Motor start wave forms

[wolfie1a]

I'm looking at some wave forms of a 4160V, 600 HP, 1200 RPM motor with 175 Kvar Caps. I'm montoring the line side of the motor starter, most of the wave forms look like I'm seeing a ringing form the caps. How ever look at the start on 12_16-2008. No too sure what I'm seeing here. Any help would be gratly appreciated.

 

[Skogsgurra]

I think that currents upstream the breaker would have told the whole truth. But, without that information, we can still guess.

It looks like a rather normal interruption where there is some arcing in C and then ringing that is over (in C) about half-way through the recording.

Before that, A and B seem to restrike and start some arcing and ringing. It does so only when the difference between phases A and B has reached a certain voltage.

The three-phase voltages you see after the break are the voltages induced by the DC field of the rotor in the stator winding.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[electricpete]

Gunnar - do your comments apply to a motor start or motor stop? (I'm reading the question as a start).

I agree currents would have helped and voltage downstream if accessible would have helped in interpretation.

wolfie1a - Are these phase to phase voltages or phase to some derived neutral? I assume the caps are downstream of the contactor and switched on with the motor? How far from contactor to caps and from caps to motor? And from the breaker back to the transformer?

It seems that possibilities to consider during start might include contact bounce, non-simultaneous closure, reflections, and possibly arcing.

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[wolfie1a]

Phase to Phase. We are feeding the motor starter with a contactor that is approx. 1000 ft. Note the feed contactor stay energized all the time. The caps for the motor come directly off the motor stater (vac bottles) and the motor feeder is approx. 50 foot long. The waveforms are of a motor start. We are using a Dranetz to capture this wave.

On a side note the reason we are looking at this is on occasion, probably 2 time a year we loose other 4160 volt motors that are on the same buss. We orginally thought we had a voltage sag problem, but after monitoring the system we found nothing. We did a motor staring simulation with E-tap to see what a locked rotor would do the voltage and even in this case the voltage drop would not be enough to cause any problems. We had our motor rebuild shop come in and test motor, we have tested the cables, contactors, multilin and every thisng else we coudl think of. The only strange event we have captured is 12-16-2008, the jagged edges on the ringing.

 

[electricpete]

Whenever you mention a side note, you always get more questions. Feel free to ignore these questions if these are outside of the direction that you want the thread to go:

The motors trip during start?
What type of trip?
Have you checked your capacitor sizing (motor steady state magnetizing current some margin above your 175kvar) ?

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[electricpete]

We had a 60hp motor that tripped on instantaneous overcurrent spuriously twice six months apart. Couldn't find anything wrong either time. Contactor and breaker were changed in between those two trips. We monitored the starts after the 2nd trip but never caught it during the act of tripping. One thing we did notice was this contactor had more separation time between contact closure and more often bounced than a monitored sister motor. That might tend to point towards contact bounce as a cause of our trip, but it would have had to have been two different contactors installed in the same position sincce we changed contactor between those two trips. We never figured out the definitive cause.

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[electricpete]

My gut feel is you are seeing non-simultaneous closure and contact bounce. I think the first spike of the blue waveform is where the second of the two poles associated with the blue voltage measurement closed. Then the 3rd contact closed and bounced where the red and green curves spike. At least that's my wild guess... not really sure.

The question of whether this type of behavior can cause an instantaneous trip has been asked and discussed before. Here is the thread:
thread238-226727

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[electricpete]

One more comment. The consultant that we brought in on the case was of the opinion that contact bounce was the cause of our trip described above. I was never as sold on the theory as he was. But you never know. We don't have any caps on the circuit.

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[Skogsgurra]

Pete.

I just reread the OP. He is looking at the line side voltage. So it is not a breaking that is shown. Clearly a start. Then, the bouncing and arcing seems to be the plausible interpretation.

OTOH, don't think arcing occurs in vacuum bottles. Bounce and high frequency ringing, in addition to the cap ringing, seems to be what is going on. There is also a definitive second HF transient between A and B almost exactly at 50 % time. Could be a second bounce.

Current recordings not available?



Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Marke]

Are the capacitors connected phase to phase, or are they star connected to neutral?

Best regards,

Mark Empson
L M Photonics Ltd

 

[wolfie1a]

The caps are connected phase to phase. We tried to get current readings but our Dranetz unit has a bad input... currently (no pun intended) being repaired.

Another piece of information I should have mentioned is the 4160V is on a high resistance ground system.

I probably should have not used the the term trip. The motor just stops, almost as if you had hit the E-stop. What we are seeing in DCS from the multilins are the NC held open contacts toggle. There are no trip indications on the multilin. It just as if you pulled the power off the multilin, hence our early assumption that we were dealing with a voltage drop.

On a side note: we tested our Multilins to see how low we could go before we toggled the contacts, we were able to drop the voltage over 50% before had any problems.

 

[7anoter4]

In my opinion:
The harmonic waves are due to stator slots-space harmonics.
As this harmonics are so elevated it means no rotor reaction since there is no rotating field.
In D breaker is on [CV some arcing].
In E a short circuit between AV and BV may occur [both curves touch the 0 line].
In E and F the breaker tries to reconnect twice again but CV remaining out.
No rotating field to start and Multilin will open the breaker when the time is out.
Possible fault:
Breaker contacts do not close correctly [Contacts burnt or else].

 

[davidbeach]

7anoter4, interesting, but did you consider the time frame? That whole image spans about 12ms. No breaker could have done any of the things you suggest. What ever happened at D, E, and F, it all involves capacitors charging and/or discharging, it all LCR ringing.

 

[electricpete]

I see 2 characteristics:
1 - the ringing around 1.5khz I attribute to ringdown of the resonant cap circuit as others have said.
2 - the abrupt step changes followed by higher frequency (100khz?) ringing I attribute to contact bounce.

My experience is in monitoring start of motors without caps. I don't see item 1. So I thinki it is associated with the caps. I do see item 2 and we have monitored the load side of contactor and have confirmed it is contact bounce. If you search the forum you can probably find my waveforms.

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[electricpete]

I guess I should clarify that it looks to me that the item 2 posted here is the same as what we saw as contact bounce, but that is just my guess.

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[electricpete]

I have to backtrack again and clarify the only point I was trying to make in my previous hz is that the 1khz ringing is associated with caps.

My gut feel again is that the other item is contact bounce but I don't rule out the discussion of arcing.

It is my understanding that the TWF in question represented a successful start (no trip). The scenario of 7anoter4 seems to end in trip.

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