branch motor circuits to “torque motors” sized per locked rotor current?

I

Isaiah

Senior Member
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Baton Rouge
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Electrical Inspector
We have mixer motors and gates that, according to one of our elect engineers, are subject to a locked rotor condition and therefore, the circuit must be sized to locked rotor amps….is this legit?


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Not necessarily.
Breakers do not trip immediately at 100% of their rating. Many of them can hold 6X rated current for a few seconds.

The engineer should be looking at the Time Current Curve of the breaker and the acceleration time of his motors. In general if you follow NEC 430 sizing rules and the motors are not special designs you shouldn't have problems.
 
jim dungar said:
Not necessarily.
Breakers do not trip immediately at 100% of their rating. Many of them can hold 6X rated current for a few seconds.

The engineer should be looking at the Time Current Curve of the breaker and the acceleration time of his motors. In general if you follow NEC 430 sizing rules and the motors are not special designs you shouldn't have problems.
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From an engineering standpoint, this makes total sense. Can you confirm there is no specific NEC requirement for upsizing per Art 430?


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ptonsparky said:
How long will they be at LRA? Even a Class 30 overload gives 30 seconds. Not too many motors will handle that much longer.
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If the motor gets into a ‘stall situation’ it could last much longer than 30 seconds…would it be better to implement a VFD for these circuits?


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Isaiah said:
If the motor gets into a ‘stall situation’ it could last much longer than 30 seconds…would it be better to implement a VFD for these circuits?
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Very few general purpose motors can take stalled/jammed or locked rotor current for 30 seconds. Typically these situations require additional external cooling at a minimum.
 
jim dungar said:
Very few general purpose motors can take stalled/jammed or locked rotor current for 30 seconds. Typically these situations require additional external cooling at a minimum.
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Yes. But a torque motor has different parameters that allow for higher heat. What I’m concerned about is the cable being too small to handle the increased heat


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Isaiah said:
Yes. But a torque motor has different parameters that allow for higher heat. What I’m concerned about is the cable being too small to handle the increased heat
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For motor circuits the conductors are usually chosen to be protected from overload conditions by the motor overloads (running protection) and the short circuits by the breakers.

If the engineer wants all the circuit components to be designed using LRA that is their choice. The NEC would only be concerned with the motor running overcurrent values.
 
Isaiah said:
We have mixer motors and gates that, according to one of our elect engineers, are subject to a locked rotor condition and therefore, the circuit must be sized to locked rotor amps….is this legit?
Click to expand...
Isaiah said:
Yes. But a torque motor has different parameters that allow for higher heat. What I’m concerned about is the cable being too small to handle the increased heat
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If the conductor is sized for the locked-rotor current, then why would it be too small to handle the heat generated by the locked-rotor current?
 
david luchini said:
As part of the discussion...

How would a conductor sized for locked rotor current, overheat when carrying locked rotor current?

I don't understand what your concern is.
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Read the entire thread
The question was whether the conductor needs to be sized for locked rotor current or not -
I’m not sure why you’re struggling with this

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Isaiah said:
Read the entire thread
The question was whether the conductor needs to be sized for locked rotor current or not -
I’m not sure why you’re struggling with this

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Click to expand...
The answer is, No, it does not.
The NEC is a minimum standard. The EE can suggest/require more up to the point the customer says "No, thank you." and hires a different EE.
You do not need to substantiate the EEs request. Let him prove it with quoted Articles.
 
ptonsparky said:
The answer is, No, it does not.
The NEC is a minimum standard. The EE can suggest/require more up to the point the customer says "No, thank you." and hires a different EE.
You do not need to substantiate the EEs request. Let him prove it with quoted Articles.
Click to expand...

And this applies to torque motors right? Would the circuit be better protected using a VFD?


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Isaiah said:
And this applies to torque motors right? Would the circuit be better protected using a VFD?
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It makes no difference what the NEC says, as long as the engineer's request is greater than the minimum.
A VFD would need to be sized to the torque motor's current. The circuit would need to be sized to the input current of the VFD.
Without knowing the motor size and VFD size we cannot say which would be less.
 
'Torque motors' are rather different than most electric motors. They are specifically designed to be used at low or zero speed, and to maintain torque at zero speed (locked rotor) for indefinite periods of time.

They are essentially designed for locked rotor application.

As I recall, article 430 specifically mentions torque motors, and has specific requirements for their protection.

Don't think of this as 'going beyond code requirements for a normal motor'. Crack the book and follow the requirements for torque motors.

Depending on the specific design of the torque motor, you might get much better efficiency using a VFD, but this becomes a significant engineering project for what is probably a small motor designed to be used at 60Hz.

Edited to add: I see that 'torque motors' were not mentioned in the original post, but rather by the original poster later in the thread. My comments were directed to the use of motors designed for locked application, not ordinary motors that get locked up.
 
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