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Anonymous Poster

Synchronous Motor Coil Connection Reversal

11/19/2010 12:20 PM

We've a large synchronous motor (6000hp) that's 'Y' or star connected with the motor connections and neutral accessible. It was suggested we open the neutral and use these as the new motor connections and connect the old motor connections together to form the new neutral. The thought is that the part of the coil that was at 0V (neutral) will now see phase voltage and the part of the coil that was at phase voltage now sees 0V. Supposedly this will increase coil longevity. Anyone heard of this? I can find little online.
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North of 60
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#1

Re: Synchronous motor coil connection reversal

11/19/2010 1:13 PM

The reason AC current is called "alternating current" is because current flow reverses and flows the other way 120 times a second. Given that, what will the suggested course of action achieve? Someone is pulling your leg, or perhaps both legs, me thinks.

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Anonymous Poster
#2
In reply to #1

Re: Synchronous motor coil connection reversal

11/19/2010 2:15 PM

In a theoretical motor there should be zero voltage at the neutral point. If there's no voltage at the neutral, then no current flow. Wouldn't there be a voltage gradient across the coil? The coils connected directly to the neutral will not see the same line to neutral voltage as coils directly connected to the line side. If coil deterioration is only near the line side of the coil, then reversing connection would slow down this deterioration, no?
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JRaef
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#3

Re: Synchronous motor coil connection reversal

11/19/2010 2:24 PM

Hmmm... smells like hogwash to me too, but maybe not complete hogwash.

Is there a VFD involved? When connected to a VFD, there is a phenomenon called "Standing Wave" generation as a result of the high speed transistor switching going on and its interaction with cable capacitance. The result is high voltage pulses at up to 2.5x the peak line voltage, often much much higher than voltage impulse rating of standard magnet wire. The damaging effect is usually seen in the "first turns" of the motor winding because once it hits the windings, the high voltage pulse gets attenuated by the motor inductance itself, but the first turn is "raw" with regards to exposure to those pulses. The pulses punch through the insulation and cause turn-to-turn shorts, usually at a microscopic level at first, but the damage is incremental. So in theory, "reversing" the winding direction might delay the onset of total failure caused by this because you are moving the "fresh" insulation to the leading edge in essence. So maybe they are thinking it's kind of like rotating your tires; it doesn't stop the wear and tear but it extends the interval time between total failure.

However if there is no inverter running this motor, it's back to being hogwash. I was just stretching to see if there might be some reasonable explanation as to why someone might think this.

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North of 60
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#4
In reply to #3

Re: Synchronous motor coil connection reversal

11/19/2010 3:12 PM

Interesting... my thoughts on large synchronus motors started via an inverter are that it must be done slowy and over a longer period of time, in comparrison to a standard induction motor.

Would not the V/Hz ratio involved in low frequency starting, as required by this type of motor, limit the problem of what you speak?

OP... We need more data on how this motor is started and what is the insulation class of the motor?

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Anonymous Poster
#5
In reply to #4

Re: Synchronous motor coil connection reversal

11/19/2010 3:18 PM

No vfd's are involved. It's started across the line - 13.8kV. Is somewhat stressful on the coils...
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North of 60
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#6
In reply to #5

Re: Synchronous motor coil connection reversal

11/19/2010 3:29 PM

So there is no pony motor or other mechanical means of coming up to sync speed? Do you short out the field windings and start it as, basically, an induction motor, then unshort the field winding and inject the DC excitation voltage as the rotor nears sync speed?

If so... I dont see how the proposed rewire of the motor will do anything for you. You sure it is a synchronous motor that you have?

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Anonymous Poster
#8
In reply to #6

Re: Synchronous motor coil connection reversal

11/19/2010 4:20 PM

It's started as an induction motor using amortisseur (sp?) windings.
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ChaoticIntellect
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#7
In reply to #5

Re: Synchronous motor coil connection reversal

11/19/2010 3:32 PM

an across the line start on a synchronous 6000hp motor?! wow... I'd think "stressful" might be an understatement.

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JRaef
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#9

Re: Synchronous Motor Coil Connection Reversal

11/19/2010 4:33 PM

Stessful, yes, but swapping the lead ends will not make any difference in this case. The voltage seen by any winding in a Y configuration is the voltage from Phase to Neutral, which is the same at either end. As said early on, "it's AC"...

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Yuri B.
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#12
In reply to #9

Re: Synchronous Motor Coil Connection Reversal

11/20/2010 2:51 AM

swapping the lead ends will not make any difference in this case. The voltage seen by any winding in a Y configuration is the voltage from Phase to Neutral, which is the same at either end.

At the central of Y there is 0 potential in relation to the body of the machine - if the windings were well balanced. Ends of Y have seen the full voltage applied all the time the motor run. Now the ends which have seen no or very little potential level in relation to the body - the earth - and whose insulation might be less worn - would take upon themselves the full voltage.

It may be, however, I have missed something in this discussion.
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bsekhar
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#10

Re: Synchronous Motor Coil Connection Reversal

11/19/2010 10:40 PM

THIS IS ABSOLUTELY BULLSHIT AND RUBBISH IDEA.

THIS WILL BUGGER UP THE ENTIRE MACHINE WITHIN 1HR.

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North of 60
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#11
In reply to #10

Re: Synchronous Motor Coil Connection Reversal

11/19/2010 11:58 PM

LOL... don't hold back. Tell us what you really think.

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W. T. McGonagall
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#13
In reply to #10

Re: Synchronous Motor Coil Connection Reversal

11/21/2010 9:00 AM

I'd give it 10 seconds!

Good honest answer though

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