Electric motor protection using a virtual speed switch

1. An apparatus for monitoring and protecting an electric motor using a virtual speed switch bit, the apparatus comprising:

a sensor component in electrical communication with the electric motor for obtaining an electrical signal from the electric motor and providing a current signal representative of a current to the electric motor;

a processor in communication with the sensor component;

a computer-readable storage medium in communication with the processor comprising computer instructions for execution on the processor, the instructions comprising:

a virtual speed switch module configured to:

calculate a motor state using the current signal representing current to the motor;

when the motor is in a starting state:

calculate a comparative motor load current from the current signal;

determine whether the comparative motor load current is greater than a predetermined starting current threshold;

when the comparative motor load current is greater than the predetermined starting current threshold, determine whether a shaft of the motor is rotating based on a current drop rate calculated by:

 waiting a predetermined period;

 after the predetermined period, calculating a subsequent motor load current from the current signal;

 determining whether the subsequent motor load current is less than the comparative motor load current; and,

 when the subsequent motor load current is less than the comparative motor load current after the predetermined period, set a virtual speed switch bit to indicate that the shaft of the motor is rotating; and

a locked rotor module configured to receive current signal and determine a locked rotor condition of the motor using the current signal and the virtual speed switch bit.

2. An apparatus for monitoring and protecting an electric motor, comprising:

a virtual speed switch module configured to:

receive electric motor current signals from a current sensor;

calculate a motor state using the current signals;

when the motor is in a starting state:

calculate a comparative motor load current from the current signals;

determine whether the comparative motor load current is greater than a predetermined starting current threshold;

when the comparative motor load current is greater than the predetermined starting current threshold, wait a predetermined period;

after the predetermined period, calculate a subsequent motor load current from the current signals;

determine whether the subsequent motor load current is less than the comparative motor load current; and,

output a determination of rotor speed when the subsequent motor load current is less than the comparative motor load current after the predetermined period; and

a protection module in electrical communication with the virtual speed switch module, configured to determine a locked rotor condition of the motor based on the determination of rotor speed.

3. The apparatus of claim 2 , wherein the protection module further determines a locked rotor condition using the motor state.

4. The apparatus of claim 3 , wherein the virtual speed switch module is further configured when the motor is in a starting state to:

calculate a second motor load current after calculation of the comparative motor load current;

compare the second motor load current with the comparative motor load current; and,

when the second motor load current is less than the comparative motor load current, set the subsequent motor load current equal to the second motor load current.

5. The apparatus of claim 4 , wherein the virtual speed switch module is further configured when the motor is in a starting state to:

calculate a third motor load current after calculation of the second motor load current;

compare the third motor load current with the comparative motor load current; and,

when the third motor load current is less than the comparative motor load current, set the subsequent motor load current equal to the third motor load current.

6. The apparatus of claim 2 , wherein the virtual speed switch module is further configured when the motor is in a starting state to wait a predetermined period after the motor initiates a start before calculating the comparative motor load current.

7. The apparatus of claim 2 , wherein the virtual speed switch module is further configured when the motor is in a starting state to determine when the subsequent motor load current is less than a predetermined portion of the motor load current, and when the subsequent motor load current is less than the predetermined portion of the motor load current, output a determination of rotor speed.

8. The apparatus of claim 2 , wherein the protection module is configured to determine the locked rotor condition when:

the motor is in a starting state; and

the output of the virtual speed switch module does not indicate rotor rotation.

9. The apparatus of claim 2 , further comprising an overcurrent element module coordinated with the protection module such that the overcurrent module does not issue a trip command during startup.

10. The apparatus of claim 2 , further comprising a thermal module configured to determine whether a thermal threshold of the motor has been reached, wherein the protection module is configured to determine a locked rotor condition independent of the thermal module.

11. The apparatus of claim 2 , wherein the protection module further determines a locked rotor condition based on an input from a physical speed switch.

12. The apparatus of claim 11 , wherein the protection module further determines whether the physical speed switch is faulty, and determines a locked rotor condition based on whether the physical speed switch is faulty.

13. The apparatus of claim 12 , wherein the protection module is configured to determine whether the physical speed switch is faulty using the current signals.

14. A method for detecting a locked rotor condition, comprising:

obtaining electrical signals from an electric motor;

calculating a load current of the electric motor from the electric signals;

determining a state of the motor;

when the motor is in a starting state:

calculate a comparative motor load current from the electrical signals;

determine whether the motor load current is greater than a predetermined starting current threshold;

when the comparative motor load current is greater than the predetermined starting current threshold, determine whether a shaft of the motor is rotating based on a current drop rate calculated by:

waiting a predetermined period;

after the predetermined period, calculating a subsequent motor load current from the electrical signals;

determining whether the subsequent motor load current is less than the comparative motor load current; and,

determining a locked rotor condition of the motor when the motor is in the starting state and the subsequent motor load current after the predetermined period is not less than the comparative motor load current.

15. The method of claim 14 , further comprising:

calculating a second motor load current after calculation of the comparative motor load current;

comparing the second motor load current with the comparative motor load current; and,

setting the comparative motor load current equal to the second motor load current when the second motor load current is less than the comparative motor load current.

16. The method of claim 15 , further comprising:

calculating a third motor load current after calculation of the second motor load current;

comparing the third motor load current with the comparative motor load current; and,

setting the comparative motor load current equal to the second motor load current when the third motor load current is less than the comparative motor load current.

17. The method of claim 14 , further comprising determining that the rotor is in a locked rotor condition when the subsequent motor load current is not less than a predetermined portion of the motor load current.

18. A motor protection apparatus for detecting a locked rotor condition of an electric motor, comprising:

a sensor component in communication with an electric feed to the electric motor, configured to obtain current signals from the electric feed;

a virtual speed switch module in communication with the sensor component, and configured to:

calculate a comparative motor load current from the current signals;

determine whether the comparative motor load current is greater than a predetermined starting current threshold;

when the comparative motor load current is greater than the predetermined starting current threshold, determine whether a shaft of the motor is rotating based on a current drop rate calculated by:

wait a predetermined period;

after the predetermined period, calculate a subsequent motor load current from the current signals; and,

determine whether the subsequent motor load current is less than the comparative motor load current after the predetermined period; and

a locked rotor module configured to determine a locked rotor condition when the subsequent motor load current is not less than the comparative motor load current.

19. The motor protection apparatus of claim 18 , wherein the locked rotor module is further configured to determine a locked rotor condition when the subsequent motor load current is not less than a predetermined portion of the comparative motor load current.

20. The motor protection apparatus of claim 18 , wherein the locked rotor module is further configured to determine a locked rotor condition when an output from a physical speed switch is not received by the motor protection apparatus.

21. A motor protection apparatus for detecting a locked rotor condition of an electric motor, comprising:

a sensor component in communication with an electric feed to the electric motor, configured to obtain current signals from the electric feed;

a protection module in communication with the sensor component, and configured to:

calculate a load current to the electric motor from the current signals;

compare the load current to a predetermined stopped motor current threshold;

obtain a physical speed switch signal from a physical speed switch;

determine a state of the physical speed switch based on the comparison of the load current to the predetermined stopped motor current threshold and the physical speed switch signal;

obtain a virtual speed switch signal from a virtual speed switch module; and

determine a locked rotor condition using the virtual speed switch signal, physical speed switch signal, and the determined state of the physical speed switch.