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Starting a synchronous motor with the field applied

Starting a synchronous motor with the field applied

I came across an application recently where a large 3500HP synchronous motor has its brushless excitation field applied as soon as the motor is started. This motor also does not appear to have any sort of controller to control power factor etc... but instead appears to have a manual rheostat that is set prior to starting in order to control the AC input to the rectifier on the static exciter.

I always thought that a synchronous motor was started as an induction motor by use of the armistoir winding and that the rotor field was not applied until the motor is started, reaches full speed, and is in sync with the source.

Am I looking at this application correctly where the field can be applied as soon as the motor is started? Can someone explain why the field would be applied when the motor is started as opposed to after it is up and running? What does applying the field during starting do to the motor itself, power factor, etc...?

There does appear to be a power factor relay which appears to trip the motor circuit if the power factor drops below a certain pre-set value but I'm not sure if this applies for both starting and running, or just while motor is running. There also is a timer relay that will stop the motor circuit if full load field current is not detected in a set amount of time.

RE: Starting a synchronous motor with the field applied

Is it by chance a GE brushless motor? If so, I believe they have the Field Control as part of the rotor assembly. So yes, you are energizing the field winding when you start it by having that inductive coupling system, but internally on the rotor, the field application is still controlled with thyristors, you just don't see it any more in your external control panel. I've only done one and I too questioned it, but the guy I was working with explained to me that GE does it differently than everyone else.

"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

RE: Starting a synchronous motor with the field applied

Yes these are brushless motors however I'm not certain if they're GE or not. The datasheet that I have appears to be a company called "Electric Machinery Mfg Co".

But you are correct, I did see a generic drawing that shows the motor having "internal field control" inside on the rotor. So although we are applying the field externally the field control on the rotor is ultimately what is controlling the rotor excitation field to the motor?

Does this internal controller have any intelligence? How does it know when to apply the field and how to vary the field for power factor control?

Is a power factor relay used to trip motor during the starting process or after it is running? It sounds like the internal controller may not apply the field until motor is started so motor is still starting as an induction motor and will have a lagging power factor during starting?

What is the point of the variable transformer on the external excitation circuit?

RE: Starting a synchronous motor with the field applied

On the rotor assembly the setup is basically just a rectifier using SCRs so that it is controlled. As I understand it the GE has really basic logic that is looking strictly at the rotor EMF to decide when to apply the field. EM has long been a big company in Synch motors, but they are now owned by Weg out of Brazil. I've seen plenty of their brushed type motors, but not their brushless. You can download some stuff to help explain it here though.

"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

RE: Starting a synchronous motor with the field applied

EM held the patent on the original "Polarized Field Frequency Relay".
The relay looked at the frequency of the rotor field.
When the frequency was low enough for field application the relay checked the polarity of each half cycle and applied the field when the polarity was correct for a smooth pull in.
I am sure that the internal field control circuit does the same.

"Why not the best?"
Jimmy Carter

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