Momentary Stop on Only One Phase Conductor

[Mr. Spinosa]

When working with motor controls, why would you put a momentary stop on only one phase conductor? - How could I explain that it is okay to disconnect only one phase?

 

[Joethemechanic]

Opening up one phase on a three phase motor circuit will not stop the motor. The motor will continue to run until it overloads something. (hopefully the overload protection) and then it will stop.

Are you sure this isn't some kind of circuit that opens up one phase wire, and connects the remaining two phases to a DC source for braking?

 

[hurk27]

Joe I don't think he wasn't talking about the feed to the motor but the start stop control, common to motor starters.

The reason you only need to momentary break the connection to the contactor coil is the basic operation of a start stop circuit, the coil of the motor contactor is a single phase device and only has two wires to it, one from the over loads from L2, and one from the start stop circuit, the start stop circuit has L1 running to one side of the stop button this button is a NC contact and makes contact until pushed inwhich it opens, the other side of the stop button runs to the start button which is a NO which closes when pushed, from the other side of the start button it goes to the coil, if this was all there was pushing the start button would only pull in the contactor while it is being pushed, so we have another set of NO contacts on the contactor in parallel with the start button, that when we push the start button the contactor pulls in and closes these auxiliary contacts locking the motor contactor on, this provides power to the coil through the stop button through the aux contacts to the coil, now when you push the stop button, it disrupts the current to the coil dropping out the contactor and opening the aux contacts, this stops the motor because the motor contactor disconnects 2 or 3 of the phase conductors feeding the motor.

Also the over loads each have a NC contact that when they trip they will also drop out the motor contactor and shut down the motor.

Look for a diagram on a start/stop circuit.

 

[Joethemechanic]

Joe I don't think he wasn't talking about the feed to the motor but the start stop control, common to motor starters.

The reason you only need to momentary break the connection to the contactor coil is the basic operation of a start stop circuit, the coil of the motor contactor is a single phase device and only has two wires to it, one from the over loads from L2, and one from the start stop circuit, the start stop circuit has L1 running to one side of the stop button this button is a NC contact and makes contact until pushed inwhich it opens, the other side of the stop button runs to the start button which is a NO which closes when pushed, from the other side of the start button it goes to the coil, if this was all there was pushing the start button would only pull in the contactor while it is being pushed, so we have another set of NO contacts on the contactor in parallel with the start button, that when we push the start button the contactor pulls in and closes these auxiliary contacts locking the motor contactor on, this provides power to the coil through the stop button through the aux contacts to the coil, now when you push the stop button, it disrupts the current to the coil dropping out the contactor and opening the aux contacts, this stops the motor because the motor contactor disconnects 2 or 3 of the phase conductors feeding the motor.

Also the over loads each have a NC contact that when they trip they will also drop out the motor contactor and shut down the motor.

Look for a diagram on a start/stop circuit.

Oh ok, you are thinking he means just to drop the contactor out by de-energizing the coil.

I was thinking he might have been looking at a braking circuit like this one, but what you are saying makes more sense.

 

[iwire]

How about before we go to far we wait for the OP to let us know what he means?

It could be a single phase motor he is talking about and the code allows controlling it by one leg only.

 

[Joethemechanic]

How about before we go to far we wait for the OP to let us know what he means?

It could be a single phase motor he is talking about and the code allows controlling it by one leg only.

OOPs,

How did I miss that he didn't say 3 phase?????:slaphead:

I think I must have an injection braking obsession. I wonder if there is a drug for that.

 

[Jraef]

OOPs,

How did I miss that he didn't say 3 phase?????:slaphead:

I think I must have an injection braking obsession. I wonder if there is a drug for that.

No, but there probably is a 12 step program available for it.

 

[kwired]

How about before we go to far we wait for the OP to let us know what he means?

It could be a single phase motor he is talking about and the code allows controlling it by one leg only.

Very possible. The confusing part is he said momentary stop which sets most of us to thinking control circuit, and not motor circuit.

A three phase motor controller only needs to open two legs and does not have to open all three BTW.

 

[Joethemechanic]

No, but there probably is a 12 step program available for it.

I am powerless over DC injection braking,,,,,,,,,, OK

I have come to believe that a power greater than me can restore me to sanity,,,,,,,,,,,, Ok first question?????

Is that greater power electrical? Because I ain't doing that electro-shock thing again :happyno:

 

[Joethemechanic]

A three phase motor controller only needs to open two legs and does not have to open all three BTW.

And two phase motor controls only need to open a single leg on each phase.

He didn't say it wasn't two phase either

.

 

[Mr. Spinosa]

Hey, you guys are alright... I was talking about a motor control circuit and the way I read it, or the way it looked to me, was that there was a momentary stop on only one phase. - Honestly I think it was a three phase motor but I have been looking at a bunch of material and I will have to dig it up again to know for sure. I am getting ready to take my administrator's test and I do really appreciate all your input. :thumbsup:

 

[kwired]

The control circuit is a single phase circuit, putting a contact anyplace in the series opens the circuit and the load does not operate.

Both poles need opened if we are talking about a disconnecting means but for controls all that is required is to open enough conductors to make the load non operable.