US3499297A - Variable capacity refrigeration system - Google Patents

Description

March 10, 1970 J. D. RUFF ET AL 3,499,297

v VARIABLE CAPACITY REFRIGERATION SYSTEM Filed Feb. 20, 1969 10 Sheets-Sheet 1 FIGI.

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POWER MN To (o/"R01. FRO/n INVERTER CENTRE FIG.28 BY ATTORNEY United States Patent Office I 3,499,297 Patented Mar. 10, 1970 3,499,297 VARIABLE CAPACITY REFRIGERATION SYSTEM John D. Rutf, 206 Birch St., and Phillip R. Wheeler, 209 Pine St., both of Alexandria, Va. 22305 Continuation-impart of applications Ser. No. 613,271,

Feb. 1, 1967, and Ser. No. 676,368, Sept. 22, 1967. ThlS application Feb. 20, 1969, Ser. No. 801,130

Int. Cl. F25b 13/00 US. Cl. 62160 16 Claims ABSTRACT OF THE DISCLOSURE This invention is an electrically powered mechanical refrigeration system using hermetic, centrifugal or axial flow compressors and with a variable frequency, speed control on the high speed compressor motor system. Stability in operation over a wide range of Operating conditions is accomplished by automatically modulating system capacity with this speed control.

This application is a continuation in part of our copending patents Residential Heat Pump With Centrifugal Compressor and Wide Range of Capacity Variation filed on Feb. 1, 1967, Ser. No. 613,271; now Patent No. 3,449,922 and Variable Capacity Centrifugal Heat Pump filed on Sept. 22, 1967, Ser. No. 676,368.

The object of this invention is to produce an electrically powered mechanical refrigeration system using hermetic, kinetic displacement compressors (centrifugal or axial flow) and which has very efficient capacity variation capabilities. As with our other inventions this capacity variation is achieved by variab e speed driving of the compressor system. This is done by variable frequency conversion of the electric power supplied from an external source such as electrical power mains or any other source of electrical power. The compressor motors used by this invention are of the squirrel cage induction type and their speed is dependent on the alternating frequency of the electric current supplied to them. This type also has no brushes and can be used in hermetic type systems. Hermetic systems of course do not use compressor shaft seals and with small high speed compressors this is very important.

In addition to the capacity variation capability, a desirable close control is kept on system stability by automatically varying compressor speed under the control of a compressor motor current sensor, (or feed back). In this manner overloads and underload's on the system are prevented. An alternative method of overload prevention is by the use of diflerential controllers which are an override control of the engagement of high capacity stages, and are dependent on the differential between the evaporator and condenser in terms of temperature (and also pressure). Insufficient pressure diiterential is a major cause of compressor overloading.

This invention is mainly concerned with the variable speed drive to the compressor system and the control means associated therewith. Such variable speed drives can be applied to all sizes of kinetic displacement compressors, and in all kinds of application (heat pump, air conditioning, refrigeration) This invention also provides a system of booster compression which automatically engages extra stages of compression when needed, as with a heat pump when outside temperature is very low.

Another desirable feature of this type of compressor drive is that very high compressor rotation speeds are obtainable by the use of converter frequencies considerably higher than the conventional 60 Hz. (Hertz) supply.

For example: The highest speed for a squirrel cage motor on a 60 Hz. supply is less than 3600 rpm. but if the 60 Hz. supply is converted to 400 Hz. then such a motor can be run at speeds approaching 24,000 rpm. These higher speeds are useful with the smaller centrifugal compressor applications.

A small variation in the rotation speed of centrifugal (or kinetic displacement), compressor produces very large variations in compressor capacity. For example: a 15% decrease in speed (motor current frequency), causes a 50% reduction in capacity. This allows a large measure of capacity control to be achieved without reducing the motor frequency to levels at which the reduced impedance of the motor windings is a problem. However when very large capacity reductions are necessary, the resulting decrease in the impedance of the motor windings is compensated, in this invention, by switching in extra motor windings. These multiple windings can take the form of tapped windings with as many taps as required for smooth operation.

The variations of system capacity achieved in this invention are automatic and controlled by sensing the demand on the system and the conditions in which the system is operating.

This invention comprises:

A variable capacity, variable frequency, hermetic, mechanical refrigeration system using kinetic displace ment (centrifugal or axial flow) compressor machinery and driven by a squirrel cage motor (or motors), and a variable frequency inverter to supply current to the motor (or motors).

A feedback control circuit which maintains a constant current through the compressor motor by slowing or speeding up the motor, under the control of a motor current sensor.

Capacity controllers, to control the inverter, which are sensitive to the outside (outdoor) air temperature and which control the system in both heat pump and cooling operation.

Capacity controllers, to control the inverter, which are sensitive to inside air (or hydronic water) temperature, and which control the system in both heat pump and cooling operation.

Capacity controllers to control the inverter, which are sensitive to refrigerant pressures, and which control the system in both heat pump and cooling operation.

