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Discussion Starter · #1 ·
We've recently replaced some 120V exhaust fans with larger fans and now the existing starters are too small and will also be replaced. The existing starters are 3 pole with only one pole being used for the motor, the other poles are used as auxiliary contacts for the pilot light and opening louvers. The wiring for all 3 fans leaves in the same conduit and there are only 2 neutrals for 3 fans, apparently two fans on opposite legs share a neutral.
A couple of questions for the ET control gurus:
Is using a starter to open only the ungrounded conductor an acceptable practice nowdays?
Is sharing a neutral ok in this situation?
I should add the original work is decades old and might have been good in its day.
Thanks!
 

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I see no issue with the MWBC. Don't know what your local requirements or specs on handle ties may be. There is also nothing wrong with only breaking the ungrounded, you could break the grounded conductor in this case too though because you would be breaking both simultaneously.

A little FYI though if you use starters with OL's with phase loss on single phase motors you have to jump through all the contacts. Usually when I do 120V motors on a starter I'll land the hot on L1, jump T1 to L2 and land the neut on L3, then the motor wires land on T2 and T3. You then have to use real aux contacts and/or a separate control relay or contactor for pilot lights and louvers.
 

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A little FYI though if you use starters with OL's with phase loss on single phase motors you have to jump through all the contacts. Usually when I do 120V motors on a starter I'll land the hot on L1, jump T1 to L2 and land the neut on L3, then the motor wires land on T2 and T3.
Why?
 

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The starter needs to see current on every pole. I've always done them this way as well
That does not make sense to me, why do all three "heaters" need to see current?

Edit: I'm not saying you guys are wrong (I guess I'm asking you show me what makes you correct), I just don't understand why all 3 OL's have to see current to work. I always thought overcurrent on 1 OL would open the switch the control voltage is going through.

Are we talking a different type of starter?
 

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Discussion Starter · #6 ·
Thanks for the replies, I've also wondered about why all the OL's were wired in series when using single phase on a three pole starter. In this case I used a Square D 2 pole with a solder pot OL on one pole only and looped the ungrounded conductor through the other pole just because of habit.
 

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because some overloads will trip if there is current on only one leg
Are you able to explain the science of why this would happen?
 

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That does not make sense to me, why do all three "heaters" need to see current?

Edit: I'm not saying you guys are wrong (I guess I'm asking you show me what makes you correct), I just don't understand why all 3 OL's have to see current to work. I always thought overcurrent on 1 OL would open the switch the control voltage is going through.

Are we talking a different type of starter?
Long story, but here it is if anyone is interested.

This was not an issue when we only had NEMA starters, the overload relays were not sensitive to a phase loss. You can always tell it is a NEMA starter or OL relay if the heaters are "elements" that you replace. If there is no way to replace them, just an adjustment knob, then it is not a NEMA relay. Some NEMA relays have an adjustment knob, but only for a small range WITHIN the range of the replaceable element. That's why the difference is really whether the elements are replaceable or not.

About 30 years ago when the Europeans started taking over our electrical industry though, they introduced the IEC style of bi-metal adjustable OL relays, and the heater elements are NOT replaceable, just adjustable. Because virtually EVERYTHING that would require a motor starter in IEC countries will be three phase, they designed all of their OL relays to be phase loss sensitive. It's a somewhat legitimate issue because if you single phase a three phase motor while it is already running, the current becomes severely unbalanced. When there is a current imbalance, the rotor of the motor will heat upo disproportionately to the amount of current flowing through the remaining phases. That means a lightly loaded motor that is being single phased can burn up WITHOUT ever tripping the OL relay. For the IEC people, their motors do NOT have any sort of "Service Factor" like NEMA motors do, they are all essentially "1.0SF" so there is NO room for error. Hence the emphasis on the added protection in the OL relay.

What they do is that the relay has a balancing (differential) spring inside, which opposes the mechanical force exerted by the bimetal heater elements. If current flows through all 3 elements, the force is balanced and the OL relay trips normally. But if there is no (or reduced) current on one phase, the spring force is not countered, so it shifts the trip point of the mechanism over to one side, meaning it will take LESS current to make it trip. So you may have it set to trip at 10A, but with one phase missing, it might trip at 9A, because if that was a 3 phase motor (with no Service Factor), that 9A was likely causing harm, but was still lower than 10A so the original value was fine.

With our NEMA OL relays, this single phasing current imbalance would be a problem too. It's just that here, we use NEMA designed motors, which under a certain size are REQUIRED to have a Service Factor of at least 1.15. That means the motor can survive additional current, at least for a while. So if you lose one phase on a fully loaded motor, it's not an issue immediately, you have time for some OTHER system ahead of it to detect the phase loss, react, and shut down the entire system.

We also use single phase power in a LOT more places than they would in Europe, so having an OL relay that works the same whether it has one, two or three heater elements in it makes life easier. For the IEC devices though, you are required to make sure that current is flowing, even if it is the same phase, through all three elements to avoid having it nuisance trip.

Class dismissed. :nerd:
 

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Great science lesson JRaef!

My experience was with (mostly SQ D) starters with replaceable OL "heaters" so that's why was ignorant to the problem you explained.

Thanks again (as usual). I'm so glad you come around. :)
 
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