WO2005046042A1

WO2005046042A1 - Protecting device for an electric motor

Info

Publication number: WO2005046042A1
Application number: PCT/BR2004/000220
Authority: WO
Inventors: Marcos Guilherme Schwarz; Luiz Magalhães MEDEIROS NETO
Original assignee: Empresa Brasileira De Compressores S.A. - Embraco
Priority date: 2003-11-11
Filing date: 2004-11-10
Publication date: 2005-05-19
Also published as: BR0305533A

Abstract

A protecting device for an electric motor, comprising: a stator having a running coil (11) and a starting coil (12); a power source (F) which supplies current to said running coil (11) and said starting coil (12); power switches (60), each connecting one of the running coil (11) and the starting coil (12) to the power source (F) when in a closed condition, said device comprising at least one thermal switch (70) operatively and electrically connecting the power source (F) to the power switches (60) and being thermally coupled to the latter, in order to detect the temperature in said power switches (60) and interrupt the current to the latter when the temperature in any of said power switches (60) reaches a determined value which is predefined and higher than a respective predetermined value for the maximum operating temperature of the power switches (60).

Description

PROTECTING DEVICE FOR AM ELECTRIC MOTOR Field of the Invention

The present invention refers to a protecting device for an electric motor, of the type which controls the basic operative functions of said electric motor and protects the latter against damages resulting from undue heating, particularly caused by failures in the running and starting switches of the electric motor, said solution being applied to motors of refrigeration systems, such as refrigerators and freezers. Background of the Invention

In the known refrigeration systems, the operation of the hermetic compressor is obtained by the activation of a single-phase induction motor, through electromechanical or electronic control devices which activate and deactivate said motor as a function of different operative and load conditions and different parameters of the motor. Such devices generally actuate also to protect such motors. In these refrigeration systems, the operative and stop periods of the compressor are defined as a function of a temperature detected by appropriate sensors provided in the interior of each refrigeration cabinet of refrigerators and freezers in which said refrigeration system is applied. The compressor stop occurs whenever said temperature inside the cabinet is within a pre- established temperature interval. These systems are also provided with starting and thermal protecting devices for the hermetic compressors, normally in the form of electromechanical devices, due to their strength and simplicity.

A disadvantage of the presently used systems is the number of different electrical and electromagnetic devices that are used in the hermetic compressors and that have redundant functions, further requiring individual connections and calibrations. These devices also generate acoustic noises and electromagnetic interference by being provided with movable contacts . Another disadvantage of the present systems is the insensitivity to supply voltage variations, which impairs mainly the function of thermal protection, resulting in overloading more severely the insulating materials of the motor, besides the overall malfunctioning of said motor in extreme voltage conditions. Furthermore, for carrying out other functions rather than those mentioned above, said refrigeration systems require the use of additional peripheral devices, which means a larger number of components and electrical connections, increasing the assembly cost and allowing the occurrence of failures in said connections and components .

In the present solutions, the protecting device to be mounted externally or internally of the compressor is sensitive to current and temperature. Among the known solutions for the starting and protecting devices of an electric motor are those which use a thermal protecting element, such as a bimetallic element (US4646195 and US6433975) , and others which utilize an electronic protecting element (US5488834 and US6020702) .

In one of the protector constructions, for interrupting the electric current to the motor in case of an over current therein, there are provided a bimetallic disc and a resistor in series with the motor windings. This disc is composed of bimetallic layers which are heated upon the passage of current and by influence of the resistor, opening the supply circuit to the motor. In another known construction of electronic protectors (US06020702 and US5488834) , the circuit protection is accomplished by the opening of the static switches, such as triacs.

The first known solution presents some disadvantages, such as requiring for each specific motor model a specific design for the motor protector.

Furthermore, such devices generate a life expectancy (durability) which depends on the number of actuations thereof throughout the motor life, as a function of the intense wear of said elements and of the mechanical fatigue of its material with the use of said protectors.

Another disadvantage of the known constructions refers to the generation of electromagnetic and acoustic noises, besides sparkling resulting from the opening of the inductive circuit and from the gradual heating of the motor as a function of the successive attempts to start the motor in non-equalized systems. The present electronic protections, such as that described in documents US6020702 and US5488834 lose their function if a failure occurs in the starting or running switches, a typical failure in closed circuits and which impairs safety. Objects of the Invention

Thus, it is a first object of the present invention to provide a protecting device for an electric motor of high durability which can effectively protect the motor and the other components of said system against damages, in case of failure of the current interrupting switches. A second object of the present invention is to provide a device such as mentioned above, which can be applied to a family of motors .

