KW rating of 4 pole motor at highest speed is low. 1

[edison123]

VFD duty 4 pole motor nameplate below.

Why is the KW rating at 2200 RPM is lower at 475 KW than that of 2000 RPM (500 KW)?

 

[waross]

Probably because the rated Amps are lower and the PF is lower.
A WAG:
Harmonics are frequency dependant.
Some harmonic sensitive component of the motor may be approaching a resonant frequency at that speed, causing greater harmonic losses.

 

[electricuwe]

Power is proportional to the product of speed and torque. As for electric motors torque is given by the rotor dimensions, lower speed motors have lower ratings.

 

[waross]

Power is proportional to the product of speed and torque. As for electric motors torque is given by the rotor dimensions, lower speed motors have lower ratings.

This apears to be a single motor rated for overspeed running on a VFD.
The manufacturer has chosen to reduce the current rating at higher frequencies and speeds.

 

[Gr8blu]

The machine's ability to properly cool itself is finite. Increased losses result in increased heating, which then has to be removed. Possible reasons for limiting the power (i.e. stator current) at the higher speed? 1) increased sensitivity of the machine relative to harmonic content on the incoming supply (i.e. more likely to trigger a mechanical resonance condition), 2) increase to other losses such as windage, friction, and core loss which then become a larger portion of the the total loss, and 3) the possibility that the pressure drop somewhere in the internal air circuit becomes too high at the increased flow (which is a result of moving the internal fan at higher speed), which triggers a "hot spot" condition somewhere in the winding.

 

[edison123]

Resonance is speed related, not KW related, and the machine is already rated for 2200 RPM anyway.

Like all variable speed motors, this one has an externally driven blower and hence cooling is not an issue.

V/Hz is almost the same at 6.8 app for the entire speed range.

For other speeds, the KW increases with the speed, which is correct.

So, why lower KW at 2200 RPM?

 

[waross]

Resonance, probably the wrong word. My old mind cannot find the correct word.
What I was suggesting is that there may be some winding characteristic of the motor that tunes with some harmonic frequency.
If so, the amplitude of said harmonic may increase enough at the critical frequency that the resulting increased losses require a slight de-rating.
Said another way, is it possible that the distortion PF drops disproportionately at certain frequencies?

 

[LittleInch]

Only the vendor / manufacturer will know for definite, but at >70Htz that's above what I've commonly seen as the max speed for a motor which looks like it was designed for 60 htz as that would be ~400V 3 phase.

 

[waross]

If you look closely at the ratings at 2000 RPM, you may notice that the HP is less than extrapolated and that rated current has started to drop.
I am going to stick with disproportionately higher harmonic heating at higher frequencies.
Are you out there, jraef?
Please share your thoughts on this.
Thanks.

 

[panter]

This is data from Siemens website.
There are definitely differences in the voltage and power itself, ie the choice of Smart Line Module (SLM) or Active Line Module (ALM) -

 

[edison123]

Thanks, panter.

Is SLM/ALM a drive issue or the motor issue?

The highest efficiency at 2200 RPM debunks the theory that the motor is derated because of higher frequency loss in the motor.

Here is some discussion from way back 2007 in another forum, which talks about the SLM vs ALM drive.

Active Line Module vs Smart Line Module - 10877 - Industry Support Siemens

 

[waross]

The highest efficiency at 2200 RPM debunks the theory that the motor is derated because of higher frequency loss in the motor.

The efficiency of motors typically improves with less than full load, peaking at around 75% of rated load.
I don't accept that a slight increase in efficiency with a reduction in HP rules out higher harmonic losses.
You may be correct for other reasons, but efficiency alone is not proof.

 

[Gr8blu]

What Bill said ... higher efficiency is often a function of the combined electrical and magnetic "loading" on the machine. How did the manufacturers get the "super duper premium high efficiency" designs? They didn't go to exotic materials or design geometries - they just didn't push the material or performance envelope quite so hard. In part, this is why the percentage increase for locked rotor current in these super efficient designs is greater than the original "modestly efficient" ones ... because the absolute locked rotor current (in amperes) is a function of geometry and materials - not load current.

 

[edison123]

Not disputing higher harmonic losses at 2200 RPM, but it is clearly not a factor in 5% derating of the motor at 2200 RPM given the highest efficiency.

Magnetic loading is not pushing the envelope in this case since the V/Hz is pretty much the same through 1500 to 2200 RPM (as a matter of fact it is slightly less at 2200 RPM).

If electrical loading is an issue (which I doubt), why not maintain the same 500 KW of 2000 RPM instead of derating to 475 KW at 2200 RPM?

 

[Gr8blu]

I think maybe the manufacturer had a choice related to nameplate stamping - either keep increasing the allowable load and show a (noticeably lower) efficiency and power factor for the highest speed, or derate to a lower rating and keep efficiency and power factor levels approximately equal.

It is also possible the application specified minimum acceptable efficiency, power factor, or both. In that case, the loading was adjusted to stay within that limitation.

 

[waross]

Displacement PF is given.
No mention of distortion PF.

 

[LionelHutz]

You should probably ask the mfg for the reason. Only they would know. Maybe it was simply rated to match a certain application or specification.

I don't agree with harmonics or really with losses either. This will be VFD driven, so it always seems the same PWM voltage waveform.

 

[jraef]

If you crunch the numbers, the V/Hz ratio is staying fairly close to the same at those different voltages and frequencies, approx. 8.2:1, and torque is determined by that V/Hz design ratio of the motor. So the difference in PN (which is the nominal MECHANICAL power rating of the motor) can simply be explained by the difference in speed with the same torque value. kW = Tq. (in N-m) x RPM / 9550

I didn't crunch all of the kW ratings though. But picking just the 345V and the 455V values, the Tq, at 345V & 382kW = 382 x 9550 / 1500 = 2423 N-m

So for the 455V rating, 2423 N-m x 2000 RPM / 9550 = 507kW. Pretty close (<2%) to the 500kW value on the nameplate, probably due to expected windage and friction losses at the higher RPMs, or even just margins of error.

 

[waross]

How does that explain the rated Amps dropping from 730 Amps at 1500 RPM and 730 Amps at 1750 RPM to 620 Amps at 2200 RPM?

 

[edison123]

Jeff,
The issue is 475 KW at 2200 RPM. Other speeds have proportional KW.
There is no shaft mounted fan and hence no increased windage loss.
The motor reached 1500 RPM at 25 V, 50 Hz in open shaft run and rotor rotational loss was a few watts only.