of amplifier 108, the input resistor 109 of amplifier 110, and, through the conductor 111, to the inputs of amplifier 113.

The lowering permissive circuit 106, in response to the positive input voltage, transmits a fixed negative voltage output signal to the first firing circuit 88 through the conductor 87, to the second firing circuit 91 through the conductor 89, and to the input resistor 114 of amplifier 113.

Amplifier 108 sums the positive input voltage with a negative biasing voltage applied to the input resistor 116 from the source 117 and produces a negative output voltage which diminishes in magnitude as the voltage output of the signal converter 79 increases. This negative voltage is applied to the input resistor 119 of amplifier 120 and summed with a positive biasing voltage applied to the input resistor 122 from the source 124. Since the dynamic braking contacts 320 are open, the transistor 129 is turned off and a biasing voltage from the negative voltage source 157 is applied to the input resistor 125 of the amplifier 120. This biasing voltage is sufficient in magnitude to cause the amplifier 120 to produce a positive voltage regardless of the output of amplifier 108. This positive voltage from amplifier 120 is blocked'by the diode 131 so that, until the dynamic braking contacts 320 close, the effective output of amplifier 120 is zero.

Amplifier 110 produces a negative output voltage proportional to its positive input voltage. However, the input to amplifier 110, and hence its output, is limited because it is connected through the diode 136 to the voltage limiting circuit comprising the transistor 137 and the resistor 139. When both the

contacts

30b and 320 are open, the potentiometer 141 is connected between a source of positive voltage 142 and ground through the base-emitter circuit of the transistor 167. The base of the transistor 137 is thus maintained at positive voltage which is of a value determined by the setting of the potentiometer 141 and is in turn applied, through the base-emitter circuit of the transistor 137, to reverse-bias the diode 136. Output voltage of the signal converter 79 not exceeding this reverse-biasing voltage, preferably sufficiently high as not to interfere with motor speed, is applied to the input resistor 109 of amplifier 110.

v If either the

contacts

30b or 320 close, the contacts 28c being closed throughout lowering operation, the negative voltage source ,150 is connected, through the diode 146 or the diode 147, respectively, to the junction 145 and increases the voltage drop across the potentiometer 141. The voltage at the base and emitter of the transistor 137 is reduced to a value also determined by the setting of the potentiometer 141, and the voltage to the input resistor 109 of amplifier 110 is limited to a value substantially equal to the emitter voltage of the transistor 137. Thus, when either the limit switch relay contacts 30b or the dynamic braking contacts 32c are closed, the negative voltage output of the amplifier 110 is limited to a value determined by the setting of the potentiometer 141.

The negative output voltage of amplifier 110 is applied through the conductor 159, the diode 160 and the conductor 132 to the input resistor 133 of amplifier 96. Since there are no other voltage inputs to amplifier 96 during lowering operation, its output voltage is proportional to this input signal and is applied to the first firing circuit 88 so that the first AC-DC converter 44 increases the voltage applied to the armature 11A as the operating handle of the master switch 58 is moved further in the lowering direction.

Energization of the field 11F is controlled by the output of amplifier 113. Input voltage to amplifier 113 is supplied by three sources. A constant negative voltage input is provided by the lowering permissive circuit 106 to the input resistor 114. A negative signal, proportional to current through the field 11F, is transmitted from the second shunt 52 through the conductor to the inverting amplifier 162 from which a positive voltage is applied to the input resistor 164 of amplifier l 13. This voltage is a feedback input to help ensure proper energization of the field 11F. The output voltage of the signal converter 79 is transmitted through the conductor 111 and applied to the input resistor 112 at the inverting input of amplifier 113 and, through the potentiometer 166, to the non-inverting input of amplifier 113.

When both the dynamic braking contacts 32c and the limit switch contacts 30b are open, the base of the transistor 167, connected to the junction 145, is positively biased and the transistor 167 turns on so that the noninverting input of amplifier 113 is connected to ground and the voltage transmitted from thesignal converter 79 through the conductor 111 is applied solely to the inverting input. The fixed negative voltage from the lowering permissive circuit 106 is of sufficient magnitude so that the output voltage of amplifier 113 results in maximum output of the second AC-DC converter 49. In this manner, maximum energization of the field 11F results to provide the minimum lowering speed of the motor. As the operating handle of the master switch 58 is moved further in the lowering direction, the positive output voltage of the signal converter 79 increases and is combined with the fixed negative voltage of the lowering permissive circuit 106 to form an output which decreases the voltage across the field 11F. The input voltage provided by the second shunt 52 serves as a feedback signal to aid in the stabilization of the energization of the field 11F. Thus, when the operating handle of the master switch 58 is moved further in the lowering direction, the voltage across the armature 11A is increased while the field 11F is weakened to produce an increase in motor speed.

