# Motor Contactor Coil Wiring



## Kevhuff26 (Dec 17, 2018)

Hello everyone,

I’m trying to settle a debate. There’s minimum data I can find online.
For a motor contactor or relay, you have A1 and A2 for the coil wires. I know typically A1 is for the hot (120v for example) and A2 is usually for the neutral. 

The question is can you wire the 120v to A2 and the neutral to A1 and the coil will function properly for its lasted life? I have seen it wired both ways and work both ways in many circumstances. I have heard fellow electricians say a coil is a coil no matter which way it’s wired.

Please let me know what you think and know.

Thank you!


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## Wirenuting (Sep 12, 2010)

It depends if your north or south of the equator.....
Oh wait, that only matters with toilets. 

No, a coil don’t know the difference.


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## MechanicalDVR (Dec 29, 2007)

You do realize a coil is just that, a coil of wire? 

Not a single electron flow regulator in sight....


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## dronai (Apr 11, 2011)

Other the O/L neutral passes through, it wouldn't matter


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## JRaef (Mar 23, 2009)

It would matter if it was a DC coil, but AC, no.

That said, convention is A1 is the hot, A2 is the neutral (if there is one).


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## dronai (Apr 11, 2011)

JRaef said:


> It would matter if it was a DC coil, but AC, no.
> 
> That said, convention is A1 is the hot, A2 is the neutral (if there is one).



Wouldn't a coil still create an electromagnate if it was DC ?


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## B-Nabs (Jun 4, 2014)

dronai said:


> Wouldn't a coil still create an electromagnate if it was DC ?


Yes but the polarity of the magnetic field would be reversed. 

Sent from my SM-G975W using Tapatalk


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## dronai (Apr 11, 2011)

B-Nabs said:


> Yes but the polarity of the magnetic field would be reversed.
> 
> Sent from my SM-G975W using Tapatalk


I have a DC coil at my bench here, and I reversed it, and it still pulled in, but the Pilot light didn't work. Probably led


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## dronai (Apr 11, 2011)

We had this very question, and here is the answer. Someone in the office had a brain fart, and thought it mattered.

A common question about solenoid valve coils is whether the electrical current polarity matters in a DC coil. Most coils with lead wires use the same color wire for both terminals and have no polarity markings. The answer is that polarity does not matter.


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## Kevhuff26 (Dec 17, 2018)

Thank you everyone for your replies. It seems everyone agrees that for an AC coil the polarity does not matter. It’s refreshing to read these posts. Thanks again, I feel confident now on this subject.


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## CoolWill (Jan 5, 2019)

dronai said:


> Wouldn't a coil still create an electromagnate if it was DC ?





B-Nabs said:


> Yes but the polarity of the magnetic field would be reversed.
> 
> Sent from my SM-G975W using Tapatalk


It wouldn't matter as far as magnetic force. But there is probably a flywheel diode across it, so reversing the polarity would forward bias the diode and become a short.


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## Kevhuff26 (Dec 17, 2018)

CoolWill said:


> It wouldn't matter as far as magnetic force. But there is probably a flywheel diode across it, so reversing the polarity would forward bias the diode and become a short.


Just to clarify your referencing a DC coil? Also would this be a specific type of relay and or contactor? I’m just wondering because I know someone on this thread verified a DC coil polarity change still allowed the coil to pull in.


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## Kevhuff26 (Dec 17, 2018)

dronai said:


> Other the O/L neutral passes through, it wouldn't matter


Can you please give more details. You’re referencing if there is a overload that passes through the neutral?


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## dronai (Apr 11, 2011)

CoolWill said:


> It wouldn't matter as far as magnetic force. But there is probably a flywheel diode across it, so reversing the polarity would forward bias the diode and become a short.


You can see those ! I've even installed a "snubber" 

Reverse on a diode isn't a short I don't think, it just doesn't work.


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## dronai (Apr 11, 2011)

Kevhuff26 said:


> Can you please give more details. You’re referencing if there is a overload that passes through the neutral?


If you look at an O/L on a starter, if there is an overload, it opens a normally closed contact and opens the coil


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## CoolWill (Jan 5, 2019)

dronai said:


> You can see those ! I've even installed a "snubber"
> 
> Reverse on a diode isn't a short I don't think, it just doesn't work.