Differential controllers (temperature sensitive and .pressure sensitive) which prevent heavy compressor loading when differential conditions in the system are unfavourable.

A booster compressor unit that uses a separate compressor motor and which can be started up when extra compression ratio is needed.

Control devices for use with this booster in heat pump and cooling operation.

4. Tables showing cut-in points of various con- 6. Controller (cooling only) with bulb in outside FIG. 7. Schematic diagram of system (heat/ cool operation).

FIG. 8. Controller (heat/cool) with bulb in outside air.

FIG. 9. Controller (heat only) with bulb indoors.

FIG. 10. Controller (cool only) with bulb indoors.

FIG. 11. Controllers (heat and cool) with bulbs indoors.

FIG. 12. Controllers (heat/cool) with bulb indoors.

FIG. 13. Schematic diagram of system (heat/cool) with refrigerant changeover valves.

FIG. 14. Controller (heat only) pressure sensitive.

FIG. 15. Controller (cool only) pressure sensitive.

FIG. 16. Controllers (heat and cool) pressure sensitive.

FIG. 17. Differential controller (temperature sensitive).

FIG. 18. Differential controller (pressure sensitive).

FIG. 19. Feedback circuit for current control.

FIG. 20. Compressor/motor combination, single stage centrifugal.

FIG. 21. Compressor/motor combination, multistage axial fiow.

FIG. 22. Compressor/ motor combination, multistage centrifugal.

FIG. 23. System schematic diagram (with booster compressor unit).

FIG. 24. Schematic wiring for alternative motor windings, 3-phase, Y.

FIG. 25. Schematic'wiring for tapped motor windings, 3-phase, Y.

FIG. 26. Schematic wiring for alternative motor windings, single phase.

FIG. 27. Schematic wiring for alternative motor windings, two phase.

FIG. 28. Schematic wiring for alternative motor windings, three phase, delta.

FREQUENCY CONVERTER (AND COMPRESSOR DRIVE) In this invention a variable frequency, frequency converter (see FIG. 1) is used to supply power to the compressor motor 1, which is a squirrel cage induction motor. Variation of the alternating frequency of the power supplied to this motor causes its rotation speed to vary accordingly. FIG. 1 shows a typical converter whereby a 220 v. 60 Hz. A-C supply is first rectified by the bridge rectifier 2 using solid state diodes 3, 4, 5, 6. This pulsating D.C. output is smoothed by centre-tapped filter capacitor 7 and fed to the SCRs (silicon controlled rectifiers) 8, 9, 10, 11, 12, 13 which switch into the three phase motor windings 14, 15, 16. The sequential firing and turning off of the SCRs (silicon controlled rectifiers) 8, 9, 10, 11, 12, 13 is controlled by the phase sequencer 17. This circuit is comprised of a series of solid state latching circuits '18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29 which are in turn controlled by pulses from a variable frequency pulse source 30. Firing of SCRs 8, 9, 10, 11, 12, 13 is accomplished by direct application of a signal voltage (or pulse) to their gates. However their turn-01f is achieved by forced commutation. Circuits 31, 32, 33, 34, 35, 36 are turn-off circuits which turn off their corresponding SCRs on receipt of pulses from flip- flop circuits 19, 21, 23, 25, 27, 29. Such turn-off circuits are quite common in silicon controlled rectifier applications and can be described as a method of forced commutation from an external pulse source.

Phase sequencer 17 is essentially a ring counter circuit and circuits 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29 are flip-flop circuits connected in a series.

In operation, pulse source 30 (see FIG. 1), sends a succession of pulses (A.B.C.D.E.F.G.H.I.I.K.L.M.N.O.P. in FIG. 2) to the sequencer 17 (FIG. 1) which distributes them sequentially to the appropriate SCR gates for firing, and the commutation circuits for turning off. For example: Prior to pulse A, flip-flop 18 (FIG. 1) is latched, or conditioned to conduct, all the other flip-flops in ringcounter 17 are unlatched, or not conditioned to conduct. Then:

As pulse A is transmitted from pulse source 30 it causes flip-flop 18 to send a pulse to fire SCR 8. Simultaneously flip-flop 18 is unlatched and flip-flop 19 is latched.

As pulse B is transmitted from pulse source 30 it causes flip-flop 19 to send a pulse to the turn-off circuit 35, which turns off SCR 12. Simultaneously flip-flop 19 is unlatched and flip-flop 20 is latched.

As pulse C is transmitted from pulse source 30 it causes flip-flop 20 to send a pulse to fire SCR 13. Simultaneously fiip fiop 20 is unlatched and flip-flop 21 is latched.

As pulse D is transmitted from pulse source 30 it causes flip-flop 21 to send a pulse to the turn-off circuit 31, which turns off SCR 8.

Similarly pulses E, F, G, H, etc. will condition flip-flops '22, 23, 24, 25, etc. and a continuous cycle is thus maintained (since flip-flop 29 is connected back into flip-flop 18 to complete the ring circuit).