The relatively high resistance of the isolation resistor 22 (FIG. 1) permits the armature 11A and the field 1 1F to be generally separately energized and controlled while, at the same time it maintains an electrical connection between the armature 11A and field 11F so that a dynamic lowering loop, which also functions to provide emergency dynamic braking upon power failure, may be provided. The dynamic lowering loop comprises the armature 11A, the limit switch contacts 15d, the isolation resistor 22, the field 11F, the conductor 40 and the dynamic lowering contacts 41a.

The dynamic braking resistor 31 (FIG. 1) must be connected across the armature 11A to maintain slow lowering speeds under heavy load conditions. However, the dynamic braking resistor 31 is speed limiting and must, therefore, be disconnected to obtain high speed operation, leaving only the dynamic lowering loop effective in the circuit. However, the significant change in speed which may occur when the dynamic braking resistor 31 is removed from across the armature 11A is undesirable in many applications of the hoist control. Therefore, the control circuit of the present invention provides for the removal of the dynamic braking resistor 31 from across the armature 11A with no material change in speed, regardless of the size of the load being lowered. This is accomplished by selecting a dynamic braking resistor 31 and an isolation resistor 22 which are of proper ohmic value, removing the dynamic braking resistor 31 at a predetermined value of motor current, and adjusting the outputs of the AC-

DC converters

44 and 49 to compensate for the change in speed which otherwise would occur at the moment the dynamic braking resistor 31 is removed.

If the operating handle of the master switch 58 is moved to the intermediate speed lowering range, the master switch contacts 73 close so that the dynamic braking contactor winding 32w may be energized through the second load sensing relay contacts 77a and the limit switch relay contacts 30a. If the limit switch has not been tripped, the contacts 30a will be closed so that energization of the winding 32w will be controlled by the condition of the contacts 77a. In this manner, the disconnecting of the dynamic braking resistor 31 may be controlled by the load sensing module 82.

The load sensing module 82 removes the dynamic braking resistor 31 from across the armature 1 1A when a predetermined value of motor current is detected through the first shunt 26. Because the first shunt 26 is connected in that portion of the power circuit between the brake winding 16w and the junction 20, both the armature current and. the field current pass through the first shunt 26 so that the sum of the currents of the AC-

DC converters

44 and 49 are measured.

As during hoisting, the positive voltage signal from the first shunt 26 is transmitted through the conductor 83 to the input resistor 168 of amplifier 169 (FIG. 3) which transmits a negative voltage signal to the input resistor 171 of amplifier 172. Amplifier 172 combines this signal with the positive input voltage from the potentiometer 175. During lowering, input to the hoisting permissive circuit 100 (FIG. 2) is blocked by the diode 94 so that there is no voltage signal transmitted along the conductor 84 and thus no voltage input to amplifier 172 from the potentiometer 191.

As during hoistingoperation, until the negative output of amplifier 169 exceeds the positive biasing voltage, amplifier 172 has a negative output which causes amplifier 199 to produce a fixed positive output voltage. This voltage, when inverted by amplifier 211, negatively biases the transistor 219 so that the load sensing relays 76 and 77 are not activated. However, when the negative output of amplifier 169 exceeds the positive biasing voltage of the potentiometer 175, the output of amplifier 172 becomes positive and switches amplifier 199 to its fixed negative output voltage.

Since there is no'voltage input from the reference control circuit 81 through the conductor 84, the transistor 195 is not conducting and, accordingly, the voltage divider across which the output of amplifier 199 is placed comprises the three

resistors

202, 204, and 205 so that a greater voltage is presented to amplifier 211 than was present during hoisting operation. The increased voltage input reduces the time delay provided by the integrating'amplifier 211 to an interval which is appropriate for lowering operation. The input voltage to amplifier 211 is transmitted through the diode 214 to the input resistor 215 which provides the suitable time delay after which a sufficient positive biasing voltage is applied to the base of the transistor 219 to ground its collector which in turn biases the transistor 231 into conduction and produces a short duration negative input pulse in the regenerative pulse feedback network, as previously described.

The conduction of the transistor 231 causes its collector to attain a positive voltage which biases the transistor 243 into conduction and energizes the second load sensing relay winding 77w. Transistor 241 is also biased into conduction and energizes the first load sensing relay winding 76w. However, since the contacts 76a are in the hoisting portion of the control circuit, operation of this relay has no effect upon circuit operation during lowering.

When the contacts 77a close (FIG. 1), the dynamic braking contactor winding 32w is energized opening contacts 32a to remove the dynamic braking resistor 31 from across the armature 11A and to close the auxiliary dynamic braking contacts, including the contacts 32d in the load sensing module 82 (FIG. 3). Because there is a significant reduction in motor current when the dynamic braking resistor 31 is removed from across the armature 11A, the contacts 32d complete a holding circuit for the winding 77w comprising the positive voltage source 247, the winding 77w, the conductor 249, the contacts 32d and the negative voltage source 250. This holding circuit maintains the energized state of the winding 77w regardless of the current through the first shunt 26 until the dynamic braking contactor winding 32w is de-energized by manually moving the operating handle of the master switch 58 to the low speed lowering range thereby opening the master switch contacts 73.

