Belt driven alternator - Electrician Talk - Professional Electrical Contractors Forum

subelect · 04-29-2009, 08:32 PM

Last week I brought a belt-driven generator in the shop. It bolts up to the frame of a gas engine and supplies 480V 3 phase for an irrigation pivot out in the boonies where there is no utility power to speak of.
Mice had gotten into the windings and I was getting ready to tear it apart to clean it out. I meggered the phase windings coming out of the generator and it was grounded out. I could not see any damage to the windings and made a phone call to our motor shop. I did not realize that each phase was grounded to the center-tap which I had not disconnected from the frame.
I disconnected the center-tap and all windings tested sat.
What is the advantage of center-tapping to ground? Back in the Navy, we had wye connected generators and they seemed to work just fine without a ground.
What do you gain with that connection? In this application, it is not like you are going to be using 277 volts for anything; it is specifically built for irrigation pivots.
Or are the windings universal for several applications and just use a different enclosure for pivots?
Thanks,
Rick

InPhase277 · 04-29-2009, 11:58 PM

Quote:

Originally Posted by subelect

Last week I brought a belt-driven generator in the shop. It bolts up to the frame of a gas engine and supplies 480V 3 phase for an irrigation pivot out in the boonies where there is no utility power to speak of.
Mice had gotten into the windings and I was getting ready to tear it apart to clean it out. I meggered the phase windings coming out of the generator and it was grounded out. I could not see any damage to the windings and made a phone call to our motor shop. I did not realize that each phase was grounded to the center-tap which I had not disconnected from the frame.
I disconnected the center-tap and all windings tested sat.
What is the advantage of center-tapping to ground? Back in the Navy, we had wye connected generators and they seemed to work just fine without a ground.
What do you gain with that connection? In this application, it is not like you are going to be using 277 volts for anything; it is specifically built for irrigation pivots.
Or are the windings universal for several applications and just use a different enclosure for pivots?
Thanks,
Rick

I would say it is both for circuits that require a neutral and to have a point by which to ground, for faults.

micromind · 04-29-2009, 11:58 PM

A 480 volt 3 phase system would need to be grounded (by code, as well as basic safety). If the source (a generator in this case) is a wye, the best place to ground it is the center of the wye. The fact that the voltage to ground is 277 is of no consequence.

If the source is a delta connection, and there's no center tap on any winding, then one of the phases is grounded. This is called a 'grounded B' or a 'corner grounded' system. In this case, the voltage to ground is 480.

If it's a delta, and one of the phases is center tapped, then the center tap is grounded. This results in 240 from two of the phases to ground, and 416 from the other phase to ground.

If the neutral is needed, it is simply connected to the spot where the windings are grounded.

In your case, the system needs to be grounded, and the center of the wye is the best place.

Rob

subelect · 04-30-2009, 07:04 AM

Thanks for the input.
You have to really enjoy that dead mouse and debris smell on your hands all day long. It makes for a long day.
Rick