A third object of the present invention is to provide a device as mentioned above, which does not generate electromagnetic or acoustic noises and sparkling during operation. Disclosure of the Invention

These objects are attained by a protecting device for an electric motor comprising: a stator having a running coil and a starting coil; a power source which supplies current to said running and starting coils; power switches, each connecting one of the running and starting coils to the power source when in a closed condition, said device comprising at least one thermal switch operatively and electrically connecting the power source to the power switches ,^* and being thermally coupled to the latter, in order to detect the temperature in said power switches and interrupt the current to the latter, when the temperature at any of said power switches reaches a determined value which is higher than a respective predetermined value for the maximum operating temperature of the power switches . Brief Description of the Drawings The invention will be described now, based on the enclosed drawings, in which:

Figure 1 illustrates, schematically, an embodiment of a protecting device for an electric motor constructed according to the present solution; Figure 2 illustrates, in a block diagram, another embodiment of the protecting device for an electric motor constructed according to the present invention; and Figure 3 illustrates, schematically, the temperature conditions in which the electric motor contacts are opened.

Description of the Illustrated Embodiments The protecting device for an electric motor of the present invention will be described in relation to a motor 10 of a refrigeration system, comprising: a stator having a running coil 11 and a starting coil 12 which are supplied by an AC power source F; a pair of power switches 60, one connecting the running coil 11 to the power source F and the other connecting the starting coil 12 to the power source F when in a closed condition, said power switch 60 connected to the starting coil being conducted to an open condition, interrupting the supply of electric current to the starting coil 12 upon completion of the start of the motor 10.

Figures 1 and 2 illustrate a starting circuit, including besides the motor 10, a current sensor 20 measuring the current circulating through the running coil 11 and the starting coil 12 of the stator of the motor 10, a voltage sensor 30 which measures the voltage in the power system, and a temperature sensor 40, which detects the temperature in the compressor shell to which the present electric motor is coupled, said current sensor 20, voltage sensor 30 and temperature sensor 40 being connected between the power source F and the motor 10 and operatively coupled to a control unit 50 supplied by the power source F, and receiving from the current sensor 20 and from the voltage sensor 30 signals representative of the current level and of the voltage level which are respectively supplied to the stator.

The control unit 50 is operatively connected to the power switches 60 in order to instruct the opening and closing thereof, as a function of determined operative conditions detected by the current sensor 20 and voltage sensor 30.

The processing unit 50 controls the simultaneous closing of the power switches 60, starting the motor

10 if the average current measured by the current sensor 20 is below a predetermined limit and if the temperature detected by the temperature sensor 40 is below a predetermined limit. The power switch 60 coupled to the running coil 12 remains closed during a time which is sufficient to start the motor 10, and it is opened by the control system after the start • (US 5,488,834), by instruction of the control unit 50. The electronic protection exerted by the control unit 50 through the instructions for opening the power switches 60 takes into account the current drawn by the motor and the integral of this current obtained in a predetermined time interval. Thus, overload conditions are detected, even though the current exceeds a nominal value by only a small amount of this nominal value, but during a relatively large interval or in over current situations in which the current is higher than the operative current. The motor 10 will also be disconnected from the power source F if the temperature in the compressor shell in which the motor 10 is installed and detected by the temperature sensor 40 surpasses a predetermined value which can impair the useful life and operation of the motor 10. The threshold current for opening the power switches 60 is corrected by the control unit 50, according to the voltage of the power system, compensating for the possible fluctuations of said voltage, as well as by the temperature of the compressor shell, since if the compressor is at a higher temperature, a smaller amount of current will lead to the maximum allowed temperature . The control unit 50 also has the capacity to detect a blocked rotor condition - non-equalized system, which prevents the motor 10 from starting - and to program the new start after a certain time interval has elapsed to allow the pressures of the system to equalize and guarantee the start of the motor 10, differently from the present thermal protectors that effect successive starting attempts, which gradually heat the coils of the motor 10 until the refrigeration system is equalized, delaying the effective start and wearing the assembly.

According to the present invention, the starting circuit also comprises at least one thermal switch 70, operatively and electrically connecting the power source F to the power switches 60 and being thermally coupled to the latter, in order to detect the temperatures therein, when the temperature in any of said power switches 60 reaches a determined value which is higher than a respective predetermined value for the maximum operating temperature of the power switches 60, protecting the motor 10 against over current and over temperature, in case a failure occurs in at least one of said power switches 60. In one embodiment illustrated in figure 1, to protect the motor 10 in case of failure of the power switches 60, the device of the present invention comprises two thermal switches 70, each being electrically and thermally coupled in series with a respective power switch 60, said thermal switches 70 when in a failure mode being in an open condition, so as to detect the temperature therein and interrupt the supply of current to said respective power switch 60, when the temperature therein reaches a value that is higher than the maximum operative temperature of said power switch 60. In a normal running condition, the temperature in the power switches 60 (usually triac type semi-conductor devices) reaches a maximum threshold value, in order not to impair their respective operation and reliability. If one of the power switches 60 fails, in order to maintain the current condition without the possibility of being interrupted by the control unit 50, its temperature tends to surpass the threshold value that has been reached during the normal operation, due to the characteristics of failure of the semi-conductor devices which present a relatively high electric resistance in these cases. Since the temperature in the power switches 60 surpasses the maximum operative value only in the case of failures in a closed condition, the thermal switches 70 should be adjusted to open at this temperature .

