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phase displacement

phase displacement

RE: phase displacement

You mean motor rating of 1.8MW, I suppose.
The currents of uncoupled motor (solo run), are predominantly inductive (excitation current being the major component). Further, excitation currents are known to be unbalanced and include harmonics. This is true with transformers as much as motors.
I suppose that's what we are seeing in the graph attached with your post.

RE: phase displacement

Your graph seems to be showing a dc offset current on one phase.

Everything else looks normal on that phase... same peak to peak amplitude as the others.

If this were taken immediately after start, there would be a very short term decaying dc component, but the dc component in your trace doesn't seem to diminish at all during the entire graph. So I'd assume that any starting transient has subsided at the time this was captured (as a double check, please confirm this trace was not captured within 1 second after start).

There's nothing I can think of that would induce an actual steady state dc current in one phase (and by the way even if there was, the three phase currents should still sum to zero at every instant in time, so if there was an actual positive dc offset on one phase there should be a negative dc offset on at least one of the other phases... although if you are monitoring from ct secondary we'd have to think about how the ct responds to dc.... after several L/R time constants all indicated dc on secondary goes to zero).

Based on all of that, I conclude that it must be a measurement setup error of some kind. There may be an ac coupling setting that would get rid of that.

(2B)+(2B)' ?

RE: phase displacement

Possibly a raw event where there's a bunch of calibration offset on the phases. A filtered event of the same thing would show all of the phases centered on zero.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

RE: phase displacement


The second attached picture is for the same motor with full load.
The zero off set value is declined but remains some different.
The motor now is running with full load ,170A ,but with some buzzing or hamming. CBM team check the vibration and informed me that the bearings and all mechanical sides are good.

RE: phase displacement

I'll revisit one aspect. It seems abundantly clear that the indicated currents don't sum to zero (moreso in the unloaded run, but also in the loaded run). That violates Kirchoff's current law, so it cannot happen in real life (absent a ground fault, which is not realistic with such large magnitude over such long duration, let alone dc). So it proves there is a measurement/display error of some type compared to the actual current flowing in the motor.

For clues about potential sources of measurement error: What type of sensor are you using to measure current? Are you clamped onto a cable or a ct secondary? If cable I assume you are clamping on a portion that is not shielded? What kind of data collection device? If someone is familiar with the data collection device maybe there are some relevant settings.

If you really want to have an open mind to the possibility of a bizarre actual dc current (even though it's already ruled out for me), I'll ask the question what is driving this motor? Is it fed from vfd or transformer? Is the motor neutral floating or grounded (it should of course be floating in every case, but again just trying to satisfy your questions which seems to presume the dc could be real).

> buzzing or hamming

I can't dispute that in theory dc current superimposed on the normal current in an ac device could produce buzzing due to changing the magnetic chracteristics (although in such case I'd also expect current distortion and there was no evidence of that even in the unloaded). I chalk it up to a coincidence, there are a lot of things that can cause buzzing. If you want to investigate the noise further, I like the free app spectroid on Android which can display spectrum and “waterfall” or spectrogram. In a noisy environment you can move toward/away from the noise source and see which frequencies are changing (to narrow down which frequency peaks are associated with the noise source).

(2B)+(2B)' ?

RE: phase displacement

Thank you very much .
in regard to your inquiries ,the measurements in done using bar-type CTs read by Sepam M20. the 6.6kv system is supplied by NER transformer.
the motor is protected by earth fault protection using toriodal CT .

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