When the dynamic braking resistor 31 is removed, the output of the reference control circuit 8] fed to the

firing circuits

88 and 91 must be compensated to prevent any material change in lowering speed.

If the values of the dynamic braking resistor 31- and the isolation resistor 22 are properly chosen, the speed range of the motor when lowering with the dynamic braking resistor 31 connected will overlap the lowering speed range without the dynamic braking resistor regardless of the load on the motor. Thus, at speeds within this overlap range, if motor speed is adjusted simultaneously with the change of operating mode, the

dynamic braking resistor 31 may be disconnected from the circuit without causing a material change in motor speed. As the motor load is increased, the corresponding increase in speed resulting when the dynamic braking resistor 31 is disconnected increases proportionally with the load. Therefore provision must be made for decreasing the voltage across the armature 11A by dropping the output of the reference control circuit 81 to the first firing circuit 88 and strengthening the field 11F by increasing the output of the reference control circuit 81 to the second firing circuit 91 in a manner which will prevent the material change in speed which would otherwise occur.

As has been previously indicated with reference to FIG. 2, for slow speed positions of the master switch 58, with the dynamic braking contacts 320 open, the effective output of amplifier 120 is zero so that only the negative output voltage provided by amplifier is transmitted to amplifier 96. As the output voltage from the signal converter 79 increases, the output of amplifier 110 increases causing a corresponding increase in lowering speed and in motor current. At a predetermined value of current through the first shunt 26, the load sensing module 82 activates the second load sensing relay 77 to disconnect the dynamic braking resistor 31. Activation of the dynamic braking contactor 32 closes contacts 320 and increases the voltage drop across the potentiometer 141 which clamps the voltage applied to the input resistor 109 of amplifier 110. The corresponding decrease in output of amplifier 110, applied through amplifier 96 to the first firing circuit 88, decreases the voltage across the armature 11A. The closing of the contacts 32c also turns off transistor 167 allowing the output voltage from the signal converter 79 to be applied through the potentiometer 166 to the non-inverting input of amplifier 113. This partially negates the effect of the signal applied to the input resistor 112, to an extent determined by the setting of the potentiometer 166, and increases the output of amplifier 113 to the second firing circuit 91 to increase the voltage across the field 1 IF. Without any other change in the motor circuit, these voltage changes would cause the speed of the motor to decrease. However, if the potentiometers 141 and 166 are properly adjusted, the changes in the voltages of the armature 11A and field 11F will just compensate for the increase in speed which would have otherwise resulted from the removal of the dynamic braking resistor 31 from across the armature 11A.

If the motor is operating with a load on the hook, there will be an increase in counter emf of the motor so that the current through the first shunt 26 will not reach the value necessary to operate the load sensing module 82 until the motor has attained a greater speed than with an empty hook. This requires an increased output of the signal converter 79. Thus, the output voltage of amplifier 110 will have increased and the output voltage of amplifier 113 will have decreased beyond their empty hook values before the contacts 32c close to adjust their outputs. The increase in effect on the output of the AC-

DC converters

44 and 49 as the load 14 is increased just compensates for the increase in effect on motor speed with load 14 caused by removal of the dynamic braking resistor 31.

Operation of the reference control circuit 81 during automatic transfer from dynamic braking to dynamic lowering mode may be more clearly understood with reference to the graphs of FIGS. 4-9.

Operation of amplifier 1 10 is illustrated by the graphs of FIGS. 4 and 5, each of which is a graph of the output of amplifier 110 (A-110 on the graph) versus the output of the signal converter 79 (S/C on the graph). Points a and b indicate, respectively, the minimum and maximum outputs of the signal converter 79 for which automatic mode transfer can be provided. Point c represents maximum output of the signal converter 79 during lowering operation. Point d represents the clamped value of output voltage of amplifier 110 after the dynamic braking contacts 32c close. As can be seen in FIG. 4, which illustrates empty hook operation, when the output of the signal converter 79 reaches the minimum transfer point a, the output of amplifier 110 is decreased by a small amount to the clamped level d at which it remains for all higher values of signal converter 79 output.

FIG. 5 illustrates operation of amplifier 110 during lowering operation with a load on the hook. When the output of the signal converter 79 reaches the value indicated at a, the current through the first shunt 26, due to increased counter emf in the motor, is not sufficient to trigger operation of the load sensing module 82. Therefore, the output of the signal converter 79 must be increased beyond the point a, correspondingly increasing the output of amplifier 1 10. When the current through the first shunt 26 is sufficient to trigger the load sensing module 82, the output of amplifier is diminished to the value indicated at d and held at that value for all greater outputs of the signal converter 79.

