DC powered motors' NEC compliance

[JBender]

I am integrating process equipment where a 3rd party is suppling some of the equipment incorporating a few DC controlled motors, and their control boards. I am responsible for documentating the circuits they are on, starting at the load center.

It seems that this supplier is IEC EN61010-1 compliant but doesn't own an NEC book. Beyond fusing on the line side of the power supply there is no other motor protection on the DC boards. The supplied fuses are too large for NEC compliance of Overload or even Short Circuit in one case.

My question is, from the AC side of a DC motor controller, can I properly protect these motors. They are small - 450 Watts and 95 Watts. Given that 125% FLA of the motor (430.32 b(1) Overload protection) will be enough to power the boards. Can I translate the DC power delivered to an AC power and fuse accordingly. I am aware that efficiency and power factor are involved but in both cases would increase the actual current used and so my fusing would be low enough.

Thanks for any thoughts you might have.

Jason

 

[DRWeig]

JBender,

Just surmising, but I bet motors that small are impedance-protected for overload (or possibly thermally protected). AC supply side protection would only be concerned with short-circuit protection.

Let's see if anybody else sees something.

Best to ya,

Old Dave

 

[sreid]

Is it possible that the motor drive electronics have electronic protection for the motors such as current limiting and over current trips?

 

[JBender]

The DC controllers do have some sort of current limiting, though nothing I can see that would satisfy NEC. I am starting to wonder if the motors are, like DRWeig suggested, impedance protected or the like. I'm chasing down the manufacturer now to find out for sure. Thanks for your suggestions!

Jason Bender

 

[DickDV]

NEC requirements for DC motors are rarely observed. Generally, if you protect the AC supply side of the DC drive according to Instruction Manual requirements and program the drive to properly protect the motor, you have done about the best you can.

I'd get an IM if you don't have one and be guided by that.

 

[Skogsgurra]

Side note: Impedance protection only works on AC motors (little fans etcetera).

 

[hoy7t]

Current from the AC line is exactly the same as the current into the DC motor. DC drives are FET or SCR based and either conduct current or do not conduct current.

DC drives are current controllers. All they do is control current into the motor.

If you protect the AC line against short circuit or overloead then you are protecting the DC motor at the same current level.

Usually the fuses supplied on the DC drive are there only to protect the semiconductors. Do not consider them as part of your electrical safety circuit.

Most DC drives have a current limiting adjustment. this can be set to jsut above the current required for maximum output required by the application.