Generator excitation 1

[voltages]

Im a beginer here, i just wonder why a generator should always have a DC excitation?

If in transformers dc cannot be transformed (unless its pulsating dc) why is it used in excitation for generators as well. Is it because the magnetic field is moving magnetic one due to it rotates as the prime mover rotates it?

hope someone can share ideas, thanks.

 

[itsmoked]

The excitation is just the magnetization of part of the generator. Usually the rotor. This static magnetic field is then mechanically swept past the stator coils to produce the generators output.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

Well, in a typical brushless exciter, the excitation is generated as AC. It is then rectified before being applied to the field windings.
Induction generators are excited by the AC line voltage.
respectfully

 

[TheBlacksmith]

A simple genrator model uses a permanant magnet for the rotor. However we need to be able to vary the strength to control voltage so DC is used as it approximates a varible permanant magnet.

 

[wbd]

The basic principle here is that a conductor moved thru a magnetic field will produce voltage. In the case of generators, it is the field(rotor) that moves and the wire(stator) that is stationary.
The dc voltage is used to generate the magnetic field in the rotor. This voltage can then be used to under or over excite the field to produce a lagging or leading power factor.

 

[waross]

The dc voltage is used to generate the magnetic field in the rotor. This voltage can then be used to under or over excite the field to produce a lagging or leading power factor.

This is only partly true of generators connected to the grid.
The load determines the power factor. The excitation determines what share of the total KVARs of the load is supplied by a generator. Changes in excitation also tend to change the system voltage in about the same ratio as the ratio of the generator capacity to the total system capacity.
With a stand alone generator, the excitation controls the voltage and has no effect on the power factor.
Induction generators are a special case that are probably best ignored for now.
respectfully

 

[voltages]

Hmmm. Actually im working in a power plant, hehehe. Thanks for the replies. Indeed waross is right, in an isolated plant (island operation) pf or KVAR control is not in effect including the generator real power or the MW for the load determines it. But if the generator is connected to a large grid, its a different story: the pf/kvar and MW can be adjusted to take part of ur slice in the system. If i want to reduce my MVAR contribution to the system i will reduce it and it follows that the voltage will drop and pf will go up (from .8 to .9 lag just for ex.)and excitation goes down too and of course the fuel comsumption goes down too without really affecting the output real MW delivered by ur generator. In turn other plants tends to supply what u have reduced to maintain balance in the system.

But my question really is cant we not use AC as excitation meaning we remove the rectifier diodes before going to the rotor to excite the generator? Our generator is brushless too. synchronous.

 

[waross]

If you did that, your generator would become a frequency converter. A 60 hz generator with a 60 Hz excited field would produce 120 Hz.
This principal was once used to produce drives for speeds above synchronous speed.
Practically, the efficiency of applying AC to the field of a generator would be faily low. A machine built and optimized for frequency conversion would have acceptable efficiency.
Try browsing through some old electric machinery and drive books. The books that are too old to have been scanned and posted on the net.
respectfully

 

[byrdj]

the answer as seen by a mechanical

The rotor needs to be a magnet with a N and S pole (excitations lets you vary the streght of this magnet)

As the magnet's Poles rotate inside the wire coils of the stator makes the AC for each pass of a N-S pole. for a 2 poles making 60 revoulutions a sec makes 60 htx, wher a 4 pole only needs to turn 30 rps

If AC was used then the N-S magnet poles would be switching.

If the rotor was not turning, you have a transformer with the staor as the secondary (output),

If the primary side of the transformer starts to rotate....you get a something...?

 

[voltages]

Thanks for the info u guys, hehehe. Electrical field is really indeed has lot of things u need to understand. As a matter of fact i have so many questions that i tend to look for answers, hehehe.

Thanks guys!!

 

[catserveng]

If you want a good read on generator theory, here is a link to a company we have used for training. Actually found our technicians liked this class better than the ones from the factory, and the end results better too as far as the guys having a real knowledge of the theory.

I have been to a generator class put on by these guys, and feel it's about the best one I ever went to.

http://www.hpcnet.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=eG201&Category_Code=

This etraining class is new, but is right in line with the course book from his instructor lead classes.

 

[byrdj]

I agree with catserveng. HP is an excellent instrutor.