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How does a 3 phase converter work?


Active Member
I'm building a three phase rotary converter from plans I have found on the internet.

All the circuits for them are about the same. Heres one.

I am trying to understand how exactly it works.

I understand, and have seen how a three phase motor will run on single phase 240 volts to some degree of effency if you spin it to get it started.

What does the capacitor do? I understand a capacitor allows AC current to pass through it. So what does it do that is different then just hooking the 3rd leg of the motor to one of the AC single phase lines? Does it shift the phase of the current enough to sort of mimic actual 3 phase power?

I am also wondering how the run capacitors in the circuit I posted work? How do they change the voltage between the lines?
Not all circuits I have seen use these run capacitors. I am planning to omit them, atleast at first, in my converter.



Well-Known Member
Most Helpful Member
Here is my prof's website which I'm pretty sure explains why:


THe gist of it, is that the capacitors add a phase shift between current and voltage (in the auxillary winding of the motor) which is required for the motor to run. THis phaes shift is there in a 3-phase source but does not exist in a single phaes source since there is just a single phase. But the capacitance needed for efficient steady state running operation is different than the one needed for best start-up torque performance. So two capacitors can be used, essentially switching the capacitances in and out depending on whether the motor is starting up or running. After that, you can make all sorts of compromises to reduce cost (like just one capacitor for either best startup, best running, or somewhere in between) depending on the application requirement, or go all-out and have both with switches to enable and disable them.

Or apparently from the webpage you can remove them all together but then you have worse performance compared to the other two and there are limitations. I think you need a motor designed for it, since it says something about high resistance in the auxillary windings.

On how the motor itself actually works:
THe gist of this, is that if you look at that graph, notice that at zero speed you have zero torque, therefore the motor can never startup on it's own unless you have an external force applied or do something else to it (like the capacitors). All that double rotating field stuff is a calculation method for the motors (it bears similiarities to the way the 3-phase calculations or done see).

I would not say the capacitors don't shift the phase to mimic 3-phase power. I'd say that it would mimic it enough for an induction motor to run. It will not replicate a 3-phase supply from a single-phase supply (you couldn't run other types of 3-phase motors or loads from it.
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Active Member
Thanks! I'm gonna take a look at those links right now.


Also, make sure the idler motor is much larger than the tool motor, remember it's actually acting as a generator as well as a motor, the current going in will always be higher than the current out (twice as I assuming 100% efficiency). To ensure the motor doesn't overheat as well as optimum performance, use an idler motor that's three times as powerful as the tool motor.

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