Hello,

synchronus motors have always intrigued me with how they help power factor within the plant. BUT I want to understand exactly what happens when the 2300v starter is picked up, when the DC field is applied, and what the power factor looks like whether its leading or lagging or whatever. The motors we have are all 2300 and 4160 GE vintage synchronous motors, separately excited, 1000 to 4000 HP frames. Any insight is appreciated.

 

Hello,

The simple answer to your question is that, depending on size, many synchronous motors can't be started just by applying the excitation (usually DC voltage) to the rotor and then applying AC voltage to the stator. Large synchronous motors just can't develop sufficient torque this way to spin up to synchronous speed.

So many large synchronous motors have small "pony" motors to spin them up to near synchronous speed and then excitation and stator voltages are applied. Or, there may be some damper, or amortisseur windings, may be used for starting (in effect making the synchronous motor an induction motor until it nears synchronous speed).

I would suggest using your preferred Internet search engine to search for the term:

"synchronous motor starting methods"

(with the double quotes).

www.wikipedia.org is always a good place to start, but my quick search gave several very good results.

Hope this helps!

 

Thank you for your reply CSA, yah while I was at work this evening I went and found one and broke out the prints and that clarified a couple of my questions.

Thank you.

 

As for power factor, well, that's kind of the interesting thing about synchronous motors. They can have leading or lagging power factors--when running as a synchronous motor, just by varying the excitation.

When starting, if they have amortisseur windings they will have a lagging power factor, or if the pony motor is an induction it will have a lagging power factor.

But, when running, the power factor (VArs) can be varied just by changing excitation. One of the reasons for using synchronous motors in the first place (they're much more expensive to manufacture than induction motors) is that they can be used to offset reactive power charges from the utility. When a large plant has a lot of induction motors or a highly reactive load the utility will put a VAr-hour meter on the plant and charge for VArs. So, most plants will use synchronous motors to try to compensate for their reactive load and reduce the VAr-hour charges.

They also run at a constant speed (synchronous speed).

But, as you can see--they require more auxiliaries that a comparable induction motor might. That said, large induction motors have to have special starters to reduce the inrush currents drawn by them, so their auxiliaries can also be expensive to purchase and maintain.

Glad to hear you found the prints. Again, wikipedia is a great place to begin one's investigation of many of these kinds of questions.