High current failure in starting AC Induction Motor

[ecenus]

I have met a few problems in starting a 125hp (460V) AC Induction Motor:

1. I use v/f control method. When I start the motor, the motor trips out as my inverter can only handle 400A and the startup current (Locked rotor current) is over than that. For the pull-up torque: T = k * B * I * cos(theta) and B is proportional to v/f. So, if I need to keep current lower than 400A and wanna start the motor, then I need to increase v/f. Is my understanding correct?

2. The situation is: if I keep the accel about 7Hz/sec, the motor will trip out at 0.4 sec due to the high current; If I set the accel about 1.88 hz/sec, the motor will trip out at 0.9 sec...I am confused about it...

Thanks you in advance for your answers and reply!

 

[ecenus]

I am thinking whether my starting commanded frequency is incorrect. I start from zero, so v = f * ratio, V is small. I should start the commanded frequency from 8 hz, get a higher starting voltage to start the motor...

 

[ScottyUK]

Try it and see.

In the mean time, the value of the time you spent trying to debug this design would have bought you a fully debugged design from ABB, Siemens, SSD, Danfoss, Control Techniques, etc. And any of those drives, provided that it was sized correctly for the motor current, would in all likelihood have you up and running by now if we can assume that the motor itself is healthy. Why do it the hard way?


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