Synchronous motor protection

• US 4,683,411 A
• Filed: 09/02/1986
• Issued: 07/28/1987
• Est. Priority Date: 09/21/1984
• Status: Expired due to Term

• Current Assignee: General Electric Company
• Original Assignee: General Electric Company
• Inventors: Long, Edward A., Hamilton, James W. Jr.
• Primary Examiner(s): Smith, Jr., David
• Application Number:
  US06/902,940
• Time in Patent Office: 329 Days
• Field of Search: 318/705-715, 318/783, 318/778, 318/779, 318/798, 318/806, 361/23, 361/24, 361/25, 361/29
• US Class Current: 318/706
• CPC Class Codes: H02H 7/0805 for synchronous motors
• CPC Class Codes: H02P 1/46 for starting an individual synchronous motor H02P27/04 takes precedence

Abstract

A digital controller for synchronous motors prevents excessive heating of squirrel cage windings during motor starting. It reduces motor excitation if the actual accrued excitation time exceeds the maximum allowable excitation time of the controlled motor. It provides for adjustment of allowable time responsive to changes in excitation voltage. The thermal characteristic of the controlled motor is automatically derived from merely a few manually entered set point values.

Claims

1. In a digital control system for protecting a plurality of synchronous motors having squirrel cage windings, during full or reduced voltage start-up, the combination comprising:

(a) means for storing digital data representative of a plurality of thermal limit characteristic curves corresponding to a plurality of synchronous motors each having a different starting torque, said curves having an independent function representative of instantaneous motor speed and a dependent function representative of maximum allowable motor excitation time, the thermal limit characteristic curves each covering a speed range of from substantially zero speed to substantially synchronous speed;

(b) programmable means for manual entry of data representative of at least two set points for any motor being controlled by the digital control system, each set point having a value representative of the maximum allowable motor excitation time of said controlled motor at a predetermined motor speed;

(c) motor speed signal means for deriving a motor speed signal indicative of actual motor speed from AC energization applied to any motor being controlled by the control system;

(d) digital processing means responsive to the digital data, the set points and the motor speed signal for sequentially deriving the maximum allowable excitation time for any controlled motor to attain any instantaneous motor speed during acceleration and for providing an output signal indicative of the actual accrued excitation time exceeding the maximum allowable excitation time said motor being controlled by the control system, said digital processing means comprising;

selection means responsive to at least one of the set points for selecting the tabular representation of one of the plurality of thermal characteristic curves having the closest fit to the value of the at least one of the set points; and

indexing means responsive to the value of the motor speed signal to derive a representation of the maximum allowable motor excitation time applicable to the actual motor speed from the selected one of the tabular representations of said curves; and

(e) output means for reducing motor excitation responsive to the output signal so as to prevent thermal damage to the squirrel cage windings in a motor being controlled by the control system.

2. The digital control system of claim 1 wherein said entry means comprises means for entering a first set point representative of the maximum allowable time that the motor may be energized while at stand still and for entering a second set point representative of the maximum allowable time that the motor may be energized at a predetermined percentage of synchronous speed intermediate to stand still and synchronous speed.

3. The digital control system of claim 2 wherein the second set point is representative of the maximum allowable time that the motor may be energized at fifty perecent of its synchronous speed.

4. In a digital control system for protecting a plurality of synchronous motors during full or reduced voltage motor starting, each of said motors having squirrel cage windings and also having field winding means adapted to be connected with field resistance means in a field winding circuit during initial energization of the motor, the combination comprising:

(a) means for storing digital data representative of a plurality of thermal limit characteristic curves corresponding to a plurality of synchronous motors each characterized by a different starting torque, said curves having an independent function representative of instantaneous motor speed and a dependent function representative of maximum allowable motor excitation time, the thermal limit characteristic curves each covering a speed range of from substantially zero speed to substantially synchronous speed;

(b) programmable means for manual entry of specific motor data representative of at least two set points for a motor being controlled by the digital control system, each set point having a value representative of the maximum allowable motor excitation time of said controlled motor at a predetermined motor speed;

