# Excitation Voltage

Discussion in 'Homework Help' started by Kayne, Apr 14, 2010.

1. ### Kayne Thread Starter Active Member

Mar 19, 2009
105
0
Hi there,

I am wondering if you can calculate the excitation voltage of a synchronous generator or do you just look this up on a graph that is supplied with the generator?

thanks

2. ### rjenkins AAC Fanatic!

Nov 6, 2005
1,015
69
Industrial motors and generators usually have a 'ratings plate' fixed to them that gives all the essential information (voltages & currents for armature/field or rotor/stator, speeds, duty cycle, temperature ratings, cooling requirements etc..)

Anything on a separate piece of paper can get lost, a riveted-on plate is there for anyone maintaining it in the future.

If a motor or generator is missing it's plate, it is very dodgy trying to find out by trial and error as speed, voltage and exitation current are all inter-related and without some known info you could get everything wrong and in an unsafe condition.

You need two of the three values, the third can then be determined safely by testing.
Power circuit current ratings are another thing again...

3. ### t06afre AAC Fanatic!

May 11, 2009
5,939
1,227
Also be very careful not to apply excitation voltage without some sort of current limiting. If the rotor is not moving at correct speed. The excitation windings will draw very high current and quickly burn out

4. ### Kayne Thread Starter Active Member

Mar 19, 2009
105
0
So can the excitation voltage on of a generator be really high?. Say you have a 11kV generator can the excitation voltage be say 5000V. To me this doesn't sound correct.

I have been working though a question and have calculated the excitation of a 11kV generator to be 5000V. I think that this would be too high and i have made a mistake that is why I am asking the question.

Thanks for the help

5. ### rjenkins AAC Fanatic!

Nov 6, 2005
1,015
69
Field strength is based on current not voltage (ie. ampere-turns).

The voltage needed is secondary, based on the resistance of the winding.

For example, on several similar machines of different ages, one specific large motor has a shunt field rating of 400V 0.5A, 60V 4A and 20V 12A.

They are all 500V 80A on the armature & same horsepower rating.

The field (exitation) current is regulated, but the voltage can change quite significantly as the winding resistance changes with temperature.