Since such thermal switches 70 are sensitive to the temperature of the ambient in which they are installed, the construction of the circuit should be carried out in a manner to guarantee the thermal coupling between the power switches 60 and the thermal switches 70. Upon reaching a predetermined temperature superior to that maximum temperature which the power switches 60 reach in a normal operation, the thermal switches 70 experience a deformation, opening their contacts and consequently interrupting the current supplied by the power source F to the power switches 60, as illustrated in figure 3 for the temperature T2. Such thermal switches 70 are designed to return to the original position, closing again the contacts, when the temperature diminishes from a determined value, indicated by temperature TI in said figure 3. This closing or reset temperature of the thermal switches 70 may be adjusted during the project of the latter to occur at any desired temperature.

In the proposed embodiments, the reset temperature of the thermal switches 70 is designed to occur at a temperature that is lower than the minimum possible ambient temperature, in order to guarantee that said thermal switch 70 remains indefinitely in an open condition, since the closing temperature will never be reached.

In another embodiment of the present invention illustrated in figure 2, the circuit is provided with only one thermal switch 70 which actuates on the simultaneous current conduction to both power switches 60, interrupting the supply to said power switches 60, when the temperature detected by said thermal switch 70 surpasses said maximum operating temperature in any of said power switches 60.

In a constructive option for the thermal switches 70, these are in the form of bimetallic elements, such as laminations . In another constructive form, said thermal switches 70 are in the form of thermofuses, which differ from the conventional fuses by not being designed to open upon self-heating, as a function of the conducted current, but rather to open as a function of the ambient temperature in which they are inserted. Moreover, in this embodiment, the thermofuse should be mounted thermally coupled to the power switches 60 and, as in the case of the thermal switch 70, in the form of a bimetallic element, when the power switches 60 exceed the maximum temperature of their normal operation, said thermofuse element interrupts the current of the circuit by melting the fuse link. By using the thermofuse, there will never occur reset of the device to return to the closed position, and therefore no project detail is provided to maintain the circuit opened. In both forms of circuit construction, the thermal switches 70 may remain in an open condition, interrupting the current by means of the power switches 60 and the motor 10, and guaranteeing this condition indefinitely, since the return to the normal operative condition is not required, due to the fact that at least one of said power switches 60 cannot be controlled any longer because it is in a condition of permanent conduction due to some failure. In the construction of the thermal switch 70 in the form of a thermofuse there is no return to a normal operative condition, since the rupture of the fuse link prevents the return to the normal operative condition of said thermal switch 70. However, for constructions of a thermal switch 70 in the form of bimetallic laminations, these may or may not return to a closing condition, as a function of the determined minimum temperature condition.

Claims

1. A protecting device for an electric motor, comprising: a stator having a running coil (11) and a starting coil (12) ; a power source (F) which supplies current to said running coil (11) and said starting coil (12) ; power switches (60) , each connecting one of the running coil (11) and the starting coil (12) to the power source (F) when in a closed condition, characterized in that said device comprises at least one thermal switch (70) operatively and electrically connecting the power source (F) to the power switches (60) and being thermally coupled to the latter, in order to detect the temperature in said power switches (60) and interrupt the current to the latter, when the temperature in any of said power switches (60) reaches a determined value which is predefined and higher than a respective predetermined value for the maximum operating temperature of the power switches (60) .

2. The device as set forth in claim 1, characterized in that there are provided two thermal switches (70) , each being thermally coupled to a respective power switch (60) .

3. The device as set forth in claim 2, characterized in that at least one thermal switch (70) is a thermally sensitive bimetallic lamination.

4. The device as set forth in any of the claims 1, 2 and 3, characterized in that each thermal switch (70) is constructed so as to present an opening temperature that is higher than said maximum operating temperature of the power switches (60) , and a closing temperature which is lower than the minimum temperature lower than that of the operating ambient, so as to prevent the closing of said thermal switch (70) after its opening has occurred.

5. The device as set forth in claim 4, characterized in that at least one thermal switch (70) is a thermofuse, having a respective fuse link to be melted when the temperature in at least one of the power switches (60) reaches said maximum temperature.