Although in certain applications of the crane control circuit of the present invention, it would be possible to match the speeds during automatic mode transfer solely through an adjustment of the voltage across the armature 11A, generally the voltage across the field 11F must be increased to properly match the dynamic lowering and dynamic braking mode speeds. FIGS. 8 and 9 are graphs of output of amplifier 113 (A-l13 on the graph) versus signal converter 79 output. FIG. 8 illustrates empty hook operation corresponding to that illustrated in FIG. 4 for amplifier 110. As signal converter 79 output increases, the output of amplifier 113 decreases along the curve 252 weakening the field 11F. At the minimum transfer point a, upon operation of the load sensing module 82, the signal converter 79 output applied to the non-inverting input of amplifier 113 boosts the output of amplifier 113 to the. field weakening curve 254 and thereby increases the voltage across the field 11F to 'a magnitude which will provide no material change in speed during mode transfer.

FIG. 9 illustrates operation of amplifier 113 during lowering operation with a load on the hook and corresponds to the operation of amplifier 110 illustrated in FIG. 5. When the current through the first shunt 26 reaches the value necessary for transfer, the output of amplifier 113 has decreased to less than that illustrated in FIG. 8 due to the increase in output voltage of the signal converter 79. At transfer, this increased signal converter voltage is applied, through the potentiometer 166, to the non-inverting input of'amplifier 113 so that the field is strengthened by a proportional amount to a corresponding point on the field weakening curve 254.

When the dynamic braking contacts 320 close, the transistor 129 is biased into conduction so that the negative biasing input is connected to ground and not applied to amplifier 120. Now the only inputs to amplifier are the fixed biasing voltage from the positive voltage source 124 and the negative output voltage of amplifier 108. Since the negative output voltage of ampli-,

fier 108 decreases in magnitude as signal converter 79 output increases, the combined input voltage causes amplifier 120 to produce a negative voltage output which increases in magnitude with increasing output of the signal converter 79.

The outputs of

amplifiers

110 and 120 are transmitted, through diodes and 131, respectively, so that only the negative output signal having the greatest magnitude is transmitted to amplifier 96. FIG. 6 illustrates the output of amplifier 120 (Al20 on the graph) plot ted against the output of the signal converter 79. Until contacts 320 close, amplifier 120 has a positive output which may be of any magnitude and which is blocked by the diode 131 yielding an effective zero output. When the contacts 32:: close, a negative output voltage is produced by amplifier 120 but cannot be transmitted through the diode 131 because this output voltage does not exceed that of amplifier 110, which is clamped at the indicated point d, until the output voltage of the signal converter 79 exceeds the value indicated at b. Thus, the output voltage provided by the combined outputs of amplifier 110 and amplifier 120 through diode 160 and diode 131, respectively, and transmitted to the amplifier 96 generally follows the curve indicated in FIG. 7, a plot of the input to amplifier 96 (A-96) versus the output of the signal converter 79. The voltage applied to the armature 11A during lowering operation of the motor is generally proportional to the input to the amplifier 96 and therefore, also follows the curve of FIG. 7.

Excitation of the field 1 IF, as has been previously indicated, is controlled by the output of amplifier 113. Before the contacts 320 close, the output of amplifier 113 is diminished at a rate illustrated by the curve 252 in FIG. 9. However, after the dynamic braking resistor 131 has been disconnected and the transistor 167 has been turned off, signal converter 79 output voltage applied to the non-inverting input of amplifier 113 shifts its output to the curve 254 in FIG. 9. In this manner, proper field weakening is provided throughout the lowering range of the motor.

If it is desired to rapidly obtain high speed lowering for the motor wherein the change in speed resulting from removal of the dynamic braking resistor 31 is not important, the operating handle of the master switch 58 may be moved in the lowering direction until the master switch contacts 72 close. This completes an energizing circuit for the winding 32w which bypasses the second load sensing relay contacts 77a so that the dynamic braking resistor 31 is instantly removed from across the armature llA, as long as the limit switch relay contacts 31a are closed.

As the operating handle of the master switch 58 is moved in a direction toward the OFF position, the output of the signal converter 79 decreases. This decreases the voltage across the armature 11A and strengthens the field 11F. When the operating handle reaches the low speed lowering range, the contacts 73 open and the dynamic braking resistor 31 is reconnected across the armature 11A. The dynamic braking contacts 32c (FIG. 2) now open so that the voltage across the armature 11A is now controlled by the output of amplifier 110 and the energization of the field 11F now follows the curve 252 in FIG. 8. When the master switchis placed in the OFF position, the contacts 70 and 74 open and the AC-

DC converters

44 and 49 are turned off so that the armature 11A and field 11F are deenergized. No current flows now through the brake winding 16w and the brake 16 is set. In this manner, the motor is stopped.

