Which way would the motor turn?

[davva]

This may never happen in reality but I was asked the question today by a colleague and was unsure of the answer.

SUPPOSE.. you had a switchboard with an A and B bus. A bus fed by a Dy1 transformer and the other fed by a Dy11. And the system was designed never to parallel the transformers but a changeover system was in place on the bus coupler.

If induction motors running on the A bus (which lets say normally run clockwise) were then supplied with power from the B bus would they turn in the opposite direction?

I know when you swap any 2 phases it will turn the opposite direction but what would happen when the phase shift changes by 60degrees. I sort of convinced myself it may run in either direction depending on the design of the motor but wasn't 100% sure.

 

[Marmite]

It's the phase rotation that determines the direction of rotation of a three phase motor. Assuming the A & B buses have the same phase rotation, then the motors will continue to run clockwise. The phase shift of the transformer causes the secondary voltage to lag/lead the primary voltage by 30 degrees for Dy1/Dy11, but the phase rotation remains the same. (Assuming all connections are standard)
Regards
Marmite

 

[burnt2x]

You nailed it, Marmite!