Question about a three phase motor ...

[john1]

Hi,

I want to get a three phase motor to spin,
with no load,
on a single phase supply.

Would like to do it without using a capacitor if possible.

I realise that a phase-difference has to be produced somehow,
and i was thinking of putting two of the windings in series,
to produce a different inductance from the remaining winding.

It could not be expected to do any useful work wired like that,
but that is not my intention for this.

Any suggestions ... ?

Regards, John

 

[Earthshaker]

John1, I work on three phase motors as a job, on irrigation systems.
Simply stated a three ph motor has three windings, electrically separated by 120 degrees. The only way I know to make it run on single phase without any intervening hardware is maybe "spin start" it that is apply power to the motor and physically spin the shaft, bear in mind that there are few if any 3 ph motors that are setup to run on 110 vac. There are other methods to make it work, the most obvious means is design a phase converter either static or active that is either capacitors, or a rotary
version.

 

[Exo]

The capacitor is the only non-electronical (simple) solution.
Why don't you want to use one ?

 

[john1]

Hi Earthshaker, Hi Exo,

Thank you both for your replies.

Earthshaker, you are of course quite correct, you can directly
apply motion to the shaft, and a three phase motor will then
'spin up' to speed with a single phase applied.
I do not need the motor to do any more than to spin up with no
load applied, but i do not want to physically start it turning.

Exo, yes a capacitor is a solution, by feeding a winding via a
capacitor, a phase shift is introduced which will impart a
turning force to the rotor, enough to spin it up.

Why don't i want to use one ?
Well, i may have to.
But i am toying with other possible ways of introducing a phase
shift between windings.

I was thinking of putting two of the three windings in series
across the AC supply,
and putting the remaining winding across the AC supply.

I feel the inductance of the two in series would be different
to the inductance of the single winding.

This might produce a phase shift sufficient to impart a turning
motion to the rotor, and get it to spin up.

Then again it might not.

Or there might be other ways.
Or i may have to get a suitable capacitor.
If it can be made to spin up somehow, without any extra parts
i would be interested to find out.

Regards, John

 

[agglipo]

Guys,

Running a 3 phase motor in single phase will burn the motor windings. Adding capacitor or giving it a starting rotation might do the trick but it will not deliver the power it suppose to do and eventually it will get hot.

Albert

 

[stevez]

With my assistance my brother made a rotary convertor to get from 208 volt single phase to 208 volt 3 phase. It requires a 3 phase motor that can be smaller than the motor being driven - and a pony motor that gets the 3 phase motor running- once rotating and on-line the pony motor can be taken off line.

It works but is a poor substitute because of severe imbalances - better than nothing.

 

[RODALCO]

I use one at home to run two 3 phase machines off with no problems.
Ok here in New Zealand we have 230 Volts 50 Hz.
The main motor I use is a Heemaf 220/380 Volts 5 Hp motor with a reasonable heavy pulley on it.
I start it up in delta, Motor links set for delta working.
230 Volts on U - V, 120 µF Capacitor across V - W or W - U for opposite rotation.
After 3 seconds when the motor is up to speed a timer will switch of the capacitor via a contactor.
I have used my system for quite a few years with no problems.
Circuit connected via 16 Amps MCB, start ± 20 Amps then reducing to around 3½ Amps and increasing when loads are applied.
Can run for unlimited time with no heating, 3Ø motor runs very smoothly.
At test terminals I have 3 * 230 Volts available to drive other 3 Ø motors from

A couple of things to bear in mind.
Caps to be rated at 400 Volts or higher
Main motor to be derated to 50 %
Sometimes i have to give the motor a push on the pulley to start, but no pony motor needed.
Safety. put all live wires and capacitors in an enclosure.

 

[Jfc]

Hey!
The solution for use a single phase with a capacitor is possible but its not the best way.
At tree phase motor you must get the six leads from the widings and conect in delta mode U-Z; v-X; W-Y, then connect the capacitor beetween 2 points of the delta mode and at last conect the power suply (230V 50Hz) the free point and one of the leads of the capacitor.
VERY IMPORTANT!! THIS IS ONLY POSSIBLE WITH LOW HP POWER MOTORS.