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Anonymous Poster

AC Generator Rotor

06/12/2010 9:24 AM

hi gents

can some one please help me?

its caterpillar generator Model#SR4 400kva, 460v, 60hz. the problem started six months ago when the rotating rectifier was damaged, after about 2months there was no output from the generator and the problem was the exciter stator coil was open, again after about 2months there was no output and the problem this time was the voltage suppressor fitted on the heat sink was melted.

i checked all the windings and they were ok, megger tested and they were ok, some thing i dont understand which is when i checked the resistance of the main generator rotor winding, i found it 1.4 ohms, but when i turn the engine by hand the resistance was going up and down, it reach up to 56 ohms and down again and the meter reading goes to negative sometimes. when i took out the rotor and checked the coil it was ok but still the resistance going up and down when i turn the rotor.

i checked another generator rotor and it was doing the same readings.

my question is: when i turn the rotor, why the resistance is changing and why some times i get negative readings on the meter? is it normal?

pls advice

thanks
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mjb1962853
Guru
Engineering Fields - Electrical Engineering - Analog and Digital Circuit Design Engineering Fields - Electromechanical Engineering - Transformers, Motors & Drives, EM Launchers Engineering Fields - Engineering Physics - Applied Electrical, Optical, and Mechanical

Join Date: Jan 2008
Location: NY
Posts: 1207
Good Answers: 119
#1

Re: AC generator rotor

06/12/2010 9:47 AM

Residual flux and turning rotor by hand generates small voltages which ohm-meter could see as fluctuating and/or negative resistance.
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redfred
Guru
Engineering Fields - Electrical Engineering - Been there, done that. Engineering Fields - Control Engineering - New Member

Join Date: Dec 2008
Location: Long Island NY
Posts: 15600
Good Answers: 981
#2

Re: AC generator rotor

06/12/2010 9:50 AM

Well you did say that this is a generator. By spinning the rotor you are generating electricity that is driving a very slow frequency alternating current into your resistance meter.

So yes, this is normal.

Since you've had multiple problems with this generator, I would seriously consider replacing the circuit breakers on the generator. It may not be the root of these multiple failures, but it certainly is not protecting the generator from electrical failures.

__________________
 "Don't disturb my circles." translation of Archimedes last words
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Anonymous Poster
#3

Re: AC generator rotor

06/12/2010 9:53 AM

Oh, this was in the 70's when i was a young engineer with not much experience, and not too many people who knew more . The customer's chief engineer thought he was better than Edison and Tesla combined, and shot me down when i suggested timidly a Caterpillar Generator (which i had read about). He went and did what he wanted. Now of course, one would use rotor windings in by hand rotating .

Poetic justice prevailed and i became his boss in 2001 when Caterpillar bought both our companies. Pity was that i realised i had no animosity left in me anymore . He did remember the incident and casually mentioned that all problem with the rotor windings..no hint of apology mind you Well, c'est la vie !

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Anonymous Poster
#4
In reply to #3

Re: AC generator rotor

06/12/2010 10:19 AM

Hello;

Are you the OP? If so, I'd say: Wow!, after 40 years you keep a very clear picture of details, that is amazing, specially for someone with no animosity left in him.

But what amazes me the most, is that after all this time, you ask about the effects of the so called BEMF, as if you had no idea ! Are you still trying to win an old fight? are you going to show this thread to him?.

It ain't worth, let him keep his precious ignorance, where he surely lives very confortable, and you move on to something new ! Be happy!.

Yahlasit
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Anonymous Poster
#5
In reply to #4

Re: AC generator rotor

06/12/2010 11:34 AM

hehehe..hahaha
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Anonymous Poster
#6
In reply to #5

Re: AC generator rotor

06/12/2010 12:15 PM

?????

you are copy/pasting someone's posts .. don't you have a mind of your own?
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Anonymous Poster
#7
In reply to #6

Re: AC generator rotor

06/12/2010 1:00 PM

Not intentionaly,... may be same experience for both of us,..Are you the legel advocate of other person?

There is nothing new in the world. You all are doing here also copy, pasting in different way. from books, learned knowledge, and from search results. Well, do you have threaded hole??......
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Anonymous Poster
#9
In reply to #5

Re: AC generator rotor

06/12/2010 2:04 PM

hehehe..hahaha what, you bugger !!??

You're wasting our time here! Thank God I can't be confussed with you, since the CR4 Admin can see the IP address of incomming posts.

Besides, I'm "translating" my thought from spanish to english in order to participate here, and clearly none of these is your native language.

Get the hell out of here, before you get the tons of insults you deserve !!

Yahlasit
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kvsridhar
Guru
India - Member - New Member

Join Date: Sep 2009
Location: Temporarily at Ashburn, VA
Posts: 2744
Good Answers: 164
#8
In reply to #3

Re: AC generator rotor

06/12/2010 1:13 PM

Gentlemen,

This is very much below the belt! i would request you to please take a look at this post of mine http://cr4.globalspec.com/thread/55633#comment578712 .. which had relevance to that post. It has been copied, modified and pasted here. i just happened to be trawling CR4 and i was shocked at the familiar words in this post.

This person, who hides under the anonymity of a guest, has been hounding me for quite sometime. i have no idea why. Maybe i have offended him in some way, i don't know. i have requested him to tell me how, so i can review and correct myself apologise. But he is not doing that and has made me a project for stalking.

Whatever surprise,shock and disturbance that this post has caused, though it is no fault of mine, i am sorry to all of you for the same....

i will use this opportunity again, my friend, to apologise to you with no strings attached. You don't have to reveal your identity, you don't have to tell me what i have done either... PLEAS STOP VITIATING THIS AUGUST FORUM just because you hate me.

So many peiple get so much knowledge here, please, please do not come in the way.

i humbly, sincerely, appeal to you, please let go. Forgive me.

In agony,

kvsridhar

__________________
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Anonymous Poster
#10
In reply to #3

Re: AC generator rotor

06/13/2010 7:02 AM

I knw kvs sir very well, he is a wonderful person...and this is not good u doin what u r...

saale himmat hai to samne aa ..mard jaise kar..agar mard ho to...
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