If, during hoisting operation of the motor, the load 14 is hoisted a sufficient distance to trip the powerlimit switch 15, contacts 15a and 15b close and contacts 150 and 15d open. Contacts 150 and 15d, located in the hoisting power circuit, disconnect operating voltage from the armature 11A and field 11F. At the same time a dynamic braking loop for stopping the armature is formed comprising the armature 11A, the conductor 40, the conductor 42, the limit switch contacts 15a, the second hoisting contacts 39a, the field 11F, the conductor 27, the limit switch resistor 29, the limit switch contacts 15b and the conductor 34. Tripping of the limit switch 15 removes energizing voltage from the limit switch relay winding 30w so that the contacts 30a are open and the auxiliary contacts 30b (FIG. 2) are closed until the limit switch 15 is reset.

During lowering operation with the limit switch 15 tripped, the armature 11A and field 11F are energized through the same circuit as without the limit switch 15 tripped since the energizing circuits do not include the limit switch contacts 150 and 15d. The dynamic braking resistor 31 is connected across the armature 11A. Since the limit switch relay contacts 30a in the master switch 58 are open, the dynamic braking relay winding 32w cannot be energized while lowering with the limit switch 15 tripped so that the dynamic braking resistor 31 cannot be removed from across the armature 11A until the limit switch has reset.

The closing of the limit switch relay contacts 30b (FIG. 2) clamps the output of amplifier without unclamping the output of amplifier 120. Since the dynamic braking contacts 32c cannot be closed until the limit switch has reset, the output of amplifier remains clamped and the motor can only be lowered in its low speed range, regardless of the output of the signal converter 79.

When the limit switch 15 resets, the limit switch relay 30 is activated and normal operation of the reference control circuit 81 is restored.

It should be understood that for certain applications of the circuits of the present invention it may be possible to match speeds during automatic mode transfer without strengthening the field 11F at the transfer point. The reference control circuit is readily adaptable for operation in this manner by adjusting the potentiometer 161 to connect the non-inverting input of amplifier 113 to ground.

If it is desired, the reference control circuit 81 may be provided with a current limiting circuit, as is well known in the art, to limit current from the first AC-DC converter 44. A control signal may be provided by a shunt connected between the junction 20 and the first AC-DC converter 44, in which case the first shunt 26 may be eliminated. The signal for operating the load sensing module 82 may then be obtained by combining the outputs of the two shunts.

Although the control system of this invention has been disclosed for use as a hoist control, it is useable in any motor control system wherein the force of the load opposes armature rotation in one direction of rotation of the armature 11A (non-overhauling load) and assists armature rotation in the other direction of rotation of the armature 11A (generally an overhauling load).

We claim:

1. A control system for operating a direct current motor having an armature and a series-wound field from an alternating current source, said system comprising a motor having an armature and a series-wound field, first and second-AC-DC converter means each having control and power input terminals and output terminals, said source beingelectrically connected to the power input terminals of the first and second converter means, master switch means selectively operable in a first range for controlling operation of the motor in one direction of rotation and a second range for controlling operation of the motor in an other direction of rotation, signal means producing a voltage of a first polarity when the master switch means is in its first range and producing a voltage of a second polarity when the master switch is in its second range, reference control means electrically connected to the signal means and to the control input terminals of the first and second converter means and responsive to a voltage of the first polarity from the signal means to produce a direct current output from only the first converter means and responsive to a voltage of the second polarity from the signal means to produce direct current outputs from both the first and the second converter means, means serially connectingthe armature and the field across the output terminals of the first converter means when the master switch means is in its first range, means connecting the armature across the output terminals of the first converter means and connecting the field across the output terminals of the second converter means when the master switch means is in its second range, and an isolation resistor serially connected between the armature and the field so that excitation of the field is related to the voltage across the armature when the master switch means is in its second range.

2. A control system as in claim 1 wherein said means serially connecting the armature and the field across the output terminals of a first converter means and said means connecting the armature across the output terminals of the first converter means and connecting the field across the output terminals of the second converter means maintains the polarity of the voltage impressed on the field the same for both ranges of the master switch and causes the voltage impressed on the armature to be of one polarity in the first range of the master switch and of a different polarity in the second 30 range of the master switch.

3. A control system as in claim 1 wherein said first range of said master switch means is a non-overhauling range in which a load on the motor is normally nonoverhauling and said second range of said master switch means is an overhauling'range in which the load on the motor is normally overhauling,

4. A control system as in claim 1 wherein said system is for driving a crane hoist, said first range of said master switch means is a hoisting range, and said second 40 range of said master switch means is a-lowering range.

5. A control system as in claim 4 including an overhoist limit switch having normally closed limit switch contacts and wherein said means serially connecting the armature and the field across the output terminals of the first converter means when the master switch mean is in its hoisting range includes the limit switch contacts and wherein said means connecting the arma ture across the output terminals of the first converter means and connecting the field across the output terminals of the second converter means when the master switch means is in its lowering range is independent of the limit switch contacts.

