Two Phase Power For Contactor Coil?

[jhaislip]

Is anyone able to explain why someone wired this motor contactor circuit such that activating the coil is done by connecting the two leads to the following:

contactor: Allen-Bradley 69A86
lead 1: 120V AC
lead 2: 120V AC - Phase Shifted by 120 degrees

So I guess the difference of the two gives you an AC signal with amplitude of around 209V. But doesn't the contactor only need 120VAC across the coil to close? Why wouldn't they just have used 120VAC and neutral? In case more info is useful, the phase shifted 120VAC isn't used anywhere else in the circuitry although there is a chance it was used long ago with other motors/equipment which have since been removed. The 120VAC connected to lead 1 is also connected to the motor's L1 and L2 wires.

Thanks!

Josh

 

[Mike_In_Plano]

Weird. I've seen occasions to wire between a contactor and "start" winding in odd ways. The phase and voltage on the start winding varies with speed and direction and some engineers used this to switch out the start-up cap or keep track of the motor's behavior. I once used such a "phase" detection circuits to sense when a motor started in reverse due to back pressure.
Otherwise, just weird.

 

[denni89627]

Is the thing running? I don't see how. AB contactors are good, but if you're coil is 120v you're going to ruin it if you try to operate it with 208v. (For the record AB has contactors with 208v coils, so maybe that's what you have. It should be printed on the side of the contactor) As for wiring the coil and motor lead together, it may be because phase protection was not available. If either one of the phases supplying the motor dropped out the contactor would drop out and the motor would be saved. If the thing is working you must either have a 208v coil or you're incorrect about the 2 120v supply lines. A DMM would be a wise investment.

 

[jhaislip]

Thanks to both of you for the responses. It may be that the contactor has a 208v coil. I will look again for a voltage on the side, but the only number i could find last time I looked was the AB 69A86. This circuit has been operational for quite some time, so odds are that it is in fact a 208v coil. I had no idea that they existed so thank you very much for pointing that out.

Your phase protection suggestion is exactly what I was looking for. That explains why they used a 208v coil and not a 120v coil. Also, had they used a 120v coil, wouldn't they also have needed access to a neutral line (see below for why there was no neutral line available)?

I originally stated that only one of the 120v lines was being used by the motor. Actually, both of the 120v lines are being used by the motor. Sorry about that.

So with this correction and your help, I think this all makes sense now. For future readers, I will summarize below:

In this setup, the single phase motor seems to take two 120VAC lines (one of them is phase shifted by 120 degrees). The motor does not need neutral or ground. If either of the phases drops out, the contactor will open, protecting the motor. Additionally, two conductor rails are used to form a slip-ring providing continuous power to the motor. Instead of having a 120V coil and needing to install a third conductor rail for neutral, a 208V coil is used inside the contactor such that when the two 120VAC out-of-phase lines are placed across the coil, 208V results and closes the contactor. Or at least, I think you would need a neutral if you were using a 120V coil.

the last question I have is, what type of single-phase motor takes two 120VAC lines which are phase shifted by 120 degrees? Is this common? I didn't see any markings on the motor showing that they had to be phase shifted. They are just referred to as L1 and L2.

btw, I am 100% confident in there being two 120VAC lines. I've checked it with an oscilloscope and they are clearly out of phase. DMM confirms the voltages as well.

 

[aks786]

what is the source of 120V two phase supply, is it taken from three phase system by using only two phases?
if motor is not using any neutral, i think motor is rated to operate at 208V (may be 220V - which is standard). and easy way to connect motor is to connect phase to phase with resultant voltage = 120 * 1.732. contactor is there to provide under voltage protection.

 

[dlgoff]

(For the record AB has contactors with 208v coils, so maybe that's what you have. It should be printed on the side of the contactor)

This one is being advertised on ebay for $15. The rating is for 220volts and is probably what he has.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=7532794634"

 

[gsibert]

Hello, I don't know if this helps or not but I'll give it a shot. Sounds like your building power supply is 3 phase 120/208v. This is a very typical power source for commercial buildings in the US. 120/208v three phase power is derived of 3 - 120volt legs 120 degrees out of phase of each other. So you having 2 hot legs out of phase of each other is quite normal as any two legs of your system will be 120 degrees out of phase of each other. If the two phases were in phase with each other you would not have two phases to make your motor work, you would have one phase hooked up to a motor on both sides. Motor would sit there bored as hell. Kinda like a light with the light switch off. Looks like your motor is a single phase motor using two legs of your 3 phase system. Well the reason there isn't a 120volt coil because, economically it is impractacle to run a 120v control circuit for a 208volt motor. That is one more wire from source to the coil for no other purpose than to operate a tiny coil load. So most electricians would use a 208v coil for the 208v motor. On a single phase motor like yours. Motor protection, if required, is only on one leg. Because if either leg fails the motor stops. Unlike 3 phase motors, that need protection on all three legs, because if one leg breaks the motor can still run. But poorly and ruin itself quickly. On your case one hot goes to the coil and to the motor. The other goes to the coil controller and from the overload to the motor. Which is a very normal case. I hope this helps. Good luck .
Jerry