Alternator Exciter Field Polarity

F1 is pos I think....

Check the polarity of the flyback diode, if there is one. If not, then determine if there's any circuitry beyond the leads, if none then it doesn't really matter since swapping the field leads is equivalent to rotating the shaft 180º on a 2 pole machine.

If it is self excited and the connections are reversed, it won't build up voltage on start up. The reason is that if the polarity is reversed, the residual magnetism will buck the applied field current causing it to reduce instead of build up.

http://newmachineparts.blogspot.com/2013/11/why-self-excited-shunt-generator-will.html

Sorry, that's true for a self-excited shunt DC generator where the field brushes are directly connected across the output terminals; that's not the topology of an AC alternator.

Right, I was thinking of a DC exciter driving the alternator field.

I cannot see how it will make a difference, apologies if I am wrong.....but I simply don't understand the worry.....

No forget it Andy, your response is late. OP is up to early response only. I'd like to add but, hell no.

...

If we assume, just for clarity a 3 phase stator, that is 3 fields and the rotor receives DC, variable value to handle variable loads, either via slip rings, or most probably via two cables sown the middle of the drive shaft, going to a single coil giving north and south magnetism.

If these exciter field connections are reversed, then the alternator output will be effectively "reversed" in comparison to its own output previously, but who the hell will ever notice that!!

It reverses every 1/50th of a second or 1/60 of a second either way, that is either 50 or 60 Hertz!! That the rotor shaft is simply turned 180° different to its previous orientation......

It will not make the slightest difference to anything else.......

Now if there were TWO alternators, built on the same shaft, that had to be "in sync" with each other, there would be a worry, but even then monitor the same phase on both using say a scope and see if the phasing is the same before connecting them to each other!!!

But how often are two alternators on the same shaft?????

Dear Andy,

Thanks for quick response, but still my question is not answer.

I would like to know if you have lost marking on exciter field F1,F2 (+ ,-)how to remark again, is there any way or procedure to check & confirm the polarity??

Try reading my posts better, what I was saying is "IT DOES NOT MATTER ABOUT POLARITY!!"

Try it and see!!

How about an answer from you now?

Use the residual magnetisation as an indication.

Run the machine un-loaded without excitation and isolated from any load.

Note the output voltage.

Add a small excitation current and see if the output voltage increases or decreases.

If it increases, you are in the original polarity. Mark it an test its validity with caution just in case this didn't work. (Nothing is 100% certain...)

While you are at it. Mark all the other field and phase terminals you may have in the plant at the next safe occasion to avoid this in the future...