Autotransformer-Single Phase to Run 3Phase Motors

[JimK]

Hi, I am new here and first post. I saw another post from a couple of years ago that got me thinking to make a reply, but then I thought I would just start a new post.
I would like to talk about the High Voltage Transformer Method for running 3phase motors from a single phase supply. I have built several converters of this type using used components like wire, relays, switches, terminals, capacitors etc. The most recent, an air compressor converter where I modified a 240v 140A choke welder by rewinding the secondary winding, only about 180 turns of 2.5mm squared building wire.
This motor is 2 pole (2860 R.P.M.). If the motor had been 4 pole (1420 R.P.M.) I would have only needed about 80 turns. Some advantages of this method are you can start a heavily loaded motor (like an air compressor) easily and you can run a motor at full load amps to get it's full name plate HP.
Also to modify, most welding transformers, say 240v to 415v is not very difficult or costly.

Jim

 

[AndyStobbs]

How do you emulate the extra phases? I'm interested.

 

[awemawson]

Capacitor presumably

 

[JimK]

Hi Andy. The transformer is used to increase the voltage supplied to a suitable capacitor, then connected to the "C" leg of the motor [the generated phase] "A"& "B" are allready connected to the line voltage.So for this method to work we need the transformer, capacitor, and motor all connected. The capacitor should be valued to the motor and its load.And rembering there is no 3phase until the motor is running. So just like a single phase motor you will need starting arrangments.I prefer a simple voltage reley with start capacitors.
So the increased voltage, the capacitor, and the Back E.M.F. generated from the motor is why this method starts and runs a heavily loaded motor so well and you can get full nameplate H.P. from the motor.
Now to your question. You allready have two lines "A"&"B" connected to the motor, so as the motor is started, as it gains speed, it is generating a voltage [Back E.M.F.] from the "C" leg of the motor, that voltage can not reach the line voltage, but when it is connected to the correct valued capacitor and that capacitor is connected to the higher voltage from the transformer you can then have very ballanced voltages.
Jim.