Permanent Magnet Synchronous Motor (PMSM) sizing of conductors (with a VFD)

[patm72]

Hi,

I'm trying to validate to which value I should size conductors going to a PMSM. The FLA of this motor will be 304A - and 308V (this is a high speed application, 24 000 RPM, continuous duty). Note that this voltage is developed at the motor which is driven by a VFD receiving 480V, 3 phases on the line side.

Obviously, NEC is mute on sizing conductors of such motors. Also, article 430 Part X only focuses on conductors feeding the power conversion equipment, not their load side conductors. Should I just look at Part II (430.22(B)) then and call it a day? That would set it to 125% of the FLA.

Thanks for your input.

 

[waross]

Obviously, NEC is mute on sizing conductors of such motors.

Not so.
The NEC doesn't differentiate on motor supply conductors based on supply or starting method, be they DOL, reduced current start, or VFD driven.
125% times Full Load Current.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Bill, yes it does, indirectly. The NEC requires that the 125% be calculated from the VFD rated INPUT current on the line side, then in the load side it is 125% of the motor values in a set of tables based on HP. Usually it’s not worth sweating over because the rated input current of a VFD is usually LOWER than the motor current, so if you just use the motor current you are covered. But it CAN bite you if the VFD is oversized for some future load or for some derating factor, i.e. a single phase supply, high altitude or high ambient temperature. So for example if you have a 50HP motor and a VFD rated for 100HP, the input conductors must be sized per the VFD, while the motor leads would be sized per the motor.

patm72,
When a motor does not fit the NEC table values, I always use the higher of those values closest to the HP (rounded up) or the nameplate FLA, then add the 25%. I’ve never had an AHJ challenge that.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[thinker]

We develop various test systems based on VFD driven dynamometer motors (both induction and PM, range 200-3000 kW). In most cases, the motor cable selection is very critical due to required flexibility. We use NEC Art.430.6(C). Its scope perfectly fits a definition of Adjustable Speed Drive System, and allows to use a motor name plate current without additional 25% ampacity. This approach has been used for many years in different US locations, was always accepted by local AHJ. For the first (pilot) project with that selection criteria, we requested a formal opinion from the our State Authority, and received confirmation.