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Extra Exciter On Generator


Clayts

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A quick question.

 

Worked on a Generator today. Was full of oil, basically electrosoled, dryed, megged, reassembled and loaded up/output checked. All OK.

 

Why did it have an extra permanent magnet exciter externally?

 

Asked boss, he told me it acts as a sort of tacho and will give a constant voltage dependant of speed and can be used as a benchmark of types for the AVR and something to do with it retaining it's residual magnetism (being permanent magnet). Didn't really get what he was saying.

 

Asked marine electrician. He told me that it is more suited for inductive loads!!

 

Could somebody give me an actual answer, that makes sense?

 

Clayton

 

 

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Hello Clayton

 

Many generator sets are "self excited". That means that the output voltage is used to provide the excitation for the alternator.

The alternator has a finite impedance and as the load current is increased, the excitation is increased in order to keep the output voltage at a preset level. Eventually, "saturation" is achieved where the excitation can not be increased any further. At this point, the output voltage begins to drop and this will begin to reduce the excitation and further reduce the output voltage. In effect, the alternator severely current limits. Typically, self excited alternators are good for a maximum load of around 120% for a short time. This is much too low for starting electric motors on the output of the alternator unless the alternator is severely oversized.

 

One means of overcoming the current limiting action, is to use a permanent magnet generator as the excitation source. In this case, it is possible to provide full excitation independent of the output voltage. Typically, much higher short term overload currents are available and the voltage sag under overload conditions is much reduced.

PMG excitation can provide maximum short term currents as high as 300% and this can enable a much smaller genset to be used to start a motor than would be used if the genset was self excited.

 

Best regards,

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Thanks, This definately explains the setup on boat with winch motors etc. I understand the principle of what your saying, only there are holes in my knowledge!!.

 

When you say "finite impedance", are you saying a set total resistance? including capacitive/inductive reactance Z.Your saying this because you can't change the resistance therefor you have to "up" the field voltage via AVR? to increase generator output. Sorry, I'm forever confusing myself.

 

I still don't really get why you nead BOTH PM and self excited exciter. If PM exciter is not limited by over saturation , then why bother having self excited exciter?

excuse me while I un-knot my tongue.

 

Clayton

:huh:

 

 

 

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Hello clayton

 

The self excited system is cheaper than the PMG based excitation, so on low cost units, this is what is offered.

In applications where there are no motors being started, an overload current is no required and so the self excited is adequate. The PMG excitation is a better system, but as it costs more, it is commonly not used.

Applications involving motors starting are best served by the use of PMG excitation and a three phase averaging AVR. The lower cost single phase peak reading AVRs are more commonly used.

 

Best regards,

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Hi again.

 

It seriously had TWO exciters on the shaft, both of which fed into AVR

 

It had an exciter supplied from the generated voltage from alternater itself.

It had ANOTHER exciter (with permanent magnets) on the very end of shaft, the same shaft, behind end bell.

 

It had "both" a PM exciter AND a self excited exciter. On a fishing boat. No lies...serious <_<

 

Any way, I have learnt enough from you to be satisfied, I did not know about the 120% vs 300% current. This explains one aspect of my question.

 

Cheers again

 

Clayton

 

 

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Hello Clayton

 

A generator with a PMG still takes a feed from the output, this is not an additional excitation, but is the voltage feedback for the AVR. The AVR adjusts the level of excitation applied in a manner to keep the output voltage at the preset voltage. In order to do this, there must also be a connection to the output.

Low cost AVRs typically look at two phases only and are a single phase peak reading. Better AVRs look at all three phases and are a three phase averaging.

 

Best regards,

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