Unbalanced motor starting currents

More information about the system at hand will help yield higher-quality advice.

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).

A starting point in analysing your waveform is mentally subtracting out the expected decaying dc offset components.

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)

Here is a link on checking unbalance on three phase motors...