FIG. 2 shows how the pulses A, B, C, D, E, etc. control the flip-flops, turn-off circuits and ultimately the SCRs 8, 9, 10, 11, 12, 13 so that a three phase switching sequence is provided. The wave-shape first shown in FIG. 2 is square-wave and would be typical of the currents in motor windings 14, 15, 16 if they were pure resistive loads. However windings 14, 15, 16 are inductive loads and for optimum efficiency the current through them should be of sinusoidal wave-shape. Capacitors 37, 38, 39 (FIG. 1) are used to help control the wave shape of current through these windings, to sinusoidal as shown finally in FIG. 2.

The frequency of pulses from the pulse source 30 (FIG. 1) is under the control of control centre 40 (FIG. 1) and is twelve times the AC. frequency of the current through the motor windings since twelve pulses are required to produce a complete three phase cycle. A change in frequency of pulses will cause a proportional change in motor current frequency. For example: 4,800 pulses per second will produce a 400 Hz. current through the motor. Then if the pulse frequency is dropped to 3,600 pulses per second a 300 Hz. current is produced.

Alternative available methods of producing a variable frequency current through the motor windings are numerous, including frequency converters using electronic switching components other than SCRs and rotary mechanical switching.

With any method of frequency conversion used, the circuits or components, which are the means of controlling the frequency of the converter operation, are incorporated into, or are in circuit with, the control centre 40 (FIG. 1).

The field windings of the squirrel cage compressor motor can be three phase, four wire, Y connected (as shown in FIG. 1), single phase (using starting windings), or any polyphase arrangement, provided of course that the frequency converter system used is of a type adaptable to the motor fields used. FIGS. 26, 27, 28 show single phase, two phase and three phase delta arrangements.

FIG. 1 shows the primary power source as being a single phase (220 v., 60 Hz.) power line. Alternatively this might be a polyphase power line in which case rectifier 2 could be replaced by a polyphase rectifier. Or a source of D.C. current could be used, in which case a rectifier would not be needed.

The frequency converter as already described is actually a rectifier-inverter combination and any further mention we make of this equipment, refers to the inverter part of the combination.

CAPACITY CONTROLLER OUTSIDE This invention uses, in circuit with the control centre 40 (in FIG. 1), a capacity controler 41, which is a device having variable electrical characteristics and which is sensitive (and responsive) to varying levels of temperature. The temperature sensitive part of this device is mounted in a location, the temperature level of which location is used in determining the appropriate working capacity of the mechanical refrigeration system. In FIG. 1 controller 41 is shown with its bulb 42 located in the outside air. This arrangement is useful particularly when the system is being used as a heat pump, as shown in FIG. 1.

In use, when the outside air becomes colder the capacity of the heat pump needs to be greater because of increased heat losses. The temperature sensitive device 41, then alters its electrical characteristics, and through the electrical circuits of the control centre 40, causes an increase in the frequency of current from the inverter through the compressor motor. This causes the motor to increase speed and the higher system capacity is reached. On-otf cycling of the system is controlled by a thermostat or aquastat in a conventional way.

Details of a typical controller 41, are shown in FIG. 3. Responsive to varying outside temperature at the bulb 42, the bellows 43 actuates switches 44, 45, 46 (FIG. 4 shows a table of typical capacity steps). Spring 47 keeps the actuating arm 48 against pivot 49. A high temperature at bulb 42 has all switches open and resistor 50 is at its full value and the system at minimum capacity, since variable resistor 50 is used as the means of controlling inverter frequency. As the temperature drops, switch 44 closes and shorts out part of 50 thus increasing system capacity. On further temperature drop switch 45 also closes and shorts out more of 50, and similarly switch 46 reduces resistor 50 to its minimum value and increases system capacity to the maximum. In a simple form of control, variable resistor 50 controls the inverter frequency directly by controlling the frequency of pulse source 30, (FIG. 1). To make the following description more easily understood resistor 50 will not be considered a part of the controllers shown, but part of the control centre 40, (FIG. 1). The controllers will consist only of the temperature (or pressure) sensitive switching devices. Alternatively variable resistor 50 could be replaced by a variable capacitance or a variable inductance. Or any other type of variable electrical device could be used to match the variable frequency control requirements of the inverter. Any variable electrical device used, could be variable over a continuous range rather than in steps as shown in FIG. 3, andthe sensor could be of a solid state type rather than mechanically actuated.

Cooling only (FIG. 5)

When the system is used for cooling (or air conditioning) as shown in FIG. 5 a controller 51 (detailed in FIG. 6) can similarly control the capacity but in this case the action is reversed and the switches 52, 53, 54 make on rise instead of making on fall of outside air temperature (capacity tables in FIG. 4).

