Chiller Motor Question 2

[testtech]

I recently conducted at test of a chiller. The motor is hermetic. About 175 HP, 480V. Test at 75% load.

Vibration data showed shaft speed harmonics with 2x (not 1x) slip X #poles sidebands.

Current signature showed slip X pole sidebands 50 db down.

Current waveform shows a sinusoidal notch on each phase at the peak. The notch drops the left side of the peak.

This appears to be a rotor bar issue but I have two questions: What would cause the notch on the current waveform and why 2x rather than 1x slip x pole sidebands on the vibration spectrum.

Thanks for your comments.

 

[electricpete]

Interesting problem.

I have seen multiple pole pass sidebands 1*Fp, 2*FP, 3*Fp etc, but never 2*Fp without 1*Fp.

Notching on the current waveform ? Possibly electronics directly in the supply path (vfd) or electronics fed from the same power source. In the latter case (non-vfd), investigate by monitoring the voltage with motor disconnected.

Possibly saturation of the sensing ct.

I would not make a call (other than continued monitoring) based on 50 db down in the current spectrum. Around 45db would be cause for concern and request a single-phase test. More presice evaluation of current sideband db down is possible knowing the speed and number of rotor bars.


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[testtech]

Hi Pete:

This chiller has a soft start but not a freq drive. Since the voltage waveform is normal, I assume the notch must come from the motor. Monitoring the voltage with the chiller disconnected would be difficult here.

I agree the 50 db is not enough to make a call.

I am using 5000 amp flex CT's with a very wide bandwidth. I don't think they could saturate here, unless I am missing something.

Here is another piece of info--I do spectral analysis of ultrasonics. (Essentially demodulation of 30khz vibration whre an ultrasonic listening instrument is the sensor.) It shows numerous peaks at 120 Hz harmonics with shaft speed sidebands. This suggests to me movement in the rotor or stator at field frequency that produces impacting that rings high frequency resonance (thus ultrasound). This certainly supports an electrical issue in the motor.

So, lots of hints, but nothing sufficiently concrete.

 

[jbartos]

Suggestion: The postings do not mention whether or not the power supply quality is adequate. Certain harmonics tend to be very detrimental to motors, e.g. even harmonics (second harmonic).

 

[testtech]

THD for the voltage is about 3%. It is about 10% for current. No abnormal even harmonics. Voltage crest factor is a bit low-1.37. I wonder if that is low enough to show a possible capacity limitation in the supply?

I have reviewed similar test results for a group of chillers and found two with crest factors at 1.38 but these had no notch at the current peak.

Perhaps I should repeat the readings with the chiller competely unloaded and see if the voltage crest factor and the current notch go up?

Do any of you have thoughts as to whether the crest factor is too low?

 

[busbar]

testtech, a minor variation from 1.414 should not be a big deal—probably just the ‘springiness’ of the electrical system. Compare it with the compressor off. The difference (flat-topping) is likely normal response to system impedance at the end of a larger branch circuit.

 

[electricpete]

Crest factor is ratio of peak to rms.

Take a sinusoid and cut a notch in the top... you have reduced the peak but not significantly affected the rms. So crest factor goes down.

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[jbartos]

Comment: The crest factor CF for a pure sinusoidal wave is:
CF=Vpk/Vrms=Vmax/Vrms=sqrt2 x Vrms/Vrms=sqrt2~1.414

 

[aolalde]

I have noticed that the designs of Hermetic Motors regularly push the magnetic flux densities close to the saturation limits. That could be the source of the crest factor reduction, but I don’t see the reason for a notch.

 

[Marke]

Hello testtech

I may be able to answer the notch question, but I will leave the vibration etc to Pete, he is far more qualified than me in that area.

You say that you are using a soft starter. A soft starter comprises revers parallel connected SCRs in each phase of the supply to the motor. At full voltage, the SCRs are at full conduction and will not reduce the voltage at all except for the voltage drop across the SCRs which is in the order of two volts per phase. As the current direction reverses, the current flow moves from one SCR in a pair, to the other one. The current will stop flowing through the conducting SCR when the current drops below the holding current of that SCR. This will be in the order of 0.5 Amps depending on the SCR used. Current can only begin to flow in the opposite SCR after it has had sufficient gate current to turn it on. If the Soft starter employes pulsed gate current (picket fence) which is most common, then there will be a delay until the SCR is triggered. This delay may be in the order of 200uS but is very dependent on the soft starter design. I expect that what you are seeing in the current waveform, is the transfer of the current between the SCRs on a phase. This may seem strange if it is happening on the left hand side of the current waveform, but you may find that what you are seeing is the effect of the current transfer on a phase other than that you are monitoring. Remeber, that the current going in on one phase, comes out on the other two, so on any one phase, you will find three notches, one at the current zero crossing and another two 60 degrees apart.

Just a thought!!
If this is the case,it will not cause problems with the motor. You could connect a bridging contactor across the SCRs once the motor is up to speed. This will short out the SCRs and prove whether the notching is Soft starter based or otherwise.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[jbartos]

Suggestion: Visit

http://www.reliabilityweb.com/fa/motor_testing.htm

http://www.reliabilityweb.com/excerpts/excerpts/Online CM Motors.pdf

for: Typical spectra of various motor deficiencies including cracked bars, bearings problems, etc.

http://www.reliabilityweb.com/art04/AC_motor__rotor_bar_case_history.pdf

http://www.techonline.com/community/ed_resource/feature_article/14449__DY5009542589HK

etc. for more info