6. A control system as in claim 1 and including an electromagnetically-released brake means having a release winding connected serially with said armature and field and energized by the direct current output of only said first AC-DC converter means when said master switch means is in its first range and by the direct current outputs of both said first and second AC-DC converter means when said master switch means is in its second range.

7. A control system as in claim 1 wherein said master switch means includes means for changing thevoltage' of said signal means, and wherein said reference control means includes means responsive to a change in the voltage of the first polarity from the signal means to produce a change in the direct current output of said first AC DC converter means and means responsive to a change in the voltage of the second polarity from said signal means to produce a change in the direct current outputs of both said first and second AC-DC converter means.

8. A control system as in claim 1 and including a teaser field resistor, switching means for connecting the teaser field resistor across the armature, and current sensitive means for operating the switching means at a predetermined magnitude of the direct current output of the first converter means while the master switch is in the first range, the teaser field resistor being connected across the armature at low magnitudes of the direct current output and being disconnected from the armature at high magnitudes of the direct current output.

9. A control system for operating a direct current motor having an armature and a series-wound field from an alternating current source, said system comprising a motor having an armature and a series-wound field, AC-DC converter means having control and power input terminals and output terminals, said source being electrically connected to the power input terminals of the converter means, master switch means having an operating range for controlling operation of the motor, signal means for producing a voltage of a selected polarity and of a magnitude controlled by said master switch means, reference control means electrically connected to said signal means and to the control input terminal on the converter means and responsive to the magnitude of the voltage from said signal means to control a direct current output from the converter means; at said output terminals, means serially connecting the armature and the field across said output terminals, a teaser field resistor connected across the armature, and current sensitive means for disconnecting the teaser field resistor at a predetermined magnitude of the direct current output of the converter means; said current sensitive means comprising current-indicating means interposed in the motor circuit for producing a current-magnitude signal indicating the magnitude of direct current output of said AC-DC converter means, and load sensing means connected to said current: indicating means and responsive to a first predetermined magnitude of said current-magnitude signal to disconnect said teaser field resistor and responsive to a second predetermined magnitude of said currentmagnitude signal to connect said teaser field resistor; said load sensing means comprising biasing means providinga biasing signal, comparison means connected to said current-indicating means for comparing said current-magnitude signal'to said biasing signal and producing a voltage of one polarity if said current-magnitude signal is of greater magnitude than the biasing signal and of an other polarity if said current-magnitude signal is of smaller magnitude than the biasing signal, time delay means connected to said comparison means and providing an output after a time delay, and relay means responsive to the output'of the time delay means for selectively connecting and disconnecting said teaser field resistor.

10. A control system as in claim 9 wherein said time 5 delay means comprises amplifier means having an input terminal and an output terminal, a first capacitor connected between the input terminal and the output terminal, a first input resistor providing a time delay of a first predetermined duration, a second input resistor providing a time delay of a second predetermined duration, and rectifier means applying voltage of said one polarity from said comparison means to the first input resistor and applying voltage of said other polarity from said comparison means to the second input resistor.

1 l. A control system as in claim wherein said time delay means includes switching means connected to the output terminal of said amplifier means for conducting current when a voltage of said one polarity is applied to said time delay means and blocking current when a voltage of said other polarity is applied to said time delay means, said switching means being connected to a source of voltage of said other polarity, a second capacitor connected between the source of voltage and the input terminal of said amplifier means for discharging through said amplifier means when said switching means begins to conduct.

12. A control system as in claim 9 wherein said biasing means includes means connected between said relay means and said comparison means providing a first predetermined magnitude of said biasing signal and when said teaser field resistor is connected across said armature and a second predetermined magnitude of said biasing signal when said teaser field resistor is disconnected.

13. A control system as in claim 9 wherein said reference control means includes amplifier means having an input terminal, a first input resistor connected to the amplifier input terminal for receiving voltage of said selected polarity from said signal means, a second input resistor connected between the amplifier input terminal and a source of voltage of said selected polarity, and voltage divider means connected between the first and second input resistors and having a wiper connected at a voltage of predetermined magnitude.

14. A control system for operating a direct current motor comprising an armature and a series-wound field -from an alternating current source, said system comprising a motor having an armature and a field, first and second AC-DC converter means each having control and power input terminals and output terminals, said source being electrically connected to the power input terminals of the first and second converter means, master switch means having an operating range for controlling operation of the motor when the motor is subject to an overhauling load, signal means for producing a voltage of a first polarity and of a magnitude controlled by said master switch means, reference control means electrically connected to said signal means and to the control input terminals of both the first and second converter means and responsive to the magnitude of the voltage from the signal means to control direct current output from both the first and second converter means, means connecting the armature across the output terminals of the first converter means and connecting the field across the output terminals of the second converter means, a dynamic braking resistor connected across the armature, and current sensitive means for disconnecting the dynamic braking resistor at a predetermined magnitude of direct current output from both the first and second converter means.