Convertible heat/cool (FIG. 7)

When the system is used as a convertible heat pumpair conditioning system as shown in FIG. 7, the heating and cooling controllers can be combined into a single instrument 55 (as detailed in FIG. 8). In addition to the switches 44, 45, 46, 52, 53, 54 controlling the value of resistor 50, there is a two pole switch 56 which controls the changeover from heating to cooling by controlling changeover motor 57 (FIG. 7). Motor 57 actuates changeover devices 58, 59, 60, 61 which are either fluid control valves or air shutters depending on whether the circulating medium is water or air. Capacity steps are shown in FIG. 4. Alternatively changeover can be accomplished manually rather than under the control of switch 56. On-ofi? operation is controlled in a conventional manner by thermostats or aquastats. In a hydronic system when changeover is made a timer 62 can delay the changeover and stop the system for a period of time long enough for the hydronic water to reach a neutral temperature. A typical time is 1 hour.

6 CONTROLLER BULB IN CONDITIONED MEDIUM FIG. 1 shows the compressor 63 which draws refrigerant vapor from the evaporator 64 andcompresses it into condenser 65, when the system is operating as a heat pump. Pipe or duct 66 is supplying the heated medium (air or water, depending on whether a hot air or a hydronic circulation is used) from the condenser to be used for heating purposes, and pipe or duct 67 is a return path for this medium. Pipe or duct 68 is supplying heat source medium (air or liquid) to the evaporator.

Since the primary control function is to maintain a pro-set temperature of the bulk of the conditioned, (heated), medium, the capacity controller bulb can be located in the return path of this conditioned medium. This is shown in FIG. 1 by controller 69 (detailed in FIG. 9) controlling heat pump capacity with its bulb 70 in the return flow of the conditioned medium. Or bulb 70 can be placed on a wall in the manner of a room thermostat when air is the medium being circulated and heated (conditioned) by condenser 65. In this manner the use of controller 69 is an alternative to the use of controller 41. By thus placing the bulb of the controller 69 in the circulating medium we are able to control the capacity of the system in its operation of maintaining the preset temperature of that conditioned medium. When used in this manner the range of the controller (from maximum to minimum capacity steps) should cover as small a temperature span as practicable. (Capacity tables in FIG. 4.) Controller 69 is similar in operation and construction to controller 41.

Heat pump, air circulation (FIG. 1)

A typical example of the above is the use of the system as an air circulating heat pump with the controller bulb 70 at room temperature. When maintaining a room temperature of approximately 73, suitable capacity steps are 73.0 (and over) system off, 72.7 to 73.0 minimum low capacity, 72.4 to 72.7 low capacity, 72.1 to 72.4 high capacity, 72.1 (and less) maximum high capacity. (Tables in FIG. 4.) In this manner the controller 69 is functioning as a multistage thermostat with switch points 71 (FIG. 9) turning off the system at the preset temperature.

Heat pump, hydronic (FIG. 1)

When hydronic liquid is used as the circulated medium the range of controller 69 is much higher and its function is as a hot water aquastat (tables in FIG. 4).

Cooling, air circulation (FIG. 5)

FIG. 5 shows the system in use as a cooling system (for air conditioning or for general refrigeration). When air is themedium being circulated, duct 72 is supplying the cooled air from the evaporator to be used for cooling purposes, and duct 68 is a return path for this medium. Pipe or duct 67 is supplying heat rejection medium (air or liquid) to the condenser.

Controller 73 has its bulb 74 in the return path of the cooled air. Alternatively bulb 74 can be placed on a wall in the manner of a room thermostat. Controller 73 (detailed in FIG. 10) controls the system capacity similarly to heating controller 69 (FIG. 9) except that its action is reversed. That is, higher capacity is provided on temperature rise rather than on temperature drop (tables in FIG. 4).

Cooling, hydronic (FIG. 5)

When chilled Water (or brine) is the medium being circulated, the temperature range of controller 73 is much lower than when room temperature air is being circulated. (Tables in FIG. 4.)

Convertible, air circulation (FIG. 7)

When this system is used in an air circulating, convertible heat pumpair conditioning arrangement as shown in FIG. 7, the operations of controller 69 and controller 73 can be combined in a single instrument 75 (detailed in FIG. 12) with an automatic control of changeover between heating and cooling operations incorporated into it. Controller 75 (FIG. 7) is identical to the combination controller 55 (FIG. 8) but its temperature range is much closer in the manner of all controllers mounted in the conditioned medium. (Tables in FIG. 4.) Bulb 76 is mounted in the return air duct (FIG. 7). Controller 75 then controls changeover motor 57 directly by means of switch 56 (FIG. 12).

