3 phase motors
- Thread starter friday
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- Location
- Logan, Utah
A 208 volt 3 phase motor will not run on a 208 volt single phase supply.
Chris
Chris
Steviechia2
Senior Member
- Location
- Massachusetts
Was it a neutral or a white insulated conductor
greeny
Member
- Location
- Southern NH
Obviously, the motor will not operate.
From your context, this is not hypothetical, correct? If the overloads were sized for the 3 phase connection, then it would better protected than if they were sized for single phase supply. The fuses didn't open?
From your context, this is not hypothetical, correct? If the overloads were sized for the 3 phase connection, then it would better protected than if they were sized for single phase supply. The fuses didn't open?
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
To answer your question, it depends on the type of OL but most likely the motor would fry before anything would act. If the 3rd leg was a neutral you would not have a rotating magnetic field; it would be oscillating back and forth on the other two legs but not spinning. When not spinning, a motor winding is essentially a high resistance short circuit. So technically your Short Circuit Protection Device should have operated, but there may have been enough resistance in the windings to prevent that, it depends on a lot of other factors.
As to the OLR, a solid state OLR that has Ground fault and/or Current Imbalance protection would have likely tripped immediately but a standard bi-metal or melting alloy OLR probably would not have done so, or at least not quick enough to prevent motor damage. There would have been current seen on all 3 circuits. It would have been more than what the 3 phase current should have been, but the effect of such a severe current imbalance would have caused motor heating in extreme disproportion to the same of amount of balanced current, so winding damage would be likely before the amount of current seen by the heater elements caused a trip.
So essentially you would have a race between the fuse / CB, the OLR and the motor winding insulation, with the insulation the likely loser no matter what.
As to the OLR, a solid state OLR that has Ground fault and/or Current Imbalance protection would have likely tripped immediately but a standard bi-metal or melting alloy OLR probably would not have done so, or at least not quick enough to prevent motor damage. There would have been current seen on all 3 circuits. It would have been more than what the 3 phase current should have been, but the effect of such a severe current imbalance would have caused motor heating in extreme disproportion to the same of amount of balanced current, so winding damage would be likely before the amount of current seen by the heater elements caused a trip.
So essentially you would have a race between the fuse / CB, the OLR and the motor winding insulation, with the insulation the likely loser no matter what.
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greeny
Member
- Location
- Southern NH
Jraef, I've seen a number of completely stalled induction motors and the overload and/or fuses usually let go first. I know many factors are involved in this, but I'm just speaking from experience.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Yep, but "usually" an "should" are Murphy's favorite terms of endearment.
mcclary's electrical
Senior Member
- Location
- VA
A completely stalled 3 phase induction motor, as Jraef pointed out, would at least be loaded equally on each pole, and the heat from a stalled rotor could possibly be dissapated quick enough by the motor windings to handle the heat until a fuse blows, a heater gets hot enough to trip, or a breaker trips.But With a single phase supply (like the op said) on a 3 phase motor, all bets are off, and the transfer of heat would be so unpredictable, that it would be anybody's guesss as to what would blow first. It could literally depend on what "position" the motor was in as to what would blow first.
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