Motor start wave forms
Motor start wave forms
(OP)
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.





RE: Motor start wave forms
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
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Motor start wave forms
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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RE: Motor start wave forms
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.
RE: Motor start wave forms
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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RE: Motor start wave forms
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RE: Motor start wave forms
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: Can motor instantaneous trip be caused by contact bounce < ½ cycle?
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RE: Motor start wave forms
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RE: Motor start wave forms
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
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Motor start wave forms
Best regards,
Mark Empson
L M Photonics Ltd
RE: Motor start wave forms
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.
RE: Motor start wave forms
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].
RE: Motor start wave forms
RE: Motor start wave forms
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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RE: Motor start wave forms
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RE: Motor start wave forms
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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