Single phasing of 3phase induction motor


Thread Starter


Can i still make the motor run near synchronous speed when one of the 3phase lines is disconnected? If so, how? Thanks!

Steve Snodgrass

You really need all three phases or the motor will run hot(burn up) and fail under load. If you are concerned about loosing a phase while running, then put a VFD (Variable Frequency Drive) on the motor. With this, the drive will continue to supply the motor with 3 phase AC even if you loose an incoming phase or even single phase the drive. As long as the drive's AC/DC Converter sections can supply the DC Bus with enough current to run the motor through the remaining phase(s)it will work. You might have to slightly oversize the drive depending on what you use. To keep the speed synchronous, use a vector drive.

Johan Bengtsson P&L Automatik AB

If the motor manages to continue to run it will still try to run at the same speed, but it will of course be weaker. If you only have one phase it can't start since it doesn't know the proper direction. /Johan Bengtsson ---------------------------------------- P&L, Innovation in training Box 252, S-281 23 H{ssleholm SWEDEN Tel: +46 451 49 460, Fax: +46 451 89 833 E-mail: [email protected] Internet: ----------------------------------------
Post-fault (single-phasing) performance is dependent on pre-fault operating conditions. Please provide some detail so that a meaningful answer can be provided. Regards, Phil Corso, PE Trip-A-Larm Corp (Deerfield Beach, FL)
Kind of.You can connect an ac capacitor from one of the other phases to the dead phase of the motor.This produces an artificial phase.
This idea is the basis for commercially manufactured single to three phase converters.
R F Redfern.
But you need a timer to disconect the capacitor
after motor start or your motor will be burnt.
Replying to David Brown's Jun13, 11:42 am comment:

Not necessarily! A PTC (Positive Temperature Coefficient) resistor can be used. They are even commercially available to eliminate centrifugal
switch starting of single-phase motors having start-run windings. For those interested look at Murata-Erie products as an example.

Now back to the original problem... loss of one phase. The addition of a capacitor to provide a starting "direction" assumes that the third phase
is available.

Sounds like another case of "catching the mice and letting the elephants go free!"

Phil Corso, PE
(Boca Raton, FL)
Thank you , I know another different way to use three phase motor in single phase supply but this is only talk, the fact is we should use the right motor to the right supply.