Unique Problem With Standby AC Generator

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Larry Milliken
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Unique Problem With Standby AC Generator

Post by Larry Milliken » Sun Aug 07, 2016 5:17 pm

Since this is a discussion of AC generators I was not sure in which forum to post, so here is the scenario. I have a 25KW Generac wired for closed delta. It was in use at our previous site with no issues, but since being moved by a local contractor has not been useable. What Iam going to say will sound implausible because it makes no engineering sense-but here goes.

1. Phase to Phase voltage is +/-245V
2. Each Phase to Neutral is +/-125V
3. Frequency of Phase E1 to E2 is 179 Hz
4. Frequency of Phase E1 to E3 is 59 Hz
5. Frequency of Phase E2 to E3 is 59 Hz

I am using a high quality Fluke multimeter to measure the frequency, and I do not doubt its readings since the ND-5 will not run when switched to generator, but does a very nice NO THANKYOU. The neutral tap is on the phase reading high in frequency. I know mechanical rotation determines the frequency, and it must be something obvious, but I am stuck. I have a email sitting on Generac's help desk waiting for a reply.

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Shane
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Re: Unique Problem With Standby AC Generator

Post by Shane » Sun Aug 07, 2016 9:17 pm

Well I'm hoping to get some more education on 3-phase power reading the replies here. So anything I say in this topic is coming from an amateur.

Somethin ain't hooked up right.

E1-E2 being 3x proper frequency sounds to me as if that output is getting the sum of all three phases. But you probably already figured that. I don't know enough to say exactly what that means.
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grich
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Re: Unique Problem With Standby AC Generator

Post by grich » Sun Aug 07, 2016 10:22 pm

Closed Delta with a neutral means there is a grounded center tap between two phases. You will read 120V to neutral only on the phases that are part of the center tap. The voltage from the third phase to neutral will be 208V...the "high leg". You should see this on the incoming AC service. You need an experienced hand looking at this.

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Re: Unique Problem With Standby AC Generator

Post by kcbooboo » Mon Aug 08, 2016 4:49 am

I believe it's called high-leg delta, where one of the three delta-connected windings is center-tapped, and that tap is your neutral. In that situation you'd have 120V from neutral to each end of its center-tapped winding, 240V from each corner leg to each corner leg, and 208V from the neutral to the opposite corner leg.

Standard delta service is just the three corner phases with no neutral, providing 208v phase-to-phase. Standard wye service is a neutral with three 120V legs and one neutral to which the three legs are referenced; phase-to-phase voltage is 208v.

Many generators provide three (or more) wires per phase, so they can be connected as delta, wye, or high-leg delta, with additional taps to adjust the output voltage slightly. As stated previously, something ain't wired right. You really should get 60 Hz from any point to any other point; only the phase angle should be different, and the Fluke meter won't measure that or care about it. Your 179 Hz reading on one wire must be a sum of the three phases.

Bob M.
======
In the diagram below, I presume there are additional stator wires spliced together. For example, it shows S1 and S4, but there's probably also S2 and S3 that are spliced. Sometimes there are two windings in series or parallel, depending on the output voltage requirements.
Attachments
wiring.jpg
Last edited by kcbooboo on Mon Aug 08, 2016 5:26 am, edited 1 time in total.

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Dale H. Cook
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Re: Unique Problem With Standby AC Generator

Post by Dale H. Cook » Mon Aug 08, 2016 5:02 am

kcbooboo wrote:I believe it's called high-leg delta ...
Sometimes called "wild leg" delta, and I agree that there is something wrong with that leg - as noted above, it should measure abour 208V to neutral.
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dbuckley
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Re: Unique Problem With Standby AC Generator

Post by dbuckley » Mon Aug 08, 2016 5:42 am

If a three phase genset is wired real delta, closed or otherwise, then there is no neutral. Or ground. A real, functioning ground is almost always required for safety reasons, and thus with a delta source a ground is constructed by one of a number of methods, and compatible protection has to be used.

Its damned rare to find a delta set feeding "stuff", unless the stuff is a transformer (typically delta/star), or a three phase delta wound motor, or you're in Norway. Its far more common to find star (a/k/a wye) wired sets.

I'd not proceed with this any further until it is sorted, because feeding an unbalanced load expecting a star feed from a delta source can end in big and expensive tears.

The odd frequency is third harmonic; I don't know exactly how its occurring, but mis-wiring the windings is my first thought; when star and delta windings are compared there is a phase difference between them (30 degrees I think), so I suspect this phase difference is causing a very non-sinusoidal waveform that the meter reads as 180-ish Hz. This could be because wild leg, which is a weird sort of delta, but the voltages are wrong, so maybe a miswired attempt at a wild leg delta. Be interesting to get a scope on there, preferably a battery operated scope to make sure the scope ground cant get in the way.

Edited to note: with a real resistive load on the thing, you may get a different set of readings.

Further edited to note: what supply voltage(s) do you need from this genset?

kcbooboo
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Re: Unique Problem With Standby AC Generator

Post by kcbooboo » Mon Aug 08, 2016 6:03 am

Actually here in the US, the three phase delta with center-tapped winding neutral is quite common. You get two 120V circuits for residential equipment, and three 240V circuits for three-phase equipment such as motors. Two stations near me have that exact service. One has a simple 8kw single-phase genset that powers everything but the three-phase air conditioner.

