excitation voltage

[krlyle1]

What is the purpose of an excitation voltage switch on a generator ?

 

[jwjrw]

AC generators convert rotational energy into AC by moving a static magnetic field through the vicinity of the stator windings. You create this static magnetic field with DC. If you tried to use AC, the magnetic field would be varying, and this is more corectly known as a transformer. Since you want to boost the energy using rotational energy, you want the static field, hence the use of DC of excitation.

Its not any different than the alternator in your car. It takes DC from the regulator producing a rotating field, and induces three phase AC in the stator. The difference is that the car alternator then rectifies the AC to make DC to recharge the battery and run the car's electrical system, where in a power plant you take the AC and step it up to the electrical transmission system.

I think this is what you are asking. If not one of the SMART guys will let us know.

 

[gar]

1001321-0933 EST

krlyle1:

Relative to your question I have no idea.

Now to conjecture. A generator or alternator requires a magnetic field. Two different ways to provide this field are a wire wound magnet or a permanent magnetic.

If your generator is a PM type, then from a conventional perspective there is no electrical way to turn off the magnetic field. If you have a PM DC motor it is also a generator.

If you have a wire wound field in a generator or alternator, then you can adjust the intensity of the field by the amount of current flowing thru the field coil. This also means if you reduce the field current to zero, turn a switch to the field off, then the output of the generator will be zero because the generator output voltage is proportional to the intensity of the magnetic field from the field coil. In real life it won't be zero because when you remove current from a magnetic with a magnetic core there will be some residual magnetic field.

A different issue. If you have a DC motor with a wound field and you loose field excitation, then what happens to the motor speed?

.

 

[Besoeker]

1001321-0933 EST
A different issue. If you have a DC motor with a wound field and you loose field excitation, then what happens to the motor speed?

That would depend on how the motor was loaded or what it was coupled to.
I had one a a little over a year ago back where the shunt field had gone open circuit. It was on a Fourdrinier paper (making) machine where speed holding is quite critical.
Well, it carried on running and the machine carried on making paper. The electricians knew there was a problem but production is king and they were not allowed to stop the machine to do any checks.

What happened is that armature voltage dropped to about 100Vdc whereas normal running voltage is close to 600Vdc. It is one of two 300kW motors that are effectively are mechanically coupled in that they both drive the same part of the machine.

There is a load sharing circuit between the two which looks at the current and maintains it in a set ratio. So the motor with the failed shunt field took its correct current, albeit giving much less torque.

Having been told what the symptoms were, I was fairly sure it was an excitation problem but the 100Vdc on the armature got me thinking. The answer in the end was quite simple. It was one of their older motors (c1960) and, as was the practice then, motors for this application had some compounding - there was also a bit of series field resulting the 100Vdc or so back EMF.

In summary, the motor ran that way for about a week until the next planned shut. The fault turned out to be a badly made "temporary" connection in the motor terminal box. From its appearance, it looked like it had been that way for a lot longer than most us would consider to be temporary.

 

[gar]

100131-2036 EST

Besoeker:

That was was quite and interesting situation.


krlyle1:

Thinking about your question. If the generator is always connected to a load and the mechanical power source is a gasoline engine, then the excitation might be turned off while the engine is being started, getting up to speed, and warming up.

This would reduce the load on the engine in a speed-torque area where its performance is not very good. This could be done on the output side of the generator with a much bigger switch.

Also at shut down if one wanted to remove the electrical load before the engine was shut off, the excitation switch would be useful and might reduce a problem with arcing if the load current was switched.

.

 

[krlyle1]

Thanks

Thanks

Thanks for all the great info its much appreciated.