Convertible, hydronic (FIG. 7)

When the system is used in a hydronic, convertible heat pumpair conditioning arrangement (FIG. 7) the big variance between chill water temperature and hot water temperature (tables in FIG. 4) makes it unsatisfactory to combine the heating controller 69 and the cooling controller 73 into a single instrument. In this case they are used together with their switch points in series (as shown in FIG. 11), but with only one control resistor 50. This can be done since at all water temperatures below the heating range (tables in FIG. 4), all the switches in heating controller 69 will be closed and the cooling controller 73 will then control capacity in the cooling range. Similarly at all water temperatures above the cooling range all the switches in cooling controller 73 will be closed and heating controller can control through the heating range. At the temperatures between the heating and cooling ranges, (44159 in FIG. 4), all switches will be closed and the system thus set for maximum capacity, with the type of operation, (heating or cooling), dependent on the position of the system changeover devices. The bulbs 70 and 74 of connected controllers '69 and 73 are placed together in the inside return water line (see FIG. 7). System changeover can be manually controlled or a two pole thermostat controller 77 (at room temperature) can control the changeover motor after a suitable delay by time 62.

Convertible, with refrigerant changeover valves (FIG. 13)

An alternative (conventional) method of constructing a convertible heat pumpair conditioning system is shown in FIG. 13. This method uses refrigerant changeover valves 78 and 79 which reverse the refrigerant flow through the inside coil 80 and the outside coil 81 so that their functions as condenser and evaporator are interchangeable. The placement of controller bulbs with this type of system is shown in FIG. 13. The bulb of outside air controller 55 (FIG. 7) is still in the outside air as already described. The bulbs of controllers in the conditioned medium are all placed in the return flow of the conditioned (or inside) medium. FIG. 13 shows this placement of air circulation type controller 75 and the combined controllers 69 and 73 (used with hydronic circulation). Control of heat/cool changeover is by conventional methods and a timer can be used to delay changeover on hydronic systems in the manner already described.

PRESSURE SENSITIVE CAPACITY CONTROLLERS Heat pump only Pressure sensitive capacity controller 82 (FIG. 1) can be substituted for temperature sensitive controller 69 (FIG. 1). FIG. 14 shows controller 82 with bellows 83 responding to the head pressure of the system, to which it is connected by line 84 (FIG. 1). Electrical connections to resistor 50 are the same as with temperature sensitive controllers.

Cooling only Pressure sensitive capacity controller 85 (FIG. 5) can be substituted for temperature sensitive controller 73 and responds to suction pressure to which it is connected by line 86 ('FIG. 5). Controller 85 (detailed in FIG. 15) is similar to controller 82 except that it is reverse acting and has a lower range.

The action of these controllers is similar to their temperature sensitive counterparts in their function as capacity controllers since there is a direct relationship between the temperature of the conditioned medium entering a condenser or an evaporator and the pressures within these coils. FIG. 4 shows typical control settings. However onoff cycling control, as achieved by switch points 71 in FIG. 9, (and 87 in FIG. 10) cannot be reliably performed by pressure controllers, so conventional thermostatic controls are used to cycle the system.

Convertible FIG. 7 shows how the temperature sensitive controllers 69 and 73 can be replaced by the pressure sensitive controllers 82 and 85 and how these can be similarly interconnected when used with a convertible heat pump-cooling system. FIG. 16 shows such a combination and FIGS. 7 and 13 show the application of these combined controls to the systems. The action of these controls is similar to the temperature sensitive (hydronic) combination of controls but of course their bellows are responding directly to pressure variations in the condenser and evaporator. FIG. 4 shows typical ranges of pressure control. Changeover is controlled by a temperature sensitive device, such as thermostat 77 (FIG. 7) or manually, as already described for convertible systems.

OVERLOAD PREVENTION Differential control Because a kinetic displacement compressor has a displacement which is variable and dependent on the pressure differential between its suction and discharge, its loading (or the power required to drive it) depend largely on these pressures and the temperatures which govern these pressures. For example: When a centrifugal, air to air heat pump is starting up (after a shutdown) with both inside air and outside air at 35 the temperature difierential is nil. The capacity controller is calling for a large capacity in the form of a high rotation speed. When the compressor starts, this high rotation speed combined with the low and very slowly increasing pressure differential cause a heavy pumping action which causes a high and very slowly decreasing compressor motor current. This undesirable overload can be avoided by the use of differential temperature or pressure controls which override the regular capacity controller till -a normal operating suction-discharge differential is reached.

Temperature sensitive FIG. 17 shows such a temperature sensitive control 88 over-riding controller 41. When the temperature differential between bulbs 89 and 90 is minimal, bellows 91 and 92 cause the over-ride switches 93, 94, to be open and thus prevent any decrease in the value of resistor 50 regardless of what position switches 44, 45, 46 are in. As the differential temperature increases, after some running at minimum capacity, the bellows will start to actuate the switches. First switch 93 will close. This will allow a single step increase in capacity (if controller 41 has switch 44 closed). Similarly, step by step, the over-ride control 88 will allow the system to reach full capacity. The bulbs 89 and 90 are mounted in the return flow of medium to the condenser, and the evaporator, as shown in FIGS. 1, 5, 7. This temperature sensitive device is not suitable for use with a system having refrigerant changeover valves (shown in FIG. 13) but the pressure sensitive device (a description of which follows) can be so used.

