Motor rated current
Motor rated current
(OP)
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%.
"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%.





RE: Motor rated current
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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RE: Motor rated current
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
RE: Motor rated current
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
www.gke.org
RE: Motor rated current
RE: Motor rated current
Gunnar Englund
www.gke.org
RE: Motor rated current
And this is another discussion of the same subject h
Gunnar Englund
www.gke.org
RE: Motor rated current
RE: Motor rated current
RE: Motor rated current
It is voltage that is problematic and needs extra care.
Gunnar Englund
www.gke.org
RE: Motor rated current
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)?
RE: Motor rated current
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).