Operation of three phase motor on single phase supply

[Framer Dave]

Hello,

I have been told that it is possible to run a small (about ⅛ hp) 3 phase motor on a single phase supply with the use of a "run" capacitor.

Please does anyone know if this is feasible? If so, how would I calculate the value of the cap and how connect it?

Any ideas very welcome.

Regards,

[Joe]

See post 17 here: http://golbornevintageradio.co.uk/forum/...p?tid=4262&page=2

- Joe

[Retired]

Hi,

That is a very small motor Dave. It depends on the motor regarding the three coils. If the motor has six leads from the coils then it can be connected in either Star or Delta. I'm unfamiliar with such a small 3 phase motor but on bigger motors depending on age many old 3 phase motors had their Star connection buried deep in the windings and trying to dig out this connection could well render the motor beyond use. Star generally is 415V.

However if the six leads are accessible then the motor can be connected in Delta which is what I think you require allowing it to run from 240V? I don't know if a VFD (inverter) is available for such a small size and to hook up this small motor using a start and run capacitor would reduce its output even further? The short answer is yes if it has six leads but I wouldn't recommend it.

Good question regarding the capacitors though and not an easy one to answer. The run capacitor needs to be chosen in value so that under power the third phase (supplied by the capacitors) is balanced as much as possible by taking amps readings. The start capacitor isn't critical because it is in circuit only long enough to bring the motor up to speed. You will also need means to drop the start cap off-line once the motor is up to speed; this can be done using a relay or for simplicity I use two start buttons pressing both to start the motor and hold in until the motor is up to speed then when I release both buttons the run capacitor remains connected but the start capacitor drops off-line; not easy for a novice.

I installed 3 phase 415V into our garage a few years ago winding my own transformer and this was most interesting if somewhat highly dangerous for a novice like me. I was fascinated as I balanced the third phase to the motor using capacitors; putting capacitors in and taking capacitors out changed the voltage and I was able to obtain exactly 415V with trial and error but on such a small motor as yours is it worth the time and effort unless of course it's intended for a special application?

Does your motor have a nameplate attached Dave?

As Joe kindly adds this question is covered in the link supplied but on a much larger motor.

Kind regards, Col.

[Refugee]

If the motor can be disengaged and re engaged while it is spinning it should start without a start capacitor.
This may not be practical with such a small machine though.

[Ed_Dinning]

Hi Dave, if it is rated at 440v it will never produce full power on 240v (single phase), but depending on what you want to drive it may work well enough.
If you are not conversant with electrical safety, DO NOY try this:

Connect 2 of the 3 motor wires of the motor to a 240v supply. Connect the 3rd wire through a non electrolytic capacitor of about 1uF and 1000v rating to the 240v live (L) lead. Motor should run. If direction is wrong connect cap on 3rd lead to 240v neutral (N) lead to reverse direction.
Measure volts across all 3 wires. If the volts across the cap are greater than the other 2 windings, increase the size of the cap until all windings are within about 30v or better of each other. Repeat the procedure with the motor under full load and you may find a different value of cap is needed.
You now have a system, with a start / run cap selector switch that should allow the motor to operate satisfactorily.


Ed

[Framer Dave]

Thanks for the above gents. The motor in question is part of this

http://www.ebay.co.uk/itm/141277242471?s...K:MEWAX:IT&_trksid=p3984.m1423.l2649

- a suds pump for my recently acquired lathe. Three phase ones seem to sell for less than single phase, hence my interest. The construction of pumps of this type as combined unit would seem to make replacement of the motor itself impracticable.

If I can buy this for a reasonable amount, I think I will follow Ed's advice. I can only trip a few breakers or blowup a few caps when all's said and done!

Regards,

[Retired]

Hi,

Good luck Dave. I wondered why you needed such a small motor. The suds pump motor on my Colchester was 3 phase 1/2hp.

I've never used suds on my lathes at home even when I had the Colchester.

Kind regards, Col.

[AlanBeckett]

Ah, so, I'm watching that as well
Fear not, I'll maintain a low profile.
The one on my Chipmaster is of course 3-Phase. I converted the main 3hp Motor to run on Singe Phase with the usual jiggery-pockery and simply connected the Suds Pump across it.
I bought another Pump to use with the the Milling machine and big Drill. That's 3-Phase and I run it it with a Capacitor for the 'Third' Phase. It works fine. I'll have a look what I used.

Alan

[Refugee]

A suds pump may well start without a start capacitor as it will be running almost off load until it has primed.

[AlanBeckett]

Why wouldn't it be primed?
The Impeller normally sits in the Suds.

Alan