I found this wiring diagram that may also help.

Bob M.
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ACCONN007.pdf
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Larry Milliken
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Re: Unique Problem With Standby AC Generator

Post by Larry Milliken » Mon Aug 08, 2016 6:59 am

Thank you for all for the input about my generator problem. I did check the wiring and it is correct as far as the Generac manual is concerned. But I shall revisit that to be absolutely certain, and I have the circuit breaker off so there is no load whatsoever so the readings I took are "isolated" generator readings. The wiring diagram Bob M. attached is exactly what I have here. It is the bottom right schematic of a closed delta configuration. It must be a Generac drawing since the lead numbers also match. Thanks again, and I will keep this post updated as I progress.

kcbooboo
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Re: Unique Problem With Standby AC Generator

Post by kcbooboo » Mon Aug 08, 2016 7:45 am

Yes, the diagram is from a Generac manual. The PDF file I added later looks like it's Generac but it was just those two pages. Other brands have similar wiring connections but the terminals might be labeled differently. I have others but figured they'd only confuse the issue, so I stuck with Generac.

If the genset only had four wires coming out before (three corners, one neutral center-tap), I wonder what changed.

There's usually a neutral bus or lug in the generator. Sometimes this is also connected to the earth/safety ground. NEC says this connection should be present at the service entrance only, which it probably is for a commercial power source. In some situations it makes sense for a generator power source but if your neutral isn't being switched, you'll have multiple ground connections in effect. My first whole-house genset had these separated; my second genset had them connected with a jumper that the electrician removed at my request, because the neutral is NOT switched in my transfer switch. In this fashion, the only place where neutral is bonded to ground is in the main service disconnect.

Bob M.

Larry Milliken
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Re: Unique Problem With Standby AC Generator

Post by Larry Milliken » Mon Aug 08, 2016 8:30 am

This particular Generac is a 12 wire and each wire is numbered S1, 2 and so forth. There is a neutral buss bar in the generator and so labeled. It has a wire run to the transfer switch, installed by the electrician, and like you mentioned is not switched but tied to the utility neutral. I thought I might lift the neutral wire going to the transfer switch but I cannot see that making any difference. Thanks for you interest and help, Bob.

kcbooboo
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Re: Unique Problem With Standby AC Generator

Post by kcbooboo » Mon Aug 08, 2016 12:50 pm

Generac uses S1 thru S12. Cummins/Onan uses T1 thru T12. I get the feeling that all of these generators are wired to some standard numbering scheme. There are two windings per phase, that can be connected in series or parallel, depending on the desired output voltage (240V or 120V) but you should still be able to get the proper voltages coming out of it with no load. Hopefully a generator technician will be able to figure it out. Since it worked before, it should work now.

Bob M.

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kkiddkkidd
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Re: Unique Problem With Standby AC Generator

Post by kkiddkkidd » Wed Aug 10, 2016 2:55 pm

Did you check the rotation of the 3-phase wiring?
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Larry Milliken
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Re: Unique Problem With Standby AC Generator

Post by Larry Milliken » Wed Aug 10, 2016 5:09 pm

No, I did not. I will check that when I visit the site in a few days. I appreciate it, Bob.

COMMENG
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Re: Unique Problem With Standby AC Generator

Post by COMMENG » Sun Aug 14, 2016 12:00 pm

Larry Milliken wrote:Since this is a discussion of AC generators I was not sure in which forum to post, so here is the scenario. I have a 25KW Generac wired for closed delta. It was in use at our previous site with no issues, but since being moved by a local contractor has not been useable. What Iam going to say will sound implausible because it makes no engineering sense-but here goes.

1. Phase to Phase voltage is +/-245V
2. Each Phase to Neutral is +/-125V
3. Frequency of Phase E1 to E2 is 179 Hz
4. Frequency of Phase E1 to E3 is 59 Hz
5. Frequency of Phase E2 to E3 is 59 Hz

I am using a high quality Fluke multimeter to measure the frequency, and I do not doubt its readings since the ND-5 will not run when switched to generator, but does a very nice NO THANKYOU. The neutral tap is on the phase reading high in frequency. I know mechanical rotation determines the frequency, and it must be something obvious, but I am stuck. I have a email sitting on Generac's help desk waiting for a reply.
Just curious about a couple of things.

Was the ND-5 running off this same generator at the previous site, or was some other transmitter being fed by it.

Different transmitters seem to have varying sensitivities to frequency.

Makes me wonder if one of the phase windings' in this Genset was configured incorrectly from the factory. i.e, one of the phase legs might be incorrectly wired in series when it should have been wired in parallel, whereas the other two legs with the correct frequency are wired in parallel.

COMMENG

Kelly
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Re: Unique Problem With Standby AC Generator

Post by Kelly » Sun Aug 14, 2016 6:52 pm

Just curious; is your generator neutral bonded to the building neutral, or does your Xfer switch have neutral switch contacts so as the three phases switch the neutral switches between generator and load? Is your power from the utility coming in a delta or wye?
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