Chiller Waveform Question 2

[testtech]

I obtained voltage and current waveforms from a centrifugal chiller of about 200 HP. It was running at about 70% full load. The voltage waveform looks fine. RMS voltage for A phase is 476.4V and the peak is 664V. The current waveform for each phase is flat topped at the positive and negative peaks. The A phase RMS is 161.5 amps. The peak is 230 amps. The duration of the positive and negative flat peaks is about 2.9 miliseconds each. Total harmonic distortion is 1.53% for voltage (A phase) and 7.21% for current (A phase). Here is the question: What characteristic of the motor produces the flat topped current waveform?

 

[testtech]

Here is a picture of the current spectrum and waveform from the CSI 2120.

http://i1.tinypic.com/s1u4ip.jpg

Frequency is shown here in CPM (sorry, I neglected to set unit to Hertz). Thus, 3600 CPM is 60 Hz, as you all know.

 

[dpc]

What are the dominant frequencies of your current distortion?

 

[ScottyUK]

There is a fair bit of low order harmonic in the current waveform. Hard to tell - looks like third or fifth: my guess would be fifth. Can your instrument give individual harmonic magnitudes as well as THD? I think you will find that one harmonic dominates the spectrum. You can almost pick out the peak and trough of the harmonic with a little imagination.

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I don't suffer from insanity. I enjoy it...

 

[testtech]

DPC and ScottyUK-
60 Hz- 154 A
180 Hz- 3.5 A
300 Hz- 9.4 A
420 Hz-4.5 A
660 Hz 1.9 A
2454 Hz- 1.7 A
All other harmonics are fractional amps.

 

[waross]

Hi testtech
Well your pictures convinced me that it is not a clipping problem. The cut-off is not straight enough for clipping. Clipping is an instrumentation problem and does not cause harmonics.
I think you have enough harmonics to explain the waveform. The question is Why the harmonics?

Please note friends, I'm over my head here and my following comments are questions rather than a definitive answer based on first hand experience.
I notice a particularly strong 41 st. harmonic if I have calculated it correctly. The harmonic to the left of the 150 kCPM line. I make it 147.6 kCPM or 2460 Hz., if I have calculated correctly.
I'm wondering, how many bars does the rotor have? 42 bars and a running speed of 3514 rpm may be generating 41st harmonics if a bar had a high resistance weld, or an inclusion in the casting.
If I'm completely out to lunch please be gentle with me.
respectfully

 

[testtech]

waross

I am attaching a vibration spectrum. The vibration amplitudes are extremely low, so I have not seen them as evidence of an electrical or mechanical problem. However, the key frequencies are all harmonics of 120 Hz. Unfortunately, I did not acquire vibration frequencies high enough to capture the rotor bar pass frequency. However, the rotor bar test showed showed 2x pole pass side bands at about 50 dB down, so I think the high resistance weld is less likely. Could an inclusion in the casting cause the symptoms we are seeing? A rotor issue might make sense, since all three phases are impacted.

http://i1.tinypic.com/s25iqc.jpg

 

[waross]

Hello testtech
With a cast squirrel cage, an inclusion in the casting will give one bar a higher resistance. If this is the case the unsymetrical bar may be generating the 2454? Hz. frequency. I estimated 2460, which is a multiple of 60 Hz. If the 2454Hz. is an accurate frequency and is NOT a multiple of 60 Hz. it may be caused by a bad bar sweeping the windings at the rotation frequency rather than the synchronous frequency.
Does any one have any idea how many bars this motor may have?
I'm guessing about 40 more or less. That would correlate with the 2400+ frequency.
respectfully.

 

[dpc]

I'm guessing that the current waveform is due to harmonics. The high 5th and 7th is typical of 6-pulse rectifier distortion. You probably have similar distribution of harmonics on the incoming voltage. The current waveform looks suspiciously like 5th harmonics as Scotty suggested.

 

[Marke]

Hello testech.
What is the speed of the motor? Is this a dual speed motor?
I have seen a number of chillers using two speed motors so that they can slow the compressor to get energy savings. If the motor is a 4 pole/ 6 pole motor usin PAM techniques for generating the pole changing, you can get very strange current waveforms.

If the motor is delta connected and the motor is over fluxed, you will get current peaks coinciding with the crest of the voltage waveform across the windings. This will occur at 30 degrees before the phase - Neutral crest and 30 degrees after the phase - neutral crest. i.e. at 60 degrees and 120 degrees. This could explain the current waveform and certainly the high fifth harmonic. Is the motor delta connected?? I see that the line voltage is 476 volts. If the motor is wound for say 440 volts or even 460 volts, you could be saturating the iron on the voltage peaks, especially if the motor is cooled by the refridgerant. Where the motor is liquid cooled, it is common to increase the flux in the iron (or reduce the amount of iron for the same flux)

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[testtech]

HI All

I don't know how many rotor bars.

