Single-phasing a 3 phase motor - starting?

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McCES

Member
Location
OHIO
I understand that if a 3-phase motor is running and loses a phase it will continue to run at elevated current until the overloads (if they are sized correctly) will shut it down.
If the single phasing issue is NOT corrected and the overloads are still good the motor will not start when energized.
My question is if you turn the shaft to get the rotor spinning will the motor then pick up and run while single-phased? My thiinking is that by turning the motor shaft you are manually creating the field required for rotation.

Your thoughts please.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
My thiinking is that by turning the motor shaft you are manually creating the field required for rotation.
Basically, you are correct.

The mechanical load on the motor and how fast you 'spin' it will also come into play.
 

wptski

Senior Member
Location
Warren, MI
I'm not a electrian but a retired machine repairman. I had machines with a fractional horsepower motor, I think it was something like 1/15HP. It turned a 30:1 Boston gearbox.

I had one where the motor wouldn't turn the gearbox but I could by hand. Motor would rotate by itself. I could stop the motor with my hand. It was found to be single phasing.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
A 'rotary phase converter' is essentially an unloaded 3 phase motor with some equipment to start it spinning. You supply power to 2 legs, and once the machine is spinning it develops the 3rd leg. You can supply power from this 3rd leg to other 3 phase loads.

-Jon
 

Electric-Light

Senior Member
A 'rotary phase converter' is essentially an unloaded 3 phase motor with some equipment to start it spinning. You supply power to 2 legs, and once the machine is spinning it develops the 3rd leg. You can supply power from this 3rd leg to other 3 phase loads.

-Jon

If you have a larger motor that is not heavily loaded, but it becomes single phased due to some problems, that big motor will essentially act as a RPC, so other small motors connected may continue to function normally, including starting from three phases generated from motor acting as a RPC. A current unbalance relay prevents this from happening by cutting off power to the large motor.
 

Denis

Senior Member
Location
50156
If you have a larger motor that is not heavily loaded, but it becomes single phased due to some problems, that big motor will essentially act as a RPC, so other small motors connected may continue to function normally, including starting from three phases generated from motor acting as a RPC. A current unbalance relay prevents this from happening by cutting off power to the large motor.

This is why I read here! thanks :)
 

One-eyed Jack

Senior Member
A 'rotary phase converter' is essentially an unloaded 3 phase motor with some equipment to start it spinning. You supply power to 2 legs, and once the machine is spinning it develops the 3rd leg. You can supply power from this 3rd leg to other 3 phase loads.

-Jon

I used a string to rope start my redneck phase converters.:D
 
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