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.