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Member

Join Date: Mar 2008
Posts: 8

Field flashing

04/16/2008 8:58 AM

What is field flashing in Alternator? and how it is achieved?

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Commentator

Join Date: Mar 2008
Location: South Texas - SOMETIMES
Posts: 89
Good Answers: 4
#1

Re: Field flashing

04/16/2008 10:29 AM

Field flashing is used when the exciter poles of generator lose their magnetism and will not allow gen set to build voltage.. This sometimes happens if a machine has not been run for long periods of time. To rectify this problem we use a 12vdc battery to repolarize the exciter poles by applying voltage to the exciter circuit directly. The gen-set must be offline and proper polarity must be observed.. The gen-set or voltage regulator manufacturer usually has procedure for this in manual..

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Power-User

Join Date: Dec 2006
Posts: 409
Good Answers: 5
#4
In reply to #1

Re: Field flashing

04/17/2008 3:08 AM

Nothing I have tried on Yamaha 500va alternator genset has work- this has 3 pairs of connections- 2 = ac 240v out; 2 = 12v dc out; 2 = connected to motor run 440v ac cap(new)- the rotor has 2 windings connected at 1 end & the other end has a diode across- there is no connection to energise power(some have slinger rod to energise rotor)- thus rotor energise must be intrinsic as rotor turns- well it ain't energising!. Diodes test good- no s/c to earth of windings- what,s left?- shorted turns quite possibly. I have used 12v dc on the rotor(diode disc) & the 2 pole pieces magnetise, but being laminated iron don't hold- I have charged the cap to 240v ac then connected it to it's leads- discharges with spark to windings(doesn't hold charge)- the insulation has been tested with a megger- I have put 12v batt to 12v dc out conn- still no work- ain't got maker's manual- the windings have a dc resistance of a few Ohms(all 3 pairs)- seems too low to me indicating shorted turns- I really don't know but would like to know- any help welcome.

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Guru

Join Date: Mar 2008
Location: Atlanta, Georgia
Posts: 756
Good Answers: 12
#9
In reply to #4

Re: Field flashing

04/18/2008 1:02 AM

I have just run a search for a diagram for that gen-set. I imagine others here who like to be helpful have too.

There does not appear to be anything on the net for free but you certainly can buy the manual for that unit. Flashing it, as has been outlined here is simple once you know which terminals are the proper winding ends. The manual will of course tell you that.

j.

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Anonymous Poster
#2

Re: Field flashing

04/16/2008 11:34 PM

Field flashing is a generator process, have never heard it being used on an alternator, but could be.

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Anonymous Poster
#3
In reply to #2

Re: Field flashing

04/17/2008 12:01 AM

guest; the field in the alternator is rationing. pac

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Anonymous Poster
#5

Re: Field flashing

04/17/2008 7:08 AM

On a shunt fed regulator or static exciter, when there is not enough residual magnitism on the alternator/generator to build up the voltage, an external dc source is used to "flash the field" to provide enough ac voltage off the terminal of the alternator/generator so the voltage regulator or static exciter can take over.

Basler Electric has a very good paper describing this. WWW.basler.com

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Anonymous Poster
#10
In reply to #5

Re: Field flashing

06/07/2009 2:14 PM

Where in the web site do i find this?

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Anonymous Poster
#6

Re: Field flashing

04/17/2008 9:58 AM

I have heard that a large horse shoe magnet placed in close proximity can stimulate the field however i have never observed this in person.

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Anonymous Poster
#7

Re: Field flashing

04/17/2008 2:04 PM

Field flashing is a process where during startup of large generators, there is not enough residual magnetism to get the generator to its terminal voltage.

In large power plants, the 125VDC battery supply ( or whatever voltage used) is applied to the exciter field( with limiting resistors , off course) to start the generation process.

Once the generator terminal voltage reaches a level where it can be used to get the voltage regulator going, the battery DC field flash is dropped off and the generator excited to its terminal voltage.

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Commentator

Join Date: Mar 2007
Location: Tex us
Posts: 57
Good Answers: 1
#8

Re: Field flashing

04/17/2008 3:18 PM

I own a 15 kva three phase Onan generator. It had the same problem. I obtained the correct manual, followed the procedure for troubleshooting. After checking the field exciter voltage at the regulator, the manual described how to "flash" the field. After "flashing" the field, the output came right up and operated fine.

You've just reminded me that I need to start & run & the unit under load periodically so that I don't have to repeat the procedure.

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