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Unbalanced voltages #210917 08/23/13 12:24 AM
Joined: Mar 2004
Posts: 947
T
twh Offline OP
Member
I learned a little about unbalanced voltages.

Two transformer banks sit on adjacent poles. One is wye and the other is delta. The voltages on the consumer side of the wye transformers were 208, 208 and 200. On the delta bank they were 225, 238 and 252 (approximately as measured over a couple days).

A new, balanced, unloaded motor on the wye system drew 3.6 amps, 3.6 amps and 5.4 amps. On the delta system, the motor drew 7 amps, 8 amps and 2 amps. The rated flc of the motor was 6 amps.

It seems the high-line voltage difference created a larger voltage difference on the delta system than the wye system. When I think about it, now, it makes sense.

My customer was on the delta bank and had a motor blowing fuses.

I'm guessing that the counter emf (back emf) of the motor almost cancelled the applied forward voltage on one phase but didn't come near high enough to limit the current on the other two phases.

The last thing I learned is that it's way better if someone else pulls a motor on a Friday night and sends it away to be rewound over the weekend. Whew!

Oh, and one more thing. It's a prick trick to put a wrong panel cover on a panel so that only two of the six screws hold the cover and to glue the last 4 screws in place so they just look like they are holding. I took out the middle two screws first and the cover slipped down and dragged the breakers out of the panel.

Work Gear for Electricians and the Trades
Re: Unbalanced voltages [Re: twh] #210919 08/23/13 09:27 AM
Joined: Jun 2004
Posts: 613
S
shortcircuit Offline
Member
Interesting. Was the Delta an ungrounded system? Lucky you were not hurt with that pricktrick:(

Re: Unbalanced voltages [Re: twh] #210920 08/23/13 02:06 PM
Joined: Mar 2004
Posts: 947
T
twh Offline OP
Member
One transformer was centre-tapped for 120/240. I think they also had a grounded arrestor on the line but disconnecting it didn't change the voltages. Those power guys are way too accustomed to high voltages. When they disconnected the load side of a transformer and tried to check voltage to ground, the meter probe sizzled. It took a couple tries before the meter died.

Re: Unbalanced voltages [Re: twh] #210921 08/23/13 04:32 PM
Joined: Jul 2004
Posts: 9,555
G
gfretwell Offline
Member
It makes me think the primary is unbalanced.
Is this wye or delta distribution?
It is not unusual to see phase unbalance on center tapped delta when they use 2 transformers but wye is usually pretty close. I think this might be GIGO.

Does the PoCo think they will fix this or are they trying to say it is "nominal"


Greg Fretwell
Re: Unbalanced voltages [Re: gfretwell] #210925 08/23/13 07:47 PM
Joined: Mar 2004
Posts: 947
T
twh Offline OP
Member
Originally Posted by gfretwell
It makes me think the primary is unbalanced.
Is this wye or delta distribution?
It is not unusual to see phase unbalance on center tapped delta when they use 2 transformers but wye is usually pretty close. I think this might be GIGO.

Does the PoCo think they will fix this or are they trying to say it is "nominal"

They re-fed the area from a different substation after a couple days of checking. It brought the voltages closer together and the problem was reduced enough that the equipment could run.

It's a closed delta configuration. I suspect they will be encouraging the customer to switch to 120/208. We discussed that at the time but I didn't want to start replacing old single phase compressors that might be border-line at 208 volts, especially when the wye configuration was off, too.

Re: Unbalanced voltages [Re: twh] #210926 08/23/13 08:06 PM
Joined: Jun 2004
Posts: 613
S
shortcircuit Offline
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Yes GIGO.


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