Why Use Isolation Transformer to Feed Motor

[McCircuit]

I recently worked on a project where they had an Isolation transformer 4160V to 4160V. I could not figure out why they would use this set up to feed a motor. Any thoughts?

 

[davidbeach]

What was the winding configuration of the transformer?

 

[McCircuit]

Not exactly sure but i know they were the same. There was no phase shift side to side.

 

[dpc]

Assuming no VFD, it could be serving as a type of "reduced voltage" starter. The transformer impedance buffers the system from the worst of the voltage drop caused by motor starting.

The other possibility would be to limit the ground fault current that could flow into the motor stator windings if the motor goes to ground. Even a wye-wye transformer will limit the ground fault current quite a bit.

 

[davidbeach]

I was thinking about a delta winding on the motor side, line side could be delta then if no phase change. Protection of the motor against damage due to ground faults would be the intended result.

 

[Thedroid]

We use a transformer right before a vfd for large ID fans. its 4160 delta/2305 to each phase and the symbol looks like a combination of a wye and a delta. I've looked for this symbol in several books and online but haven't been able to figure it out.

 

[Thedroid]

Kind of off topic, but the transformer I was talking about is a quadruple secondary extended delta rectifier transformer.

 

[lansford]

One other reason would be to resistance ground the motor side so that in the event of a phase to ground, fault in the motor, you would minimize stator lamination burning.
JIM