Phases contacting but don't create a problem

[RubyMantis28312]

I have a 3 wire coming into a junction box - black, red and white. I have 120v from black to ground or neutral, 120v from red to ground or neutral, and 208v from black to red. The neutral is capped. From the junction box, a 2-wire goes to a dead-end key switch that ties into the black and red. The key switch is a close-to-make style. So I have 208 going directly to the key switch. When I close the switch, it turns on a motor but I'must unable to see how it gets there. When the switch is closed, I can no longer read 208v from phase to phase but I do have 120v from each phase to ground. How is this possible? I tested for continuity and the switch is definitely causing the wires to make contact. The switch appears to be feeding in and out on the same wires.

 

[GoldDigger]

I think that the explanation is simple. The red and black are a switch leg coming from the motor.

At the motor two phases of the three phase supply drive the motor winding (single phase motor). On one end of the motor the supply is connected to one lead from the red-black pair and the other wire from the pair is connected to the motor winding.

When the switch is open you see the other leg voltage coming through the motor winding. (Sort of stray voltage on steroids). There is then no voltage across the motor winding.
When you close the switch each end of the motor is connected to a different hot leg and the motor runs.

The situation would probably have been easier for you to understand if it were a single phase 120 circuit. There you would see 120 on one wire of the switch leg and neutral on the other until you close the switch.

 

[charlie b]

I am having some difficulty following your description. Let’s start here:

I have a 3 wire coming into a junction box -

Coming from where? Are the black and red connected to a 2-pole breaker in a three phase panel, and is the white connected to the panel’s neutral bar?

I have 120v from black to ground or neutral, 120v from red to ground or neutral, and 208v from black to red.

“Ground” and “Neutral” are not the same thing. Is there another wire (i.e., a green wire) in the same junction box, or is the conduit serving as the equipment grounding conductor?

From the junction box, a 2-wire goes to a dead-end key switch that ties into the black and red. The key switch is a close-to-make style.

I don’t know what a dead-end key switch is nor what “close-to-make” means. Can you clarify?

When I close the switch, it turns on a motor but I'm unable to see how it gets there.

Are you saying that you can’t see the wires that run from the switch to the motor?

I will stop there, as I see you have another response already. I will read that before continuing.

 

[RubyMantis28312]

In response

In response

I will check to see if they are switch legs coming from the motor. If they are, is it possible to make it a 3way setup?

In response to other comments, dead end means it is a switch-loop - like a 2-wire that goes from a light down to a switch.
Close-to-make means when the switch is in the "on" position, it functions as a single pole would - it creates a continuously path.

 

[GoldDigger]

I will check to see if they are switch legs coming from the motor. If they are, is it possible to make it a 3way setup?

In response to other comments, dead end means it is a switch-loop - like a 2-wire that goes from a light down to a switch.
Close-to-make means when the switch is in the "on" position, it functions as a single pole would - it creates a continuously path.

If you really have a motor wired across two hot wires of the three phase supply there is no inherent reason that you could not make the control a three way function. But you would have to be able to run travelers from your current switch location to your second switch location or else add an additional conductor to the run between the current switch and the motor to be used as the common lead.
A breaker feeding the motor would have to be a two pole breaker with common trip, but the control switches could interrupt only one side of the line. However such switches could not serve as a disconnect for the motor if one is required.

 

[kwired]

I have a 3 wire coming into a junction box - black, red and white. I have 120v from black to ground or neutral, 120v from red to ground or neutral, and 208v from black to red. The neutral is capped. From the junction box, a 2-wire goes to a dead-end key switch that ties into the black and red. The key switch is a close-to-make style. So I have 208 going directly to the key switch. When I close the switch, it turns on a motor but I'must unable to see how it gets there. When the switch is closed, I can no longer read 208v from phase to phase but I do have 120v from each phase to ground. How is this possible? I tested for continuity and the switch is definitely causing the wires to make contact. The switch appears to be feeding in and out on the same wires.

The following is a description of how to draw out the circuit

Supply L1

Junction point - supply L1, connects to Switch loop L1

switch contact - L1 one side, switch loop return to other side

Switch loop return connects to motor

Motor winding(s)

Supply L2


L1---------junction(begin switch loop)---------switch----------(end switch loop)motor---------L2

Your white wire may be connected to N, but has no function in this circuit as is, and can be ignored.


When the switch is open you have 208 across it because each side connects (indirectly) to L1 and L2. If there is no current flowing there is no voltage drop across the motor so you see full 208 volts across the open portion of the circuit (the switch)

When the switch is closed you have 120 to ground because both sides of the switch are now at same potential as L1. You have 208 volts drop across the motor because it has resistance/impedance and you applied 208 volts across it. Before you closed the switch you had no voltage drop cross the motor - but did have L2 connected to it which also is at 120 volts to N.

Draw it out and examine where/what the source and load is and hopefully that helps. Any open point in the circuit will see full applied voltage across it until it is closed. Then resistance/impedance will determine where voltage drop occurs.