synchronous motor starting

[mersin33]

I am trying to do a voltage dip study of a large synch. motor start using PSS/E. So far, all the documentation I have gone through is about induction motor starting. Any help will be highly appreciated.

Thanks in advance,

saltan

 

[rconnett]

Salt - synchronous motors are fitted with amortisseur (spelling?) windings which are embedded in the rotor and are used for starting the motor. They make the synch motor act exactly like an induction motor during starting.

The main difference is that for normal induction motors require ~6 times full load current during starting, a synchronous can only require 3.5 times full load current, so they can be easier to start (i.e. lower voltage dip).

The reason for this is that the synch motor establishes its full load torque via flux linkages provided by the field, whereas an induction machine must establish these linkages via the same rotor bars it uses for starting, so the rotor design cannot be madjusted too much to reduce the inrush characteristics.

For modelling, I'd check the locked rotor amps on the machinfe or estimate them at a lower value than an equivalent hp induction machine, but otherwise model it as an induction motor for starting purposes.
Hope this helps.

 

[mersin33]

rconnett-first of all thank you very much for the quick reply.

I am not educated about motors, particularly synch motors, therefore forgive me if this question is really stupid:

From your input, I understand that you are suggesting me to model the amortisseur (spelling??? but I know what it means )windings which will make the synch motor equivalent to an induction motor until the synch motor synchronizes. Or am I mistaken?

If I am not mistaken, what happens to the motor when it is loaded, after it is synchronized?

The motor in my case starts without load, and is later coupled to the load via a clutch.

Regards,

Saltan

 

[jraef]

You are correct in your interpretation of his response. Model it as if it were an induction motor. The synch motor will be slightly better (easier on your system) which gives you a built-in "fudge factor".

However, why are you concerned with connected the load for a voltage dip study? Once running it should not be a problem (otherwise you would never get it started). In response however, just look at the Full Load Amps of the motor for your worst case scenario. If coupling the load makes it draw more power thant that, your motor is too small!

"Venditori de oleum-vipera non vigere excordis populi"