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Old September 14th, 2008, 11:23 PM   #1
Marc
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Single phase motor wired to 3 phase overload

HI all,

Can anyone explain to me the correct way to wire a single phase 120 volt AC motor to a three phase solid state overload/contactor?

I have seen L1 through contact 1 to the motor with neutral to contact 2 looped back throught contact 3 out to the motor.

I have also seen L1 through contact 1 looped through contact 2 out to the motor and neutral through contact 3 out to the motor.

Also I have seen L1 looped from contact 1 through contact 2 and contact 3 out to the motor with a direct connection to neutral (not through contactor/overload).

Having seen all of these in the field, I am not sure which is correct and why.

Any help would be appreciated.

THX

Marc
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Old September 14th, 2008, 11:29 PM   #2
leitmotif
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Seems kinda expensive to me. Why not get a current sensor relay from Grainger for about 60 to 70? Be a bit simpler.
Dan Bentler
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Old September 14th, 2008, 11:32 PM   #3
Marc
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Hi Dan

Customer requirement.

Besides, I deal with stuff in the field, and have seen all previously mentioned scenarios and am wondering which is correct.

Thanks for the response.

Marc
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Old September 15th, 2008, 07:43 AM   #4
Tom Jenkins
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I usually run L1 through contact 1 looped through contact 2 out to the motor and neutral through contact 3 out to the motor. Any f these will work, and I'm not sure there is any advantage of one over the other, although I like to run the neutral through the overload.
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Old September 15th, 2008, 08:03 AM   #5
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Quote:
Originally Posted by Tom Jenkins
I usually run L1 through contact 1 looped through contact 2 out to the motor and neutral through contact 3 out to the motor. Any f these will work, and I'm not sure there is any advantage of one over the other, although I like to run the neutral through the overload.
Why do you like to run the neutral through the overload? Just curious, because I have not seen that before.
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Old September 15th, 2008, 08:27 AM   #6
TConnolly
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The only reason for looping back is if the OL relay is an electronic OL with phase loss sensing. Otherwise, looping back doesn't gain you anything other than extra work. Run L1 through contact 1, N throught contact 3, and be done with it. Contact 2 is not going to get emotional about being left out.
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Old September 15th, 2008, 09:00 AM   #7
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OK, easy bit first, we know you have to use all three circuits through the overload otherwise it would trip on single phasing, this applies to both electronic and thermal overloads. If it is an overload directly joined to a contactor then you would want to take the neutral through the combination otherwise the neutral wouldn't be switched which leads to potential danger because Neutrals aren't always at low voltage to earth and I have seen Lines and Neutrals crossed during installation. And my definition of single phase is different to American single phase so that Neutral may not be a Neutral at all. A 230V panel in the UK would be Line to Neutral but if it was moved to the States it may be connected Line to Line for 220V. So I reckon that passing the phase and Neutral through the overload is the smart thing to do. Though either could be used for the second loop through I would go with the Line as, if the Neutral faulted to earth you get greater breaking capacity if the Line goes through the overload twice. So after that meanering ramble I am going for Line looped through twice with the Neutral taking up the third connection.

Bryan
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Old September 15th, 2008, 09:52 AM   #8
Tom Jenkins
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Quote:
Originally Posted by Binaural
Why do you like to run the neutral through the overload? Just curious, because I have not seen that before.
See BryanG's answer.
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Old September 15th, 2008, 11:37 AM   #9
Marc
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Thanks for the replies,


What about the addage that you should never break the neutral? I only ask because someone else mentioned this as a no no and that the line should loop through all three and the neutral should be unswitched.

Not trying to be a pain, just looking for a better understanding.

Marc
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Old September 15th, 2008, 11:48 AM   #10
leitmotif
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Quote:
Originally Posted by Marc
Thanks for the replies,


What about the addage that you should never break the neutral? I only ask because someone else mentioned this as a no no and that the line should loop through all three and the neutral should be unswitched.

Not trying to be a pain, just looking for a better understanding.
Marc
Breaking the neutral is OK
IF
all conductors are broken simultaneously. Breaking the neut is (was at one time in past ??) required on the circuit breakers supplying gasoline pumps.
The other possibility is where you have a generator isolation (anti parallel) switch. Some utilities may require you to break the neutral to ensure no part of the generator circuitry can connect to their lines.

Dan Bentler

Last edited by leitmotif; September 15th, 2008 at 11:58 AM.
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Old September 15th, 2008, 01:21 PM   #11
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Never break the neutral may be a good rule of thumb when neutral is bonded to ground for control circuits.

For motor circuits, you want complete disconnection for a variety of reasons, not the least of which, is that many of your applications for this standard circuit may be 220-240vac single phase motors with a center tapped neutral which is grounded.

You then certainly want to disconnect both motor power legs since one of them is no longer necessarily at ground potential.

I question BrayanG statement about thermal overloads. If only two heaters are warmed below their current trip rating, why would the overload mechanism open? How do thermal/mechanical overload relays detect single phasing?

It has been my belief, (not necessarily fact) that this only holds for a single phasing three phase motor. The overload device will detect it, not because of missing current on one pole, but because of excess current suddenly appearing on the other two.

If all three heaters, (two at the correct load level and one cold one) are happy, and properly matched to the trip current curve of the individual poles, won't it still work correctly?

Help me understand, please.
Thanks.
Paul
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Old September 15th, 2008, 02:04 PM   #12
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OkiPC I believe that you are correct. If a 3 phase motor is happily running and then looses a phase or two the remaining windings will rise to the occasion and try to keep up with the load. This increase in current will trip the overload device and disconnect the motor from the line.
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Old September 15th, 2008, 08:40 PM   #13
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breaking the grounded conductor isn't just a no-no...it's against the NEC.

just my 2 pesos

Last edited by stasis; September 15th, 2008 at 09:04 PM.
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Old September 16th, 2008, 02:56 AM   #14
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Loosing one phase may make the other two phases see more current in a three phase system, I think it will depend very much on the load attached to the motor. For instance I would expect the current for a fan to drop on the loss of a phase. One thing I do know is that the thermal overloads in Europe look at the current in each phase and the differential to other phases. I have taken a few to pieces to have a play, they are a bit agricultural when you get inside them with springs and bits of plastic.

Bryan
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Old September 16th, 2008, 05:14 AM   #15
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Quote:
Originally Posted by BryanG
Loosing one phase may make the other two phases see more current in a three phase system, I think it will depend very much on the load attached to the motor. For instance I would expect the current for a fan to drop on the loss of a phase. One thing I do know is that the thermal overloads in Europe look at the current in each phase and the differential to other phases. I have taken a few to pieces to have a play, they are a bit agricultural when you get inside them with springs and bits of plastic.

Bryan
Loosing one phase WILL make the other two phases see more current in a three phase system

I think it will depend very much on the load attached to the motor. For instance I would expect the current for a fan to INCREASE 3 TO 5 TIMES FULL LOAD CURRENT on the loss of a phase.
As the motor is about to stall.

--------------------------------------------------------------------------




With thermal Bimetal overloads, If a motors Overload is set to the motors full load current setting and loses a phase, a standard 3 phase overload ( as per the brown bar only ) It would trip in about 1 minute, where as if it had a single phase trip capability ( as per the brown and the purple bar ) it would trip in about 15 seconds.


Last edited by Gil47; September 16th, 2008 at 05:29 AM.
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