(c) means for deriving a motor speed signal from the field winding circuit so as to have a periodic time duration representative of the instantaneous actual percentage of synchronous motor speed, said motor speed signal being indicative of actual motor speed from AC energization applied to a motor being controlled by the control system;

(d) digital processing means responsive to the digital data, the set points and the motor speed signal for sequentially deriving the maximum allowable excitation time of a controlled motor being stimulated at full or reduced voltage startup in order to achieve its actual allowable instantaneous motor speed during acceleration and for providing an output signal indicative of the actual accrued excitation time exceeding the maximum allowable excitation time of the controlled motor, said digital processing means comprising;

selection means responsive to at least one of the set points for selecting the tabular representation of one of the plurality of thermal characteristic curves having the closest fit to the value of the at least one of the set points; and

indexing means responsive to the time duration of the periodic signal for deriving the maximum allowable motor excitation time applicable to the actual motor speed for the motor being controlled by the digital control system and being stimulated at full or reduced voltage startup;

(e) output means for reducing motor excitation responsive to the output signal so as to prevent thermal damage to the squirrel cage windings in a motor being controlled by the control system; and

(f) control means for protecting synchronous type motors from thermal damage upon energization of the motor for excessive time intervals at operation below synchronous motor speeds when motor excitation occurs at a full or a reduced voltage magnitude to extend the maximum allowable running time at any instantaneous actual percentage of synchronous speed with respect to the actual running time permitted with excitation at full rated voltage so that the allowable stall and run time of the motor being controlled is automatically extended under said reduced voltage excitation, said control means comprising an output means for removing motor excitations. wherein

5. The digital control system of claim 4 wherein said entry means provides for manual entry of a set point value representative of the percentage of synchronous motor speed at which DC excitation is to be applied to field winding means, to attain synchronous speed of a motor being controlled by the control system,

said digital processing means further comprising means coupled to said analog means and responsive to said motor speed signal for providing a second output signal responsive to the actual motor speed attaining the percentage of synchronous motor speed represented by the set point value, and

means responsive to the second output signal for applying DC energization to field winding means of a motor controlled by the control system.

6. In a digital control system which utilizes reduced starting votages for protection of synchronous type motors having ac energized stator winding means, and amortisseur winding means that are subject to thermal damage upon energization of the motor for excessive time intervals at operation below synchronous motor speed, the combination comprising:

(a) first signal means for determining a first electrical signal indicative of a comparison of the actual accrued motor running time and the allowable motor running time at any instantaneous actual percentage of synchronous motor speed;

(b) circuit interruption means for removing ac energization from the motor when the first electrical signal is indicative of actual accrued motor running time exceeding allowable motor running time at any instantaneous percentage of synchronous motor speed;

(c) first power control means for providing ac energization to the motor for a first start time period at a reduced voltage magnitude that is less than the full rated voltage; and

(d) first means for modifying the first electrical signal in response to reduced voltage energization of the motor, said modifying means extending the maximum allowable running time of the motor for minimizing unecessary removal of ac energization by said circuit interruption means.

7. The digital control system of claim 6 further comprising:

(a) second power control means for providing ac energization to the motor for a second start time period at full rated voltage; and

(b) second means for modifying the first electrical signal responsive to ac energization being provided at full rated voltage to reduce the actual accrued running time permitted at any synchronous speed prior to removal of ac energization from the motor to that permitted with ac energization at full rated voltage, whereby start protection is improved during the second start time period.

8. The digital control system of claim 6 wherein the first modifying means extends the allowable actual running time to substantially the product of the actual running time permitted with ac energization at full rated voltage and the squared ratio of locked rotor current at full rated voltage to inrush current at the reduced voltage magnitude.

9. The digital control system of claim 7 wherein the first modifying means extends the allowable actual running time to substantially the product of the actual running time permitted with ac energization at full rated voltage and the squared ratio of locked rotor current at full rated voltage to inrush current at the reduced voltage magnitude.

10. The digital control systems of claim 8 further comprising:

(a) second signal means for providing an electrical locked rotor signal representative of locked rotor current at full rated voltage;

(b) means for detecting inrush current upon ac energization of a motor at the reduced voltage magnitude; and

(c) first deriving means for deriving the squared ratio of locked rotor current at full rated voltage to inrush current at reduced voltage magnitude.

