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Discuss I have a motor supplied by 2 hot legs from 3 phase 480v. One line to motor is interrupted by a pressure switch. With one line to the motor hot at all in the Electrical Forum area at ElectriciansForums.net

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I have a motor supplied by 2 hot legs from 3 phase 480v. One line to motor is interrupted by a pressure switch. With one line to the motor hot at all times. Why does the motor not burn up?
 
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James

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Is this a 3 phase motor with 3 hot wires?

I suspect it is a single phase motor with 2 wires supplying it, when 1 is disconnected there is no path for the current to flow.

If it was a 3 phase motor and you removed 1 then there would still be current flowing through the remaining 2 phases but the motor would be stalled and get hot.
 
Two phase motors do exist although are a rarity. It is probably single phase. What is the purpose of the pressure switch.
 
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  • #4
Is this a 3 phase motor with 3 hot wires?

I suspect it is a single phase motor with 2 wires supplying it, when 1 is disconnected there is no path for the current to flow.

If it was a 3 phase motor and you removed 1 then there would still be current flowing through the remaining 2 phases but the motor would be stalled and get hot.
Yes it’s a single phase motor supplied by 2 hots. The A phase is hot at all times to motor and B phase is opened until pressure switch closes. The lead coming off motor for B phase will have voltage to ground even with switch open. I’m assuming it’s coming from A because of windings in motor?
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Two phase motors do exist although are a rarity. It is probably single phase. What is the purpose of the pressure switch.
It’s a fan motor on a dryer for a air compressor.
 
Where is this pressure switch, within the motor or externally. A pic would be nice, we like pics.
 
As is #2 the pressure switch interrupts one hot breaking the circuit and no current flow. I assume the motor does stop.
 

Megawatt

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Arms
I have a motor supplied by 2 hot legs from 3 phase 480v. One line to motor is interrupted by a pressure switch. With one line to the motor hot at all times. Why does the motor not burn up?
@Bassman welcome to the forum What you have is a single phase motor but what you are saying is you are just using A and B phase and not C phase. I have never heard of a single phase motor @ 480vac motor but the question is why don’t it burn up since you have A phase controlling the motor through the pressure switch and B phase is present on the motor at all times. The way the motor windings are they will not let the motor try and start. That motor is old school and their needs to be a relay added that when the pressure switch cuts the motor off B phase will also drop out. When no power is present B phase needs to be wired N/O and when the pressure switch is activated the relay will energize and close the contacts on the relay for the motor to start, with the motor stops no power will be present on the motor.
 

Megawatt

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Arms
@Bassman welcome to the forum What you have is a single phase motor but what you are saying is you are just using A and B phase and not C phase. I have never heard of a single phase motor @ 480vac motor but the question is why don’t it burn up since you have A phase controlling the motor through the pressure switch and B phase is present on the motor at all times. The way the motor windings are they will not let the motor try and start. That motor is old school and their needs to be a relay added that when the pressure switch cuts the motor off B phase will also drop out. When no power is present B phase needs to be wired N/O and when the pressure switch is activated the relay will energize and close the contacts on the relay for the motor to start, with the motor stops no power will be present on the motor.
Baseman I got the 2 phases crossed up but it still doesn’t matter A or B same thing
 
@Bassman welcome to the forum What you have is a single phase motor but what you are saying is you are just using A and B phase and not C phase. I have never heard of a single phase motor @ 480vac motor but the question is why don’t it burn up since you have A phase controlling the motor through the pressure switch and B phase is present on the motor at all times. The way the motor windings are they will not let the motor try and start. That motor is old school and their needs to be a relay added that when the pressure switch cuts the motor off B phase will also drop out. When no power is present B phase needs to be wired N/O and when the pressure switch is activated the relay will energize and close the contacts on the relay for the motor to start, with the motor stops no power will be present on the motor.
I agree a pressure switch will normally break the control circuit.
 
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  • #11
External in line before motor. Sorry no pics.
I won’t be back at work until next week. Maybe I can get a pic then.
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As is #2 the pressure switch interrupts one hot breaking the circuit and no current flow. I assume the motor does stop.
Yes it stops. I have another question. The other lead coming off motor (B phase). When pressure switch is open for it, the lead coming from motor still has voltage to ground. Is it coming from A phase because of the windings in motor? Thank y’all so much for the replies.
 
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I don't know the type of motor but the voltage is likely to be carrying on through the run winding. Does it have capacitors not that this will make much difference.
 

Megawatt

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Arms
I won’t be back at work until next week. Maybe I can get a pic then.
No pictures are necessary it can’t burn up wired the way it is. I told you how to wire that motor. What bothers me the most is voltage being present in the motor when stopped, that’s not UL listed and we are trying to get these motors fixed. If someone starts doing work on this motor when it is stopped and not lock it out that motor is back feeding the other windings, that is a dangerous situation
 
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I don't know the type of motor but the voltage is likely to be carrying on through the run winding. Does it have capacitors not that this will make much difference.
It does have a capacitor.
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No pictures are necessary it can’t burn up wired the way it is. I told you how to wire that motor. What bothers me the most is voltage being present in the motor when stopped, that’s not UL listed and we are trying to get these motors fixed. If someone starts doing work on this motor when it is stopped and not lock it out that motor is back feeding the other windings, that is a dangerous situation
Ok. Thanks very much for the replies.
 

Lucien Nunes

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The motor sees a single circuit from A, through itself, through the switch and back to B.
As far as the motor is concerned it functions exactly as if it was from one hot to neutral rather than one hot to another hot. As far as the user is concerned, the difference is that with only the B phase switched, the windings (and, as you noted the B lead from the switch to the motor) remain hot relative to ground all the time, so for safety it is better to switch both legs.

All normal motors will show continuity from all terminals to all other terminals, regardless of what type of starting etc. they use. Therefore if any one terminal is made hot, they will all read hot to a multimeter.
 

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