Motor rated current

[thinker]

We are testing VFD fed AC induction motor designed to MG-1, Part 31 for dynamometer application .The name plate ratings (at base speed 6000 RPM): 260 kW power, 450V, 436 FLA. This motor was loaded using eddy-current dynamometer at the rated torque of 413 Nm (measured by the load cell). The actual motor current measured and displayed by VFD (at rated torque) was around 500A. Motor vendor people witnessed the test and suggested that they re-rate the motor current to higher value. The power interface to the motor was designed based on current name plate value of 436A. When we blamed the motor vendor that motor re-rating would cause us significant re-work of infrastructure, their response was:
"Please note that NEMA allows the actual motor amps to deviate from the name plate amps by 10% when operated at rated voltage, power and speed".
Please advise what standard allows for that (if the above statement is true). Motor cable selection was based on name plate data, and we not recall any requirement to count for additional 10%.

 

[electricpete]

I have never heard of a 10% NEMA allowance for FLA at rated volts/frequency/load.

There is a 10% allowance for change in voltage which may be excpted to cause a 10% change in amp draw, but that's a different animal.

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[waross]

Hello thinker
I have several suggestions;
1, Measure the current with a known good RMS ammeter.
2, Measure the voltage with a known good RMS voltmeter.
3, Go to

http://nema.org/

and search "motor tolerance", Down load "Free Standard: Condensed MG 1-2002
Information Guide for General Purpose Industrial AC Small and Medium Squirrel-Cage Induction Motor Standards".
yours

 

[Skogsgurra]

No, do not measure voltage with an RMS voltmeter. You will then measure not only fundamental, but also all higher frequency components that result from the PWM switching (assuming a PWM inverter). There are instruments that can measure PWM voltage without error from higher frequency components. Fluke has one and Metrix also.

If you do not have access to such an instrument, you can roll your own. Make a first order low pass filter with, say, 1 Hz corner frequency. Arrange the R in two halves so that the filter is symmetrical. Some DMMs are sensitive to the HF as well and it is better to filter both COM and V terminals.

Then read the voltage. What you see now is V/Hz from the fundamental and since you know Hz, you can easily get motor voltage.

What about slip? Does slip correspond to nameplate at this load? If it doesn't, you may have more than one problem.

Gunnar Englund

http://www.gke.org

 

[aolalde]

As far as I know heating on the winding and conductors is the result of total RMS current (including harmonics) why should we avoid to mesure them?

 

[Skogsgurra]

I said "do not measure voltage with an RMS voltmeter". That makes more sense.

Gunnar Englund

http://www.gke.org

 

[Skogsgurra]

This reference explains it.

https://www.ohiosemitronics.com/

pdf/tech_papers/Monitoring-VFDs(F).pdf

And this is another discussion of the same subject

http://www.reed-electronics.com/tmworld/article/CA180848.html

Gunnar Englund

http://www.gke.org

 

[CJCPE]

Per NEMA MG1-2002 Condensed: "When operating at rated voltage, rated frequency and rated horsepower output, the input amperes shall not vary from the nameplate value by more than 10 percent. [MG1-12.47]"

 

[CJCPE]

In order to properly test the motor, the fundamental value of the voltage should be adjusted to the rated value at the rated frequency as described by skogsgurra. When rated load is applied, the motor should operate at the rated RPM (at the rated slip) and the fundamental value of the current should be at the rated value. Does MG1 part 31 require the fundamental value or the total RMS value of rated current to be marked on the nameplate?

 

[Skogsgurra]

Short note: Current is seldom a problem. The motor winding's inductivity filters all PWM component out. Current measurement can be done with any current clamp.

It is voltage that is problematic and needs extra care.

Gunnar Englund

http://www.gke.org

 

[thinker]

CJCPE, thanks for the reference. The 10% variation allowed by MG1 Standard effectively means that motor conductors have to be selected based on 1.1 x (Rated nameplate current). On the other hand, NEC 430.6(C) requires: "For motors used in AC adjustable voltage,variable torque drive systems, the ampacity of conductors...shall be based on the maximum operating current marked on the motor nameplate".
May be for motors used in VFD, it makes sense to request a motor vendor to put on the nameplate not rated current but "maximum operating current" (which would include 10% MG1
deviation)?

 

[CJCPE]

I believe that NEC 430.6(C) refers to motors that operate at line frequency with speed control by reducing the voltage below line voltage thus increasing motor slip.

I think the following applies fo VFDs:

430 X. Adjustable-Speed Drive Systems

430.120 General. The installation provisions of Part I through Part IX are applicable unless modified or supplemented by Part X.

430.22 (A) General. Conductors that supply a single motor used in a continuous duty application shall have an ampacity of not less than 125 percent of the motor's full-load current rating as determined by 430.6(A)(1).

430.6(A) General Motor Applications. For general motor applications, current ratings shall be determined based on (A)(1) and (A)(2).