Voltage THD is 1.55%. 60 Hz-476V
3rd-.84
5th-6.97
7th-.76
11th- 1.63
13th-1.13
41st-.43

Motor is 2 pole, <3600 RPM. Should be single speed. It is star-delta connected.

Marke--If the motor is overfluxed, does this have any implication for reliability, especially when operated for long periods at full load? I would guess over heating is not an issue here. This client is trying to push off a 10 year tear-down for another year. Except for this issue, I see no reason not to wait another year.

 

[ozmosis]

Could the 2.454kHz be a frequency imposed on the network from other sources such as the switching frequency of a VFD?

 

[waross]

I have been thinking about my last post and have a correction. The frequency of a bar problem would not be related to the number of bars, but may be related to the number of coils in each winding. Probably not.
There are to many millions of 460 volt motors running on 480 volts for this to be an issue. This is standard industry practice and within the allowable motor tolerance.

dpc makes a very good point. Can you do a test on the voltage without the chiller on-line/ The chiller may be the victim of power line distortion rather than the cause.

sed2developer has a good point.
We had a discrepancy on the 41st harmonic. Can you verify the exact frequency? 2460 is 41st harmonic, 2454 may be externally generated.
respectfully

 

[testtech]

waross

This is the 41st harmonic. It seems to me that a rotor bar issue that produced magnetic field imbalances would show up as side bands or been seen through demodulation processing, but not as a harmonic of 60 Hz. Further, it is unclear how high frequency current distortion that is not present to a significant extent on the voltage (where it is .43V) can produce the flat topped current waveform.

At this point, the the concept described by Marke may be most relevant to the flat topping. I hope he or someone else can weigh in and elaborate a bit.

As a digression, the most frequent source of flat topping I see is in electrical transformers that are running at or above full load. The core saturates and secondary voltage and current show flat peaks. This should not be happening here since the chiller is running at about 70% load.

 

[Marke]

Hello testech

You have confirmed that the motor is connected in delta.
When I look at the current waveform, it is apparent that the whole waveform is distorted, not just the flattened top.

The current flowing in phase 1 is made up of the sum of two currents, the current flowing through a motor winding betwteen phases 1 and 3, and the current flowing through a motor winding between phases one and 2. These two currents are displaced by 60 degrees from each other. If you are able, measure the current waveform flowing in one winding. This current will be less than the line current, but I expect that this current will show the peak on the crest of the waveform.
This flux saturation of the iron will result in an increase in the heat dissipated in the iron, but provided that there is adequate cooling, there is no issue. In an air cooled motor, the onset of saturation will cause a rapid rise in frame temperature.
It is common for very small air cooled motors to be operated in the region of the onset of saturation. This increases the heat, but due to the ratio for volume to surface area, the effective cooling is much higher and no problem results except where these motors are operated above their design voltage. Submersible pumps also operate with a much higher flux density than air colled motors because they are liquid cooled.

I look forward to seeing the winding current waveforms.

Best regards

Mark Empson

http://www.lmphotonics.com

 

[testtech]

Marke:

I posted two images of currents. One is 3 phases in a single plot. It occurs just above the one you downloaded. Let me know what you think.

 

[Marke]

Hello testtech

The current waveforms are the line curents and are what I would expect to see for a delta connected motor that is a little overfluxed. you can see a peak occuring at 60 degrees and another at 120 degrees coincicing with where the peak would be in the current flow in the individual windings.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[testtech]

Thanks everyone.

 

[electricpete]

I think Mark figured it out and testtech waded through all the possibilities to find that right answer.

That's a great case study.

I think the the explanation is a little more complex than was mentioned.

If you were looking at an individual leg current of an UNLOADED motor, we would expect to see spike from saturation right at the "90 degree" sin wave peak as described above. This is because the saturation occurs at highest flux, highest flux occurs at excitation current, and total current is equal to excitation current in the unloaded motor.

But if you are looking at a loaded motor such as posted, total current is not the same phase as excitation current. Total current leads excitation current by an angle (90-arccos(p.f.)).

A minor correction but still probably similar result.

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[electricpete]

Just to be clear, 90-arccos(p.f) is close to 90 degrees for a fully loaded motor. For this motor p.f. angle looks like maybe 20 degrees so the spike in the leg current from saturation would occur approximately 70 degrees after the peak in the leg current.

Then combining leg currents to phase currents. The peak of the phase as mentioned differs 30 degrees from the peak of each of the peaks of the two associated legs.

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[aolalde]

It looks to me that this curent distortion is produed by the low order harmonics, 3rd, 5th and 7th. the 3rd harmonic is mainly produced by winding distribution and saturation of the magnetic circuit; the 5th and 7th mainly due to uneven leakage of flux due to a reduced and uneven airgap. It does not matter if the motor winding is wye or delta connected although normally hermetic motors are designed for wye start, delta run. The more you increase the operating line voltage the more distorted current will result.