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Old 09-28-2012, 01:23 PM   #16
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AC 220 Shut off?


It's not good for them to be sure but unless they were left this way for long periods it shouldn't burn them out. They'll try and work but they just can't. Your HVAC units most likely just have seen their day. The contactor and relay idea is probably the best for protection and most cost efficient as well. Any sparky should be able to do you one up no problem.

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Old 09-28-2012, 02:25 PM   #17
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AC 220 Shut off?


This doesn't make sense. A device that uses two phases of a 3-phase system is not a "2-phase" device. It is single phase. Upon a moment of reflection, it will be clear that losing one leg on a 2-pole circuit doesn't result in partial power, it results in NO power AT ALL. Without both phases hot, there is no circuit and no current can flow. Are you absolutely sure these are not 3-phase units? The symptoms you describe are exactly what's expected of a 3-phase unit with a lost phase.
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Old 09-28-2012, 02:26 PM   #18
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Quote:
Originally Posted by andrew79 View Post
It's not good for them to be sure but unless they were left this way for long periods it shouldn't burn them out. They'll try and work but they just can't. Your HVAC units most likely just have seen their day. The contactor and relay idea is probably the best for protection and most cost efficient as well. Any sparky should be able to do you one up no problem.
What are you thinking is going on here electrically?
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Old 09-28-2012, 02:38 PM   #19
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AC 220 Shut off?


I've worked with three phase power for quite a few years and I have never heard a single phase motor hum when it looses one leg on a three phase system. Three phase motors yes.
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Old 09-28-2012, 03:03 PM   #20
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AC 220 Shut off?


I am not an electrician.
I dont profess to be.
I am a business owner and I had a question.

I can tell you that the fans were moving - but not normally - instead very slowly.
I can tell you that when the power was restored fully, all of them worked normally.
I can tell you that the breaker is a DOUBLE breaker, not a triple to the condensor units, so I assume that means it cannot be 3 phase ...

I did NOT measure the voltage across the service disconnect, so I cannot attest to voltage.
I assume that if I checked each leg of the service disconnect to earth, I would have found one out and one hot.
I gather from the forum comments that a measurement across the 2 should have been zero.

I will take the comments from this forum and show them to my electrician when I have a meeting with him next week, and we'll go from there.

Thanks to all for the comments.

Scott
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Old 09-28-2012, 04:23 PM   #21
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Quote:
Originally Posted by jrclen
I've worked with three phase power for quite a few years and I have never heard a single phase motor hum when it looses one leg on a three phase system. Three phase motors yes.
Your basically giving a 208 motor 120. It's not going to like that. Depending on the motor if its got a big enough start up you likely won't hear a thing. For a simple fan it'll hum. Next time you come across a 208/240 forced air furnace turn off one line and see what happens. I heard the last one I worked on across a room through a closed door. I'm sure it doesn't happen all the time but its like theres enough of a magnetic field there for it to try and wind up but it just can't get the initial surge to get over the hump.
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Old 09-28-2012, 05:06 PM   #22
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AC 220 Shut off?


Andrew, it's not at all like giving a 208V motor 120V, current would still flow in that case. It's like giving a 208V motor nothing. But you bring up a very good point,

Escotmm, perhaps that second hot leg (the one with no power) was grounded on a wye connected 3 phase system. (Through the 120V control circuit for instance). Let us know what your electrician finds when he makes voltage readings the next time it happens.

I am not saying you didn't see what you say you saw. We are all trying to help.
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Old 09-28-2012, 05:55 PM   #23
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Quote:
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Your basically giving a 208 motor 120.
NO, you are not. There is NO neutral to reference 120 volts. So as the others are saying, 208 or 240v, minus one leg = 0V.
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Old 09-28-2012, 05:56 PM   #24
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Quote:
Originally Posted by escottm View Post
I am not an electrician.
I dont profess to be.
I am a business owner and I had a question.

I can tell you that the fans were moving - but not normally - instead very slowly.
I can tell you that when the power was restored fully, all of them worked normally.
I can tell you that the breaker is a DOUBLE breaker, not a triple to the condensor units, so I assume that means it cannot be 3 phase ...

I did NOT measure the voltage across the service disconnect, so I cannot attest to voltage.
I assume that if I checked each leg of the service disconnect to earth, I would have found one out and one hot.
I gather from the forum comments that a measurement across the 2 should have been zero.

I will take the comments from this forum and show them to my electrician when I have a meeting with him next week, and we'll go from there.

Thanks to all for the comments.

Scott
If all this is true then something VERY weird is going on. Please do post back and let us know what you guy find.
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Old 09-28-2012, 06:15 PM   #25
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AC 220 Shut off?


This could be a corner-grounded delta system. That's the only way you get a 2-pole breaker for a 3-phase load.
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Old 09-29-2012, 09:39 AM   #26
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Quote:
Originally Posted by Speedy Petey View Post
NO, you are not. There is NO neutral to reference 120 volts. So as the others are saying, 208 or 240v, minus one leg = 0V.
How about this. Phase A and phase B are connected to the motor windings. Phase B is lost. There is a 120V load connected to phase B. A light bulb or control transformer. Couldn't current flow from phase A through the windings, through the 120 load to the neutral point? Just an idea.
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Old 09-29-2012, 03:59 PM   #27
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Quote:
Originally Posted by jrclen View Post
How about this. Phase A and phase B are connected to the motor windings. Phase B is lost. There is a 120V load connected to phase B. A light bulb or control transformer. Couldn't current flow from phase A through the windings, through the 120 load to the neutral point? Just an idea.
Nice. Now get this: let's say there are three single-phase units, #1 wired phase A to phase B, #2 B-C, #3 A-C. Phase B of the utility service goes out. Unit #3 runs normally, but now units #1 and #2 are in series across phases A and C! Each unit gets approximately 104V and behaves exactly as the OP describes. And this will happen even if the units are on different circuits.

Mystery solved, I believe. We were wrong - phase loss protection matters on a 3-phase service even if there is no 3-phase equipment!
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Old 09-29-2012, 08:57 PM   #28
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Quote:
Originally Posted by mpoulton View Post
Nice. Now get this: let's say there are three single-phase units, #1 wired phase A to phase B, #2 B-C, #3 A-C. Phase B of the utility service goes out. Unit #3 runs normally, but now units #1 and #2 are in series across phases A and C! Each unit gets approximately 104V and behaves exactly as the OP describes. And this will happen even if the units are on different circuits.

Mystery solved, I believe. We were wrong - phase loss protection matters on a 3-phase service even if there is no 3-phase equipment!
Excellent. This makes perfect sense.
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Old 09-30-2012, 02:37 AM   #29
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AC 220 Shut off?


That's exactly it. Your not loosing a leg branch wise your loosing it service wise. The motor winding end up in series with the circuit and backfeed the panels lost leg. As long as there's a neutral running back to the panel somewhere on that lost leg current will flow. Or in a case like just posted by m poulton it will also flow.
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Last edited by andrew79; 09-30-2012 at 02:41 AM.
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Old 10-01-2012, 09:42 PM   #30
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AC 220 Shut off?


Hey guys,
Really interesting stuff.
I am still waiting for the electrician, he'll be by in a couple more days.
But I would like to make s comment on the recent threads.
There are 6 220v HVAC units in the panel.
And there are several more 220v devices also.

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