Two speed AC motor fusing

[thinker]

We received from Germany equipment with two speed AC motor(3 phase, 60 Hz, 460V, Y/Y, 3.5 FLA (low speed winding) and 14.5 FLA (high speed winding). We plan to use for its low speed winding branch circuit protection Class CC fuse 7A, as recommended for DOL starting. This motor must always start via low speed winding, and then switch to high speed. The question is: what kind of transient in the motor occurs during switching from low to high speed? Also, what should be the criteria for high speed winding feeder protection? It seems that no in-rush currents (like at DOL start) are present, rotor is running with low slip at half of the max speed before switching. We have no practical experience with such motors. Please advise!

 

[waross]

I would protect the high side as if it was a single speed motor. There may be transients when the motor is switched from low speed to high speed. The switching transients may be more than Locked Rotor Amps, but probably of shorter duration. The transients may cause unexpected tripping of instantaneous protective devices. These problems usually manifest on larger motors. This size of motor may not experience serous switching transients.
Accelerating current from 50% speed to 100% speed may be more than you realize, however at 10 HP (more or less) dual element fuses should give good protection without failure.

Bill
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"Why not the best?"
Jimmy Carter

 

[thinker]

Thank you, Bill. To my surpise, the motor manufacturer could not provide any data on switching transients.