3000 hp 1800rpm, 4160v Synchronous (manufacturer Ideal) motor with 90 volt excitation system.

[Simon05511]

We just removed our motor and sent it off to the shop due to not being able to Sync. When the Rotor was removed and each of the 4 coils was tested, it was found that one of the coils on the rotor had a lower reading than the other three. Three of the rotor coils were .306-.310 ohms, and the fourth coil was .270.
My questions,
1. Would this cause the motor to not sync?
2. Is there anything we could do to help it sync? increase Dc voltage during start up?

 

[petersonra]

What do you mean by "not being able to sync"?

Synchronous just means the rotor and stator are moving at the same rpm. I don't know that if they are not in sync that the motor can even turn.

But, I am not familiar at all with synchronous motors because they are not common. Hopefully someone who knows a lot more can answer your question.

 

[Jraef]

Yes, if one coil is weak, it may not "pull in", which is the process by which the synchronous part of the motor takes over from the amortisseur winding that is used to accelerate it from a stop.

If you don't understand the terminology, a synchronous motor will not start by itself from a dead stop, because it has no "slip" with which to generate torque. So synchronous motors are built with special bars in the rotor, called the "amortissuer winding" that has two functions: it acts as a dampener (the definition of "amortissuer") for torsional oscillations of the load once it is running in synch, but also functions to make the rotor act like an induction motor to accelerate the motor from a dead stop, until it gets to around 97% speed. Once it gets to that speed, the rotor winding coils are energized and their magnetic fields "pull" the rotor poles into alignment with the stator fields and start rotating the rotor in synchronism with the stator fields. If one of the stator windings OR the rotor coils is weak, the critical strength needed for that final pull-in can't happen.