11. The digital control systems of claim 9 further comprising:

(a) second signal means for providing an electrical locked rotor signal repesentative of locked rotor current at full rated voltage;

(b) means for detecting inrush current upon ac energization of a motor at the reduced voltage magnitude; and

(c) first deriving means for deriving the squared ratio of locked rotor current at full rated voltage to inrush current at reduced voltage magnitude.

12. The digital control system of claim 10 further comprising means to delay detecting inrush current upon ac energization for a time interval sufficient to permit initial dc offset components of the motor current to diminish.

13. The digital control system of claim 11 further comprising means to delay detecting inrush current upon ac energization for a time interval sufficient to permit initial dc offset components of the motor current to diminish.

14. The digital control system of claim 6 wherein the first signal means includes:

(a) means for storing digital speed and time data representative of the thermal limit characteristic curve of a motor defined by an independent function of the percent synchronous speed of the motor and a dependent function of allowable motor running time;

(b) second deriving means for deriving a second electrical signal from ac energization applied to the motor that is a function of the instantaneous actual percentage of synchronous speed of the motor; and

(c) third deriving means which utilizes the second electrical signal to sequentially obtain from the stored digital data an updated indication of the allowable motor running time at the instantaneous actual percentage of synchronous speed, and continually modifies said updated indication as a function of actual accrued motor running time.

15. The digital control system of claim 7 wherein the first signal means includes:

(a) means for storing digital speed and time data representative of the thermal limit characteristic curve of a motor defined by an independent function of the percent synchronous speed of the motor and a dependent function of allowable motor running time;

(b) second deriving means for deriving a second electrical signals from ac energization applied to the motor that is a function of the instantaneous actual percentage of synchronous speed of the motor; and

(c) third deriving means which utilizes the second electrical signal to sequentially obtain from the stored digital data an updated indication of the allowable motor running time at the instantaneous actual percentage of synchronous speed, and continually modifies said updated indication as a function of actual accrued motor running time.

16. The digital control system of claim 14 for use with synchronous type motors having field winding means adapted to be connected with field resistance means in a field winding cirouit during an initial start interval, and further comprising:

(a) third signal means which utilizes the second electrical signal from the field winding circuit to provide periodic waveforms having a time duration representative of the instantaneous actual percentage of synchronous motor speed;

(b) third modifying means for converting the second electrical signal to a digital speed representative of instantaneous actual percentage of synchronous motor speed; and

(c) fourth deriving means which utilizes said digital spped representation to derive from the digital data representative of the thermal limit characteristic curve of a motor an allowable motor running time at the instantaneous actual percentage of synchronous speed.

17. The digital control system of claim 15 for use with synchronous type motors having field winding means adapted to be connected with field resistance means in a field winding circuit during an initial start interval, and further comprising:

(a) third signal means which utilizes the second electrical signal from the field winding circuit to provide periodic waveforms having a time duration representative of the instantaneous actual percentage of synchronous motor speed;

(b) third modifying means for converting the second electrical signal to a digital speed repesentative of instantaneous actual percentage of synchronous motor speed; and

(c) fourth deriving means which utilizes said digital speed representation to derive from the digital data representative of the thermal limit characteristic curve of a motor an allowable motor running time at the instantaneous actual percentage of synchronous speed.

18. The digital control system of any of claim 6 wherein the first electrical signal determining means further comprises:

(a) means for storing a first digital time datum having a first value representative of maximum allowable stall time of a motor at full rated voltage.

(b) fourth modifying means for sequentially modifying the value of the stored digital time datum as a function of actual accrued motor running time; and

(c) fifth modifying means for sequentially modifying the value of the stored digital time datum as a function of the instantaneous actual percentage of synchronous motor speed so that the resulting stored digital time datum is indicative of whether actual accrued motor running time has exceeded allowable motor running time at any instantaneous actual percentage of syncnronous motor speed.