The diode across a coil is reversed and doesn't conduct until the power is switched off, then it momentarily conducts and dissipates the field energy.

If You reverse the power supply polarity, the diode will now be forward biased and will conduct when thebpower is on, and is therefore a short.


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## Kevhuff26 (Dec 17, 2018)

dronai said:


> If you look at an O/L on a starter, if there is an overload, it opens a normally closed contact and opens the coil


Thank you for the clarification. I to have seen that many times on overloads.


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## dronai (Apr 11, 2011)

CoolWill said:


> The diode across a coil is reversed and doesn't conduct until the power is switched off, then it momentarily conducts and dissipates the field energy.
> 
> If You reverse the power supply polarity, the diode will now be forward biased and will conduct when thebpower is on, and is therefore a short.


Diode snubbers
When the current flowing is DC, a simple rectifier diode is often employed as a snubber. The snubber diode is wired in parallel with an inductive load (such as a relay coil or electric motor). The diode is installed so that it does not conduct under normal conditions.

Freewheeling Diode: It is called freewheeling because an inductor can be likened to a flywheel which wants to continue turning after it has been driven up to speed. A freewheeling diode is like a valve that allows that flywheel energy to be coupled to a load and do useful work.

Two different animals, you get 50% for your try :smile:


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## CoolWill (Jan 5, 2019)

dronai said:


> Diode snubbers
> When the current flowing is DC, a simple rectifier diode is often employed as a snubber. The snubber diode is wired in parallel with an inductive load (such as a relay coil or electric motor). The diode is installed so that it does not conduct under normal conditions.
> 
> Freewheeling Diode: It is called freewheeling because an inductor can be likened to a flywheel which wants to continue turning after it has been driven up to speed. A freewheeling diode is like a valve that allows that flywheel energy to be coupled to a load and do useful work.
> ...


Nah bruh. Flywheel, flyback, snubber, relay diode.... It's all the same in most texts on electricity.

It doesn't matter. The point is, if a diode is across the terminals of a coil, reversing the power supply polarity will cause a short.


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## dronai (Apr 11, 2011)

CoolWill said:


> Nah bruh. Flywheel, flyback, snubber, relay diode.... It's all the same in most texts on electricity.
> 
> It doesn't matter. The point is, if a diode is across the terminals of a coil, reversing the power supply polarity will cause a short.



Sounds good :vs_laugh::vs_cocktail:


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## micromind (Aug 11, 2007)

CoolWill said:


> if a diode is across the terminals of a coil, reversing the power supply polarity will cause a short.


Long ago, this cost me a couple of fuses to find out.........

The issue here is when power is suddenly removed from a DC coil, there will be a serious spike of voltage as the magnetic field collapses. The polarity of this spike is opposite of normal. 

If a simple diode is placed across the coil and is reverse biased during normal operation, it doesn't conduct. It will be forward biased for the spike and will short it out and protect anything electronic upstream. 

If it is installed 'backward', (or the polarity is backward) it'll be forward biased during normal operation and short out the circuit. At best, this will blow a fuse; at worst, it'll blow more expensive stuff......

This effect was discovered many years ago in automotive ignition coils. Odd as it may sound, the spark will occur when the points (who uses those anymore.....) open, not when they close. 

When they close, current is converted to magnetism in the iron core. When they open, this magnetic field will collapse, aided by the capacitor (usually called a 'condenser') connected across the points. When this happens, it'll cause a spike to develop in the secondary winding and flash the spark plug.


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## B-Nabs (Jun 4, 2014)

dronai said:


> We had this very question, and here is the answer. Someone in the office had a brain fart, and thought it mattered.
> 
> A common question about solenoid valve coils is whether the electrical current polarity matters in a DC coil. Most coils with lead wires use the same color wire for both terminals and have no polarity markings. The answer is that polarity does not matter.


I wasn't suggesting a DC solenoid or relay wouldn't work with reversed polarity, I was simply saying the magnetic field reverses polarity when the electrical polarity is reversed. In the case of a solenoid or relay, with a spring-loaded ferromagnetic core, it will pull in regardless of polarity. But if the core were a permanent magnet, it would travel in the opposite direction if the polarity of the coil were reversed. I'm speaking just about the physics of electromagnets, not the functionality of actual devices. 

Sent from my SM-G975W using Tapatalk


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