Electric motor protection using a virtual speed switch

Patent metadata

Field	Value
Publication number	US-9939492-B2
Application number	US-201514710796-A
Country	US
Kind code	B2
Filing date	May 13, 2015
Priority date	May 13, 2015
Publication date	Apr 10, 2018
Grant date	Apr 10, 2018

Abstract

Detection of a locked rotor condition of an electric motor during startup herein uses a virtual speed switch. The methods and systems herein determine whether the motor is in a starting state, and the monitor the motor current to determine if it is greater than a predetermined threshold, and that the motor current decrease exceeds a predetermined threshold after a predetermined amount of time. If the decrease in motor current is not less than a predetermined threshold, then a speed switch bit is set, indicating that the rotor is turning. The methods and systems herein further determine whether a physical speed switch is faulty, and determine a locked rotor condition using the virtual speed switch signal, a physical speed switch signal, and whether the physical speed switch is faulty.

First claim

What is claimed is:

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,
settin

Assignees

• Schweitzer Engineering Lab Inc

Inventors

• Patel Subhash C
• Alexander George E

Classifications

• G01R31/343 Primary

  in operation · CPC title

• H02H7/0855

  avoiding response to transient overloads, e.g. during starting · CPC title