Unbalanced motor starting currents
Unbalanced motor starting currents
(OP)
We have a 460V/3ph/60hz 125/62.5 hp 143/99amp dual speed motor that is regularly tripping a main breaker on startup in low speed. The circuit breaker trips about once every 5 low-speed starts. We have recorded amp readings of 1700+ on A phase, 1300 on B phase, and 1100 on C phase.
The startup amps are consistent amoung starts. "A" phase is always the highest.
I believe this is a new phenomenon to the motor, but I can't be sure. The breaker just started tripping a couple of days ago.
Once the motor gets past the starting, the voltages and currents are all relatively the same. A phase is 495V/76.9A, B phase is 497.8V, 79.9A, C phase is 494.2V, 77.7A.
You can see a picture of the startup voltages and currents at http://www.ico-inc.com/l3m3.jpg
Any ideas about what to check or what could be wrong?
The startup amps are consistent amoung starts. "A" phase is always the highest.
I believe this is a new phenomenon to the motor, but I can't be sure. The breaker just started tripping a couple of days ago.
Once the motor gets past the starting, the voltages and currents are all relatively the same. A phase is 495V/76.9A, B phase is 497.8V, 79.9A, C phase is 494.2V, 77.7A.
You can see a picture of the startup voltages and currents at http://www.ico-inc.com/l3m3.jpg
Any ideas about what to check or what could be wrong?
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RE: Unbalanced motor starting currents
What is the normal time of acceleration to nameplate speed?
What is time to trip during problem?
Are there similar motors with like applications in the facility?
Have you compared low-speed startup currents to those of high-speed?
Is this a molded-case breaker? What type and rating? What is the setting of instantaneous elements?
What is the type of starting?
What is the motor code letter?
It may be worthwhile rolling phases [ABC:123/231/312] to determine whether problem stays with the (source) bus or follows motor (load).
RE: Unbalanced motor starting currents
Discussion showing typical waveforms with decaying dc offset is posted at: Thread237-9886
There is a different decaying dc offset on each of your three phases, with one being the highest magnitude and the other two smaller magnitude dc offsets in the opposite direction. But once the decaying dc part decays away, the remaining ac component is balanced.
All of the above is as-expected.
The parts of your example that are not expected:
#1 - There is no reason I can see that the same phase should consistently have the highest dc offset. Perhaps might have something to do with relative timing of closing of the three phases (one phase always first)? But still I'd expect it to be random since there should be no consistent relationship between power system phase and closing time.
#1B - Is this by chance closed in by a static (solid state) circuit breaker?
#2 - The ac component is decaying (in addition to the dc component) during the 1st 5 cycles. The ac component should stay constant at approx LRA. (see my example). I'm at a loss for an explanation.
2A - For 100A FLA motor, LRA should be in the ballpark of 500A-700A. I see that after 5 cycles it appears to be 300A peak ~ 200Arms. Why is that? Was this recorded waveform from a successful start?
2B - Does your motor come up to speed in 5-10 cycles (I doubt it)
2C - Was there a change in winding configuration during the start? (shift between dual winding speeds)
RE: Unbalanced motor starting currents