Generator Theory

[big john]

I got into the utility industry a year ago when electrical construction tanked, and what I knew about generators was what I'd read in Article 445, so bear with me:

We have synchronous generators ranging in size from a couple-hundred KW to many tens of MW. All these machines have brushed excitation systems.

My main question: Is excitation current a linear function of excitation voltage? If not, why?

I've been told that Ohm's law will not work to calculate excitation current.
I've been told that applying excitation to a stationary field winding would result in overcurrent.
I've also seen much lower field-winding resistance measurements than Ohm's law would dictate.

I would've thought field-current could be determined just like you would in an electromagnet, and that we regulate it during operation simply to change power-factor. What the heck am I missing?

-John

 

[Robert Martin]

Generator theory

Generator theory

The testing I have done in the past in regard to field current was under no load. It was linear until it was close to the saturation voltage where the current begins to drop.
As the field current increases the output voltage increases.
This is an induction alternator so the field is stationary.
I'm not sure why you would want to put excitation voltage on the field when the machine isn't in motion but unless you put to high a voltage (exceed the wire current rating) it won't hurt.
Excitation current calculations may not be as easy as ohms law as with a magnetic field that is variable the current could be differant than ohms law dictates.
I am unaware of changing the PF by varing the field current. I changed the excitation to adjust the voltage.
As all machines are differant I would check with the manufacturer for the specifactions. E-I-R of all the windings.
Hope this little bit helps.

 

[rcwilson]

It's non-linear due to saturation of the field and change of field resistance due to temperature, plus other effects. At operating temperature it is relatively linear up to about 90% voltage.

Putting full excitation on a generator at standstill will probably burn up the rotor windings since there is no cooling air flow. It will also overheat portions of the generator since the V/Hz ratio will be way off. The exciter will put out the current and magnetic field to maintain full voltage but the frequency is zero. The high V/Hz ratio will saturate the iron and force magnetic field into alternate paths like hold down bolts that are not designed for the magnetic fields. Most exciters have a built-in V/Hz limiter.

 

[big john]

So, it really is nothing more than a big electromagnet, because any electromagnet will saturate. I think the guys I've spoken with were thinking it operated sort of like the rotor in an AC motor, where if if wasn't turning, it would result in significantly higher current flow than if it was.

Thanks for clearing up the misinformation.

-John

 

[richxtlc]

We have synchronous generators ranging in size from a couple-hundred KW to many tens of MW. All these machines have brushed excitation systems.


I would've thought field-current could be determined just like you would in an electromagnet, and that we regulate it during operation simply to change power-factor. What the heck am I missing?

-John

Not all generators have brushed excitations systems, some are brushless.

When a generator is connected to a grid, trying to adjust the voltage above the system voltage will on changed the VARS being produced by the generator. Increasing the VARs decreases the PF and if not carefully watched can cause heating in the stator ends. If you try to lower the voltage below the system voltage, the VARS change in the other direction, but a greater problem is the generator can become unstable due to a weak field and slip a pole in the event of a system fault.