3 phase power is something (one of many really) that I have difficulty grasping.
Suppose one has a 3 phase delta connected electric motor with a need to run it from single phase, this is something that is often done when building a rotary phase converter to create 3 phase power from a single phase supply. Single phase, one neutral and one phase line from the mains supply.
There are different ways of getting the 3 phase motor up to synchronous speed. One way is a single phase pony motor, another is a pull rope to spin the motor manually and another common way is to create a phase shift by adding a capacitor in series with a connection to the third motor terminal. The capacitor is disconnected as soon as the motor is up to speed.
Now, as far as I know the desired phase angle to the third motor terminal is 120 degrees and the maximum phase angle to be had from a capacitor (in theory) is 90 degrees which I assume is always considerably less in practice.
So, I am thinking that if I used a 1:1 transformer (i.e. old isolating transformer) across the line I would have full line voltage at 180 degrees phase difference and if that was fed to the third terminal via a capacitor the resultant total phase shift would be 180 degrees minus something less than 90 degrees. Hopefully something close to 120 degrees phase angle.
My theory is that the motor would be self starting and all three coils of the motor would be directly energised from the mains. Furthermore it would appear that the rotary phase converter idler motor would not be required at all.
Any comments please?
john