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?

 

[electricpete]

If you look at it only in the frequency domain, it doesn't matter whether wye or delta. However if you look at it in the time domain, you expect to see the current spike from s of saturation at a given point in the timewaveform (at the current peak if motor is unloaded and we are looking at leg current). Where in the time waveform it shows up depends on the connection.

I do not think that winding distribution would result in 3rd time harmonic of current in the absence of saturation.

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[electricpete]

Well, I tried to recreate the waveforms of this case from the harmonics with some assumptions about power factor using a math program Maple.

http://home.houston.rr.com/electricpete/thetapfrun2.PDF

If you scroll through the graphs, you see the the first graph (MagnetizingCurrentsLeg) shows magnetizing current which was created with phase relationships to put the "spike" associated with saturation at the peak of the fundamental magnetizing current, which is where I think it should go.

The second graph "TotalLegCurrents" shows the sum of the above magnetizing currents plus a load current (2 graphs = 2 different legs). More detials is provided in the third graph titled Aphase_exciting_load_total_legcurrents which shows for one LEG (I shouldn't have labled it phase), the magnetizing and load components and their sum.

The power factor determines the relationship between the magnetizing component and the load component (the phase difference and the magnitude ratio). Power factor angle in this file (run2) was assumed to be 10 degrees.

The last plot (PhaseCurrent) is determined as the difference of two leg currents (as we expect in delta). This plot has an appearance somewhat similar to what was posted by testtech.

Now in another run (run1)

http://home.houston.rr.com/electricpete/thetapfrun1.PDF

I changed the powerfactor to 35 degrees.

The results are kind of intriguing.... the LEG currents very closely resemble testtechs results for this configuration. The phase current does not.

There is one error I made in both files. Looking at the phase relationship between load component and fundamental exciting component, the load component should lead the exciting component by 90-thetapf. It looks like the opposite. I don't think this affects the final shapes (flip it mirror image around the peak). I'm a little bit confused why my equations didn't produce the phase relationship I was looking for:

Iam(t):=A1m*cos(1*w*t)+A3*cos(3*w*t)+A5*cos(5*w*t)+A7*cos(7*w*t);

Iaload(t):=A1L*cos(w*(t+Pi/2-thetapf));

Both A1m and A1L are positive.
The fundamental Iaload should lead by Pi/2-thetapf according to the equations but according to the graph it looks to lag. Any ideas where I went wrong?






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[electricpete]

Actually the magnetizing current in the first example (run2) doesn't have a pretty shape to look at to show relationship of fundamental to the "spike". The fundamental is too small and the waveform doesn't look realistic to me (I expect one spike at the peak of the fundamental).

The magnetizing current in the second example (run 1) has the shape I think it should have. The second example (run 1) also produces a waveform which much more closely looks like testtech's.

The second example (run 1) would make sense if we were looking at a wye motor. But not if star-delta meaning star-start and delta run with the phase (not leg) currents measured while running. Can you clarify the configuration again and did you measure phase or leg currents?

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[electricpete]

"The second example (run 1) also produces a waveform which much more closely looks like testtech's."

(the waveforms I'm referring to are labeled as leg currents, not the final phase current).


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