# Excitation Current

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what happens if we give alternating current as excitation instead of dc? any changes in generating voltage? and how to calculate excitation current for a 2.5 mw 440 v 50 hz alternator?
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what happens if we give alternating current as excitation instead of dc? any changes in generating voltage? and how to calculate excitation current for a 2.5 mw 440 v 50 hz alternator?

Probably the same thing that would happen if you were to fill your automobile battery with tomato juice !

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• 2 weeks later...

Hello boby

If you have a standard alternator with a single rotor winding and apply and AC voltage to the rotor, you will have two rotating magnetic fields in the rotor, equal in magnitude and frequency, but rotating in the opposite direction.

If you had a thre phase winding on the rotor and applied an ac waveform, you would have a single rotating field. The speed of the field will be determined by the frequency of the AC that is applied to the rotor. If the rotor was stationary, then the generator would act like a transorfmer and the output frequency would equal the input frequency. If we now spin the rotor, the output frequency will be the sum of the rotor speed and the applied frequency.

For example, if we have a 4 pole 50Hz generator, then the rotor speed will be 1500RPM. If we apply 50Hz excitation (assuming a three phase rotor winding) we will have an output frequency of 50Hz plus 50Hz = 100 Hz. NB if the rotor field was spinning in the opposite direction, we would have zero Hz.

In the case of a single rotor winding, we would have to apply a single phase voltage and this would result in two fields, one forward rotating and the second reverse rotating. This would result in two output voltages, one at the sum of the two frequencies and the other at the difference.

Best regards,

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