15. A control system as in claim 14 wherein said current sensitive means comprises current-indicating means for producing a current-magnitude signal indicating the magnitude of the direct current output from both said first and second AC-DC converter means,

and load sensing means connected to said currentindicating means and responsive to said currentmagnitude signal produced at said predetermined magnitude of direct current output to disconnect said dynamic braking resistor.

16. A control system as in claim 15 wherein said load sensing means comprises comparison means connected to said current-indicating means comparing said current-magnitude signal to a biasing signal of predetermined magnitude and producing a voltage of one polarity if said current-magnitude signal is greater than the biasing signal and of an other polarity if said currentmagnitude signal is less than the biasing signal, time delay means connected to said comparison means and providing an output after a time delay, and relay means responsive to the output of the time delay means for disconnecting said dynamic braking resistor.

17. A control system as in claim 16 wherein said time delay means comprises amplifier means having an input terminal, a first input resistor providing a time delay of a first predetermined duration, a second input resistor providing a time delay of a second predetermined duration, and rectifier means applying voltage of said one polarity from said comparison means to the first input resistor and applying voltage of said other polarity from said comparison means to the second input resistor.

18. A control system as in claim 14 wherein said reference control means includes matching means preventing material change in speed of said motor when said dynamic braking resistor is disconnected.

19. A control system as in claim 18 wherein said reference control means includes first amplifier means having an output terminal connected to said first AC-DC converter means and an input terminal, second amplifier means having an output terminal from which an output voltage is transmitted to the first amplifier means in response to an input voltage from said signal means, and third amplifier means having an output terminal and from which an output voltage is transmitted to the first amplifier means in response to an input voltage from said signal means, and said matching means includes switching means limiting the output voltage of the second amplifier means when said dynamic braking resistor is connected across said armature and limiting the output voltage of the third amplifier means when the dynamic braking resistor is disconnected, and means connected between the input terminal of the first amplifier means and the output terminals of the second and third amplifier means permitting the output voltage of greater magnitude to be applied to the first amplifier means.

20. A control system as in claim 19 wherein said reference control means includes fourth amplifier means having an output terminal connected to the control input terminal of said second AC-DC converter means and inverting a non-inverting input terminals, the inverting input terminal being connected to receive said voltage of said first polarity from said signal means, a source of voltage of a second polarity, opposite from said first polarity, connected to the inverting input terminal of the fourth amplifier means, and resistance means providing a portion of said signal means voltage to the non-inverting input terminal of the fourth amplifier means, and wherein said switching means includes means diverting said signal means voltage from the non-inverting input terminal when said dynamic braking resistor is connected across said armature.

26 min'al of said fourth amplifier means for applying a voltage of said first polarity indicating the magnitude of direct current output of said second converter means to the non-inverting terminal.

. a t a

Claims

1. A control system for operating a direct current motor having an armature and a series-wound field fRom an alternating current source, said system comprising a motor having an armature and a series-wound field, first and second AC-DC converter means each having control and power input terminals and output terminals, said source being electrically connected to the power input terminals of the first and second converter means, master switch means selectively operable in a first range for controlling operation of the motor in one direction of rotation and a second range for controlling operation of the motor in an other direction of rotation, signal means producing a voltage of a first polarity when the master switch means is in its first range and producing a voltage of a second polarity when the master switch is in its second range, reference control means electrically connected to the signal means and to the control input terminals of the first and second converter means and responsive to a voltage of the first polarity from the signal means to produce a direct current output from only the first converter means and responsive to a voltage of the second polarity from the signal means to produce direct current outputs from both the first and the second converter means, means serially connecting the armature and the field across the output terminals of the first converter means when the master switch means is in its first range, means connecting the armature across the output terminals of the first converter means and connecting the field across the output terminals of the second converter means when the master switch means is in its second range, and an isolation resistor serially connected between the armature and the field so that excitation of the field is related to the voltage across the armature when the master switch means is in its second range.

2. A control system as in claim 1 wherein said means serially connecting the armature and the field across the output terminals of a first converter means and said means connecting the armature across the output terminals of the first converter means and connecting the field across the output terminals of the second converter means maintains the polarity of the voltage impressed on the field the same for both ranges of the master switch and causes the voltage impressed on the armature to be of one polarity in the first range of the master switch and of a different polarity in the second range of the master switch.

3. A control system as in claim 1 wherein said first range of said master switch means is a non-overhauling range in which a load on the motor is normally non-overhauling and said second range of said master switch means is an overhauling range in which the load on the motor is normally overhauling.

4. A control system as in claim 1 wherein said system is for driving a crane hoist, said first range of said master switch means is a hoisting range, and said second range of said master switch means is a lowering range.

5. A control system as in claim 4 including an overhoist limit switch having normally closed limit switch contacts and wherein said means serially connecting the armature and the field across the output terminals of the first converter means when the master switch mean is in its hoisting range includes the limit switch contacts and wherein said means connecting the armature across the output terminals of the first converter means and connecting the field across the output terminals of the second converter means when the master switch means is in its lowering range is independent of the limit switch contacts.

