Self Starting Synchronous Motor

It's tricky, you're right, that's why you ned a protection system for them. You have to time it right, but when you find the sweet spot, usually at around 95% speed, then the poles will pull it into line. That spot is always slightly lower than the slip speed of the amortisseur winding, so it's not really that hard to get there if the load is consistent, but it definitely warrants watching it. That is sometimes done by ear, but most often it's a function built into the protection relaying system, the same one that is watching out for pole slip.

Thank you both, it's going to take a while for this to sink in.

I think you can understand my question about pole slip and the damage it could do.

My only experience of synchronous machines has been believe it or not with rotary converters. (I'm showing my age.) They used a pony motor which with the converter acting as an inductor would run up and self synchronise.

Don't be surprised if I come back with more questions.

One of the places I worked had four old 1500HP synchronous mill motors, in the year I was there I never even saw them run. The new plant had two 6000HP mills, but they were standard slipring machines. The bigger mills were far more efficient.

The little mills would run at night when I would be many miles away.

TonyS,

One side benefit from a synchronous motor is that by varying the DC field, you can have it correct the plant's power factor (the details would require a method of measuring the power factor and automatically changing the field as needed). That is why synchronous motors have been called "rotary condensers" (an old school term for capacitor), and sometimes are run without any load.

--John M.