Excitation of synchronous machine

[MartinBruha]

I develop a simulation model of synchronous machine including excitation winding and damper winding. Everything works more or less reasonable except of the excitation.

Let's say that I have rotating diodes excitation. Is it possible to have pretty high excitation current in the excitation (field) winding while the input voltage is zero?
By input voltage I mean the voltage on the terminals of the diode rectifier. I would suggest that at zero-voltage input it is kind of open circuit so there should be no excitation current. Am I right or is it completely wrong point of view?

I can get lot of measurements from the site, unfortunately not this kind.
Do you have any suggestions?
Thanks for all your tips.

Best regards
Martin

 

[waross]

With no excitation current, residual magnetism will give up to a few percent of rated output voltage. Once you apply excitation current, the output voltage at no load becomes fairly linear with excitation current over the normal operating range. When you apply a load, the Automatic Voltage Regulator responds to the voltage drop and things start to change dynamically.

Bill
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"Why not the best?"
Jimmy Carter

 

[ScottyUK]

The rotor is a large inductance, hence it stores a significant amount of energy in the magnetic field. It is entirely possible to have the field voltage applied to the exciter winding at a zero or even a negative value and still have a large field current in the main rotor. This situation is not sustainable in the long term because the stored energy is finite, but in the transient case yes it is possible.


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If we learn from our mistakes I'm getting a great education!

 

[MartinBruha]

Thanks a lot for your replies. I had similar opinion that it is possible to have high excitation current at zero field voltage applied to the exciter winding, but only during transients. The excitation current should be falling so that the voltage on the magnetizing inductance (-L*dif/dt) is equal the voltage drop on the exciter winding resistance (Rf*if).
In my case this behavior can take 2 seconds or more and the excitation current is nearly constant or even growing so I guess it is a mistake in the modeling.