19. The digital control system of any of claim 7 wherein the first electrical signal determining means further comprises:

(a) means for storing a first digital time datum having a first value representative of maximum allowable stall time of a motor at full rated voltage.

(b) fourth modifying means for sequentially modifying the value of the stored digital time datum as a function of actual accrued motor running time; and

(c) fifth modifying means for sequentially modifying the value of the stored digital time datum as a function of the instantianeous actual percentage of synchronous motor speed so that the resulting stored digital time datum is indicative of whether actual accrued motor running time has exceeded allowable motor running time at any instantaneous actual percentage of syncrhonous motor speed.

20. The digital control system of claim 18 wherein the first modifying means includes a means for determining a second value representative of allowable stall time at a reduced voltage magnitude.

21. The digital control system of claim 19 wherein the first modifying means includes a means for determining a second value representative of allowable stall time at a reduced voltage magnitude.

22. The digital control system of claim 20 wherein the means for determining the second value representative of allowable stall time at a reduced voltage magnitude comprises deriving the third value other than unity and multiplying the first value representative of maximum allowable running time at full rated voltage by the third value.

23. The digital control system of claim 21 wherein the means for determining the second value representative of allowable stall time at a reduced voltage magnitude comprises derving the third vlaue other than unity and multiplying the first value representative of maximum allowable running time at full rated voltage by the third value.

24. The digital control system of claim 22 wherein the magnitude of the second value is limited to a predetermined magnitude to limit the maximum allowable running time attainable during reduced voltage energization.

25. The digital control system of claim 23 wherein the magnitude of the second value is limited to a predetermined magnitude to limit the maximum allowable running time attainable during reduced voltage energization.

26. The digital control system of claim 20 further comprising a means for sensing ac energization being increased to fully rated voltage as a means for determining a fourth value representative of maximum allowable running time at full rated voltage.

27. The digital control system of claim 21 further comprising a means for sensing ac energization being increased to full rated voltage as a means for determining a fourth value representative of maximum allowable running time at full rated voltage.

28. The digital control system of claim 22 further comprising:

(a) means for sensing ac energization being increased to full rated voltage;

(b) means for modifying the third value to unity; and

(c) means for determining a fifth value representative of maximum allowable running time at full rated voltage.

29. The digital control system of claim 23 further comprising:

(a) means for sensing ac energization being increased to full rated voltage;

(b) means for modifying the third value to unity; and

(c) means for determining a fifth value representative of maximum allowable running time at full rated voltage.

30. The digital control system of claim 22 further comprising:

(a) means for detecting ac energization being increased to full rated voltage; and

(b) means for dividing the second value by the third value.

31. The digital control system of claim 23 further comprising:

(a) means for detecting ac energization being increased to full rated voltage; and

(b) means for dividing the second value by the third value.

32. The digital control system of claim 18 for use with synchronous type motors having field winding means adapted to be connected with field resistance means in a field winding circuit during an initial start period and wherein the fith modifying means further comprises:

(a) means for storing a plurality of digital speed and time data representative of the motor thermal limit characteristic curve of a motor defined by an independent function of the percent synchronous speed of the motor and a dependent function of the allowable motor running time;

(b) means for obtaining a second electrical signal from the field winding circuit that is representative of the instantaneous actual percentage of synchronous motor speed; and

(c) a fifth means for deriving from the stored plurality of digital data a digital representation of the allowable motor running time at the instantaneous actual percentage of synchronous motor speed for modifying the stored first digital time datum.

33. The digital control system of claim 19 for use with synchronous type motors having field winding means adapted to be connected with field resistance means in a field winding circuit during an initial start period and wherein the fifth modifying means further comprises:

(a) means for storing a plurality of digital speed and time data representative of the motor thermal limit characteristic curve of a motor defined by an independent function of the percent synchronous speed of the motor and a dependent function of the allowable motor running time;

(b) means for obtaining a second electrical signal from the field winding circuit that is representative of the instantaneous actual percentage of synchronous motor speed; and

(c) a fifth means for deriving from the stored plurality of digital data a digital representation of the allowable motor running time at the instantaneous actual percentage of synchronous motor speed for modifying the stored first digital time datum.