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Old 12-03-2012, 06:42 PM   #16
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Peerless 5 HP 3 phase motor hums...


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Originally Posted by Poodlehead View Post
I thought all 3 phases were the same, about 110-125, with respect to ground. I was measuring 230-247 between phases, which is double a one phase measurement. I guess I'm a bit 'corn-fused'...
It's not possible to have a 3-phase system with 240V between phases and 120V from each phase to ground. The voltages don't add up. If you have 120V from each phase to ground, then you have a wye system, and the voltage between phases must be 208V. This is a common type of system, but isn't what you have. If you have 240V between phases, then you have a delta system and the voltage from phase to ground is either 120-208-120, or 240-0-240, depending on whether it's a high-leg delta service (most common, and what you have) or a corner-grounded delta (very uncommon).

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Old 12-03-2012, 07:00 PM   #17
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Peerless 5 HP 3 phase motor hums...


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Originally Posted by mpoulton View Post
If you had check each phase to ground with a light bulb, you would get a burned out light bulb! One of those phases is 208V to ground.

To get around this problem your test lamp should be
rated for 240v !
Either use a 240v lamp or use 2 x 120v lamps in series.

This is common practice in Australia,
but we use 2 x 240v lamps
incase we find 415v.

But because you use 120v,
then you would use 2 x 120v lamps
incase you find 220v.

Test lamps are a simple but very effective tester.
You can put them in a clear acyrilic tube
for protection.
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Old 12-03-2012, 07:33 PM   #18
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Peerless 5 HP 3 phase motor hums...


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It's not possible to have a 3-phase system with 240V between phases and 120V from each phase to ground. The voltages don't add up.
Now that the 3rd phase is connected again, I will re-measure.

Quote:
Originally Posted by mpoulton View Post
If you have 120V from each phase to ground, then you have a wye system, and the voltage between phases must be 208V. This is a common type of system...
I don't understand why each phase can be 120V to gnd, but going across two phases is only 208V and not 240V. Maybe the addition that I'm missing has to do with rms, avg, or peak readings?

I'm pretty sure that I have a 4 wire wye config and maybe my high readings earlier of 247V across two phases was due to one phase not connected? I will re-measure tomorrow because I would like to understand this better.

Thank you for your help AND patience!

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Originally Posted by dmxtothemax View Post
To get around this problem your test lamp should be... ...a 240v lamp or use 2 x 120v lamps in series... You can put them in a clear acyrilic tube
for protection.
Great ideas! Thanks much!
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Old 12-04-2012, 12:32 AM   #19
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Peerless 5 HP 3 phase motor hums...


I will give you a quick drawing what the typical 3 phase 4 conductor service look like so you will get the idea.

The upper one is WYE connection and the lower one is DELTA connection with wild leg.



Hope that clear up on that one but beware the drawing letters are not the excat the same as it supposed to be but the key item is you find the netural on B-C then you know the A is wild leg per picture in real life normally B phase is wild leg if connected to the netural.

Merci,
Marc
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Old 12-04-2012, 02:15 AM   #20
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Peerless 5 HP 3 phase motor hums...


If you had a 4-wire wye service and phase-to-phase voltage of 247V, then the phase to ground voltage would be 143V (by definition - there's no room for variation on this) and that's way too high for things to work normally. If you had 247V between two phases, you must have a high leg delta service.
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Old 12-06-2012, 10:12 PM   #21
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Folks, Sorry for the delay, but the same phase was out the following day. The utility people were out and looks like they fixed it for good this time. A brace had come loose and they could see where it arced on the wire causing the same fuse to blow again.

Now when I measure voltages, they are much different and as you guys explained before. From gnd to each phase I measured: 212-123-123 and 245 between all phases which matches the DELTA config shown before.

I still have a hard time understanding the "212" reading because it doesn't seem like that number adds up right, but I guess I should just remember the "by definition" comment and be happy...

Thanks again for the excellent lesson!
Pat
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Old 12-06-2012, 11:36 PM   #22
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Peerless 5 HP 3 phase motor hums...


If the two 120 volt legs of a 4 wire delta system are actually 120, the high leg would be (theoretically) 207.84.

If they're both 123, the theoretical high leg voltage would be 213.036.

Rob
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Old 12-07-2012, 06:33 AM   #23
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If the two 120 volt legs of a 4 wire delta system are actually 120, the high leg would be (theoretically) 207.84.

If they're both 123, the theoretical high leg voltage would be 213.036.

Rob
Rob, would you please tell me the math that you are using to come up with those numbers and then I think I'll be good...

Thanks in advance,
Pat
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Old 12-07-2012, 08:08 PM   #24
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Peerless 5 HP 3 phase motor hums...


Actually, the math is pretty simple.

The square root of 3 (1.732) is a very common figure for 3 systems.

If the system is delta, multiply the low leg voltage by 1.732 and you'll have the high leg voltage.

If it's a wye, multiply the phase to neutral voltage by 1.732 and you'll have the phase to phase voltage.

These figures apply only in theory, in real life they're only close. Factors like phase angle, power factor, and a bunch of other variables skew the results a bit.

Rob
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Old 12-07-2012, 09:32 PM   #25
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Peerless 5 HP 3 phase motor hums...


This may make more sense.



The phases are 120 degrees out from each other, not 180 degrees as 220V single phase is. This is how you can have a rotating magnetic field without a capacitor for 3-phase because it's an inherent trait.
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Old 12-08-2012, 08:09 AM   #26
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Peerless 5 HP 3 phase motor hums...


You guys rock! Thank you very much for the lesson AND patience! Pat
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Old 12-09-2012, 06:59 PM   #27
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Peerless 5 HP 3 phase motor hums...


Thats a nice looking old motor !
Built when they really cared about quality !
Look after it, and your children could be using it too.
GOOD STUFF !

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