Pressure sensitive A similar differential control 96 is shown in FIG. 18 but it is sensitive to discharge and suction pressure to which it is connected by lines 97 and 98 as shown in FIGS. 1, 5, 7 and 13. Its action and its electrical connections are otherwise identical to the temperature sensitive control 88 (detailed in FIG. 17).

FIG. 17 shows control 88 over-riding the switching of the taps on resistance 50, by controller 41, but it, (or control 96) can be used similarly to over-ride the switching of the taps on the resistor 50, by any of the other controllers shown. For example: Controller 41 (FIG. 3), controller 51 (FIG. 6), controller 55 (FIG. 8), controller 69 (FIG. 9), controller 73 (FIG. 10), controller 75 (FIG. 12), combination controller (FIG. 11), controller 82 (FIG. 14), controller 85 (FIG. 15), and combination controller (FIG. 16), are all switching devices which switch the taps on resistor 50 similarly. And the differential control (88 or 96) can be connected between any of these, and resistor 50, in the manner shown in FIG. 17.

OVERLOAD PREVENTION Current feedback An alternative method of stabilizing system operation in this invention is by directly modulating the speed of compressor motor 1 (FIG. 1) (by motor current frequency variation over a continuous range) so that the exact motor current level desired, is maintained at all times. This is possible because the power requirements (and therefore the motor current) of a centrifugal (or axial flow) compressor vary as the cube of the rotation speed (or motor frequency), so that a relatively small increase in motor supply frequency causes a considerable motor current increase and a decrease in frequency causes a sharp falling off of motor current. The actual motor current level is detected by pickup device 99 (FIG. 1) and a balance circuit in control centre 40 balances the level of the voltage from the pick-up device, against a voltage of constant value. Any variance from the desired motor current is thus detected and the balance circuit then causes the frequency of the motor current to increase or decrease till the desired current is reached. In this arrangement then, the motor current frequency is a floating value and the motor current (and power output) is stabilized.

FIG. 19 shows a typical balance circuit. The current flowing through conductor 100 induces an AC. voltage in pickup coil 99 which is placed around or near this conductor.

Rectifier 101 and capacitor 102 rectify and filter this voltage and resistor 103 provides a load. On the other side of the balance circuit constant voltage VI is causing current to flow through a similar load 104 (assuming points 105 and 106 are connected together). Bi-directional potentiometer motor 107 will start to turn if there is a difference between voltage VI and the voltage on capacitor 102. Movement of motor 107 will cause an increase or decrease in the resistance of potentiometer 108 (depending on whether the voltage at 102 was greater or smaller than VI). Potentiometer 108 is the means of controlling the frequency of pulse source 30. The movement of potentiometer 108 causes the compressor motor to start slowing down or speeding up till the desired current through conductor 100 is reached at which time balance is regained in the circuit and motor 107 stops moving.

In the simple form of stabilization shown the desired current is one single value. But actually, with the use of capacity control (as shown with controller 41 in FIG. 19) the various stages of capacity call for various currents through conductor 100 (tables shown in FIG. 4). By inserting connections from variable resistor 50 into the balance circuit (FIG. 19) at points 105 and 106 we are able to control the circuit operation for various voltage levels from pickup 99 (or current levels through conductor In this way the various capacity requirements called for by the switching actions of controller 41 will cause the circuit to be set for various operating currents, and the balancing circuit will cause the system to operate at whatever frequency is required to maintain these currents. There are of course high and low limits to the frequency range.

This type of current control can be used in conjunction with resistor 50 and all the capacity controllers already shown: 41 in FIG. 3, 51 in FIG. 6, 55 in FIG. 8, 69 in FIG. 9, 73 in FIG. 10, 75 in FIG. 12, 82 in FIG. 14, 85 in FIG. 15, and the combination controllers in. FIGS. 11 and 16. This type of control is an alternative to the differential overload controls 88 and 96 shown in FIGS. 17 and 18.

A variation of this type of control might be the use of solid state balancing circuits rather than the mechanical potentiometer (107, 108) shown. Also this balance circuit could control pulse source 30 by a variable capacitance, a variable inductance, a variable voltage or any other electrical, or mechanical, device having variable characteristics suitable for the control of pulse source 30. Or when pulse source 30 is not used the balance circuit would control the output frequency of whatever machinery was used to provide variable frequency motor current. This would be the case if mechanical switching, or a different type of electronic inverter were used.

COMPRESSOR ARRANGEMENTS This invention uses hermetic compressors. That is, compressors which have their drive motors included within their enclosures and do not use shaft seals. Such seals are a frequent cause of trouble and are particularly unsatisfactory with kinetic displacement compressors having high rotation speeds.

This invention uses kinetic displacement compressors such as the centrifugal (single stage) unit, with rotating impeller 109, shown in FIG. 20 and the axial flow unit shown in FIG. 21.