7. A control system as in claim 1 wherein said Master switch means includes means for changing the voltage of said signal means, and wherein said reference control means includes means responsive to a change in the voltage of the first polarity from the signal means to produce a change in the direct current output of said first AC-DC converter means and means responsive to a change in the voltage of the second polarity from said signal means to produce a change in the direct current outputs of both said first and second AC-DC converter means.

9. A control system for operating a direct current motor having an armature and a series-wound field from an alternating current source, said system comprising a motor having an armature and a series-wound field, AC-DC converter means having control and power input terminals and output terminals, said source being electrically connected to the power input terminals of the converter means, master switch means having an operating range for controlling operation of the motor, signal means for producing a voltage of a selected polarity and of a magnitude controlled by said master switch means, reference control means electrically connected to said signal means and to the control input terminal on the converter means and responsive to the magnitude of the voltage from said signal means to control a direct current output from the converter means at said output terminals, means serially connecting the armature and the field across said output terminals, a teaser field resistor connected across the armature, and current sensitive means for disconnecting the teaser field resistor at a predetermined magnitude of the direct current output of the converter means; said current sensitive means comprising current-indicating means interposed in the motor circuit for producing a current-magnitude signal indicating the magnitude of direct current output of said AC-DC converter means, and load sensing means connected to said current-indicating means and responsive to a first predetermined magnitude of said current-magnitude signal to disconnect said teaser field resistor and responsive to a second predetermined magnitude of said current-magnitude signal to connect said teaser field resistor; said load sensing means comprising biasing means providing a biasing signal, comparison means connected to said current-indicating means for comparing said current-magnitude signal to said biasing signal and producing a voltage of one polarity if said current-magnitude signal is of greater magnitude than the biasing signal and of an other polarity if said current-magnitude signal is of smaller magnitude than the biasing signal, time delay means connected to said comparison means and providing an output after a time delay, and relay means responsive to the output of the time delay means for selectively connecting and disconnecting said teaser field resistor.

10. A control system as in claim 9 wherein said time delay means comprises amplifier means having an input terminal and an output terminal, a first capacitor connected between the input terminal and the output terminal, a first input resistor providing a time delay of a first predetermined duration, a second input resistor providing a time delay of a second predetermined duration, and rectifier means applying voltage of said one polarity from said comparison means to the first input resistor and applying voltage of said other polarity from said comparison means to the second input resistor.

11. A control system as in clAim 10 wherein said time delay means includes switching means connected to the output terminal of said amplifier means for conducting current when a voltage of said one polarity is applied to said time delay means and blocking current when a voltage of said other polarity is applied to said time delay means, said switching means being connected to a source of voltage of said other polarity, a second capacitor connected between the source of voltage and the input terminal of said amplifier means for discharging through said amplifier means when said switching means begins to conduct.

14. A control system for operating a direct current motor comprising an armature and a series-wound field from an alternating current source, said system comprising a motor having an armature and a field, first and second AC-DC converter means each having control and power input terminals and output terminals, said source being electrically connected to the power input terminals of the first and second converter means, master switch means having an operating range for controlling operation of the motor when the motor is subject to an overhauling load, signal means for producing a voltage of a first polarity and of a magnitude controlled by said master switch means, reference control means electrically connected to said signal means and to the control input terminals of both the first and second converter means and responsive to the magnitude of the voltage from the signal means to control direct current output from both the first and second converter means, means connecting the armature across the output terminals of the first converter means and connecting the field across the output terminals of the second converter means, a dynamic braking resistor connected across the armature, and current sensitive means for disconnecting the dynamic braking resistor at a predetermined magnitude of direct current output from both the first and second converter means.

15. A control system as in claim 14 wherein said current sensitive means comprises current-indicating means for producing a current-magnitude signal indicating the magnitude of the direct current output from both said first and second AC-DC converter means, and load sensing means connected to said current-indicating means and responsive to said current-magnitude signal produced at said predetermined magnitude of direct current output to disconnect said dynamic braking resistor.

16. A control system as in claim 15 wherein said load sensing means comprises comparison means connected to said current-indicating means comparing said current-magnitude signal to a biasing signal of predetermined magnitude and producing a voltage of one polarity if said current-magnitude signal is greater than the biasing signal and of an other polarity if said current-magnitude signal is less than the biasing signal, time delay means connected to said comparison means and providing an output after a time delay, and relay means responsive to the output of the time delay means for disconnecting said dynamic braking resistor.

21. A control system as in claim 20 including an additional current-indicating means serially connected with the output terminal of said second AC-DC converter means and wherein said reference control means includes means connected between said additional current-indicating means and the non-inverting input terminal of said fourth amplifier means for applying a voltage of said first polarity indicating the magnitude of direct current output of said second converter means to the non-inverting terminal.