NEC definition of torque motor

[actnup]

I am looking for a definition or application of a torque motor per the NEC. I work at a valve actuator manufacturing facility and I am have a problem rating citcuit protectors, etc.. According to the NEC a torque motor should be rated at its locked rotor current instead of the full load current. In my application the motors have torque switches in the circuit to keep the motor from seeing the full locked rotor current. But I also see starting currents that are sometimes above locked rotor. My problem is that other manufaturers suggest ratings per FLA only and I can't justify that to myself. I need to know if the motors are in torque applications are they considered torque motors?

 

[digidyn]

Locked rotor currents are a rating only. If you are looking at constant torque motors, they you may have a type F motor. Look at the nameplate and see if it has a TYPE anywhere on it. Under it's full load HP you will see the full load current. Startups are very current intensive and often peak over this.

For a motor, I suggest using time delay current limittin fuses such as CC or J type. They will take the start currents better and react faster in a short circuit. DO NOT USE CIRCUIT BREAKERS as a primary protection.

On type F motors, I usually have to go up to 2.5 times the FLS rattings with a time delay current limitting fuse. The code allows for this. This is also why I use the current LIMITING time delay fuses. They reduce the voltage spike and limit the current during a fuse blow. They also react much faster that fast acting fuses in a short circuit. They usually have an IR rating of 200KAmps as well. They are where I put my money when I do motor protection.

Marc

 

[macmckim]

You may want to contact Bodine Electric, Chicago. They have great hand book on small electric motors. In this manual, explains "torque motor's, construction and application therory. Good luck