FIG. 22 shows a unit with a single motor and multistage centrifugal compression. Any number of stages can be thus used, with all impellers mounted on the same shaft.

FIG. 23 shows two compressor units operating together so that the low pressure unit 110 is acting as a booster. This arrangement is used when the system is operating at low temperatures for heat pump applications. The low pressure booster unit can be shut off when not needed (since it has its own motor) and the normal pressure range unit 111 run alone, drawing vapor through the idle compressor (or through a bypass). Capacity controller 112 with its bulb 113 in the outside air, is similar in construction and operation to controller 55 (detailed in FIG. 8) except that an additional switch 114 is used to switch the pulses from pulse source 30 into an additional inverter 115 to supply low temperature compressor 110. When switch 114 is not made, pulses do not reach inverter 115 and compressor 110 does not run. Tables in FIG. 4 show typical control steps. The compressor machinery in each of the units (110 and 111) can be either of the centrifugal or axial flow type with one or more stages of compression. Any time that both compressors are running their speeds will be similar since both inverters are responding to the same pulse frequency.

Capacity controller 112 as shown in FIG. 23 consists of capacity controller 55 (detailed in FIG. 8) with additional switch 114. But any of the heating capacity controllers already shown could be substituted. That is with any of the systems of capacity control already shown (which have heating function), additional low pressure booster compressor unit 110 can be added; with switch 114 added to the capacity controller used, so that the booster unit can be controlled. A refinement in the use of the booster compressor is to have more than one stage of capacity at which it is operative.

11 VARIABLE MOTOR WINDINGS When a large reduction in motor speed is required (for example 50%) the necessary reduction of motor current frequency causes the impedance of the motor field windings to be reduced proportionally. Also when capacitors 37, 38, 39 are used on the windings (FIG. 1), they also are then found to be inappropriate in value at this lower frequency. The motor efficiency consequently is adversely effected. When this invention is used under these conditions additional motor windings with appropriate capacitors (when needed) are switched into the circuit. FIG. 24 shows high speed motor windings 14, 15, 16 with capacitors 37, 38, 39. Also shown are a set of low speed field windings 116, 117, 118 with matching capacitors 119, 120, 121. Switching of the appropriate motor windings is achieved by contactor 122. The holding coil 123 is energized from control centre 40 at the times when the motor frequency is maintained at low levels.

An alternative arrangement is the use of more than one set of low speed windings so that more steps of field winding adaptation can be accomplished. Alternative switching methods also might be used; such as SCR circuits.

The use of capacitors 37, 38, 39 and 119, 120, 121 are not necessary when some forms of frequency conversion are used (such as motor generator set).

The variable windings can alternately take the form of tapped windings as shown in FIG. 25, with the contactor 122 connecting either to high speed windings 124, 125, 126 or to the full windings (low speed operation) which are a combination of windings (124 and 127), (125 and 12-8), and (126 and 129).

FIGS. 26, 27, 28 show various forms of motor field winding arrangements and switching methods for their variable frequency adaptation. FIG. 26 shows a single phase arrangement (with starting winding 130 controlled by starting relay 131). Contactor 122 connects the output of the converter to either high speed winding 132 or low speed winding 133. FIG. 27 shows a two phase arrangement. Contactor 122 connects the output of the converter to either the high speed windings 134, 135 or the low speed windings 136, 137. FIG. 28 ShOWs a three phase delta arrangement. Contactor 122 connects the output of the converter to either the high speed windings 138, 139, 140 or the low speed windings 141, 142, 143.

FIG. 19 shows switch 144 which energizes the holding coil 123 in contactor 122 at the times when the inverter frequency is below a predetermined level.

We claim:

1. In combination, a variable capacity mechanical refrigeration system regulating the temperature of an air filled space and comprising kinetic displacement vapor compression means direct driven by variable speed, alternating current, electric motor machinery of the squirrel cage induction type and the said compression means and motor machinery being combined in a common hermetic enclosure, to pump refrigerant Vapor from within an evaporator and into a condenser, and the said evaporator having a flow'of fluid medium contacting it, and the said condenser having another flow of fluid medium contacting it, a variable frequency inverter means using sequential switching equipment to switch electrical current from an external source into the windings of the said motor machinery in such a manner to provide a variable frequency flux through the said windings, and thus achieve a variable speed rotation of the said motor machinery, since the ro tation speed of this type of motor is dependent on the frequency of the current supplied to it, a temperature sensitive means that senses the temperature at a location whose temperature is associated with the operation of the said system, and when variations of this temperature bring cause for changes of the system capacity, such changes are automatically performed by the said temperature sensitive means which controls the frequency of the said inverter and thence the capacity of the system.

2. Claim 1 and the said temperature sensitive means automatically selecting a suitable capacity loading of the system, this loading consisting of a current level in the motor windings, and a feedback control means of sensing the actual current level in these motor windings and automatically adjusting the frequency of the said inverter till the selected suitable capacity, or motor current, is reached, and then continually maintaining this motor current in this manner.

