Incorrect phase voltage on 3 phase circuit?

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Old 01-20-09, 08:00 AM
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Incorrect phase voltage on 3 phase circuit?

I have a pair of 3-phase circulator pump motors. When one died I checked for voltage at the disconnect panel and found that phases 1 & 3 are both reading 120 volts but phase 2 on both disconnects is reading 210 volts. Is this normal? I always thought that all three would be at 120 volts.

The remaining motor seems to be running fine with no overheating.

Thanx - Rick from Farmington Hills
 
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Old 01-20-09, 08:17 AM
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How did you measure the voltage? From what point to what point using what type of meter?
 
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Old 01-20-09, 09:49 AM
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I had over-ridden the disconnect box ON-OFF safety latch and energised the box, then measured from the load-side terminals of the disconnector assembly (the terminals at the bottom of the disconnector assembly) to the conduit running into the box making sure to get good contact on the conduit. My meter is a digital VOM set on the 700v AC range.
 
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Old 01-20-09, 10:26 AM
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Measure phase-to-phase voltages. E.g., A-B, A-C, and B-C. I take it that you have 3-phase service from the utility, not a converter?
Doug
 
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Old 01-20-09, 11:45 AM
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Our 3-phase is from the utility.

Measuring as you suggest I get 244v on all three measurements (A-B, A-C and B-C). Am I right in assuming that this method shows the combined voltages of the two circuits?

So why the difference in voltage when I measured from A-(or C)-to-ground and B-to-ground? My (limited) understanding was that 3 phase power consisted of three 120v circuits 33 degrees out of phase with each other.

This may be another in a long line of "wrong things I thought I knew." LOL
 
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Old 01-20-09, 01:23 PM
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You have what's called a 3-phase corner grounded delta service. That one phase is nominal 208 volts to ground. The other 2 will be 120 volts to ground. Phase to phase is 240 volts. Wherever used, it should be identified as "orange".

It won't effect any three phase loads or straight 240 volt loads, just 120 volt loads that are using that phase.

You'll notice that, in the electrical panel for lighting/receptacle loads, that every third breaker is not used or they've fed a single phase panel off of 2 legs of the service panel.

Here's a link that can explain it better than I.http://www.phaseconverterinfo.com/ph...r_deltawye.htm
 
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Old 01-20-09, 01:34 PM
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Originally Posted by roach711 View Post
Measuring as you suggest I get 244v on all three measurements (A-B, A-C and B-C). Am I right in assuming that this method shows the combined voltages of the two circuits?
Your measurements are all normal for a type of service known as a corner grounded delta. Unlike residential single-phase services which are all basically the same, there are many different ways that three-phase power can be supplied.

So why the difference in voltage when I measured from A-(or C)-to-ground and B-to-ground? My (limited) understanding was that 3 phase power consisted of three 120v circuits 33 degrees out of phase with each other.
It's not quite that simple. Your understanding reflects a different type of three-phase service called a center-grounded wye which is becoming more common than the delta configuration you have. It's acutally 120 between phases (360 in the full circle).

This may be another in a long line of "wrong things I thought I knew." LOL
It's not really wrong, just a simple view of what can be a pretty complicated topic.
 
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Old 01-20-09, 02:24 PM
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Originally Posted by wirenut1110 View Post
You have what's called a 3-phase corner grounded delta service. That one phase is nominal 208 volts to ground. The other 2 will be 120 volts to ground. Phase to phase is 240 volts. Wherever used, it should be identified as "orange".

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Originally Posted by ibpooks View Post
Your measurements are all normal for a type of service known as a corner grounded delta.
What? Come on guys. He has 120 to ground on 2 phases and 210 (208) on the third. That is not a corner grounded delta. That is a center tapped delta (wild leg and all that) or high leg delta.

You still have 240 phase to phase.


corner ground delta has no 120 volt available and will read 240 from 2 of the phases to ground and 0 on the third.
 
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Old 01-20-09, 02:27 PM
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Good call, nap.
 
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Old 01-20-09, 02:32 PM
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Sorry for the improper terminology. Bottom line is, his readings are normal and have no effect on his motor's demise.
 
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Old 01-20-09, 02:36 PM
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Thanks all, for your input. I now see that something I thought was fairly simple is anything but.

Most important to me is that the service is OK as is and that I don't have to call in the utility guys to have a look.

I'll have to read up on these wiring methods.
 
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Old 01-20-09, 02:41 PM
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Originally Posted by ibpooks View Post
Good call, nap.

I knew you knew better.:bad poster: so you don't forget.


You too wirenut.
 
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