3. Claim 1 and the said temperature sensitive means being sensitive to the temperature of the air outside of the said space whose temperature is regulated by the said mechanical refrigeration system, since the temperature of this outside air affects capacity requirements.

4. Claim 1 and the said temperature senstive means being sensitive to the temperature of the said fluid medium moving toward contact with the said evaporator.

5. Claim 1 and the said temperature sensitive means being sensitive to the temperature of the said fluid medium moving toward contact with the said condenser.

6. Claim 1 and the said mechanical refrigeration system being a heat pump with selective heating and cooling cycles.

7. Claim 6 and the said vapor compression means being in two parts and each part having a separate electric motor and only one part of thi compression means is used during moderate and less than moderate capacity requirements, but when there is a need for a large capacity, as when using air as the source of heat in heat pump operation at low outside temperatures the other part of the compression means is then used as a booster by switching power into its motor.

8. Claim 1 and the said electric motor machinery having field windings in parts, selective switching means wherein some parts of these field windings are disconnected from the electrical circuit at times when the inverter frequency is at a level at which less windings are more efficient.

9. Claim 1 and a device that senses the temperature differential between the media in the two flows of fluid media that are moving toward contact with the said evaporator and the said condenser, and this temperature differential sensing device being used to limit the said means which controls the frequency of the said inverter, in a manner to prevent the application of excessive sys tem capacity at the times when the said temperature dif ferential is too low, thus preventing compressor overload- 10. In combination, a variable capacity machanical refrigeration system comprising kinetic displacement vapor compression means direct driven by variable speed, alternating current, electric motor machinery of the squirrel cage induction type, and the said compression means and motor machinery being combined in a common hermetic enclosure, to pump refrigerant vapor from an evaporator to a condenser, a variable frequency inverter means which uses sequential switching equipment to switch electrical current from an external source into the windings of the said motor machinery in such a manner to provide a variable frequency flux through the said windings, and thus achieve a variable speed rotation of the said motor machinery, since the rotation speed of this type of motor is dependent on the frequency of the current supplied to it, a pressure sensitive means that senses the refrigerant pressure at one part of the said system and when variations of this pressure 'bring cause for changes of the system capacity, the said pressure sensitive means automatically selects a suitable capacity loading of the system, this loading consisting of a current level in the motor windings, and a feedback control means of sensing the actual current level in these motor windings and automatically adjusting the frequency of the said inverter till the selected suitable capacity, or motor current level, is reached, and then continually maintaining this motor current level in this manner.

11. Claim 10 and the said pressure sensitive means sensing the system discharge pressure when the system is being used as a heat pump, in heating operation.

12. Claim 10 and the said pressure sensitive means sensing the system suction pressure when the system is being used in cooling operation.

13. Claim 10 and the said mechanical refrigeration system being a heat pump with selective heating and cooling cycles.

14. Claim 13 and the said vapor compression means being in two parts and each part having a separate electric motor and only one part of this compression means is used during moderate and less than moderate capacity requirements, but when there is a need for a large capacity, as when using air as the source of heat in heat pump operation at low outside temperatures the other part of the compression means is then used as a booster by switching power into its motor.

15. Claim 10 and the said electric motor machinery having field windings in parts, selective switching means wherein some parts of these field windings are disconnected form the electrical circuit at times when the inverter frequency is at a level at which less windings are more efficient.

16. In combination, a variable capacity mechanical refrigeration system comprising kinetic displacement vapor compression means direct driven by variable speed, alternating current, electric motor machinery of the squirrel cage induction type and the said compression means and motor machinery being combined in a common hermetic enclosure, to pump refrigerant vapor from within an evaporator and into a condenser, and the said evaporator having a flow of fluid medium contacting it, and the said condenser having another flow of fluid medium contacting it, a variable frequency inverter means using sequential switching equipment to switch electrical current from an external source into the windings of the said motor machinery in such a manner to provide a variable frequency flux through the said windings, and thus achieve a variable speed rotation of the said motor machinery, since the rotation speed of this type of motor is dependent on the frequency of the current supplied to it, a pressure sensitive means that senses the refrigerant pressure at a part of the said system and when variations of this pressure bring cause for changes of the system capacity, such changes are automatically performed by the said pressure sensitive means which controls the frequency of the said inverter and thence the capacity of the system, and a device that senses the temperature differential between the media in the two flows of fluid media that are moving toward contact with the said evaporator and the said condenser, and this temperature differential sensing device being used to limit the said means Which controls the frequency of the said inverter, in a manner to prevent the application of excessive system capacity at times when the said temperature differential is too low, thus preventing compressor overloading.

References Cited UNITED STATES PATENTS 3,355,906 12/1967 Newton 62228 XR MEYER PERLIN, Primary Examiner US. Cl. X.R.

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