Re: OT - Single Phase to Three Phase RPC's


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Posted by David Sherman on Tuesday, October 03, 2006 at 4:35PM :

In Reply to: OT - Single Phase to Three Phase RPC's posted by jack cain on Tuesday, October 03, 2006 at 2:46PM :

There are basically two ways to do it. A "static converter" is basically just a box of capacitors and a switch that provides enough phase shift to get a 3-phase motor running. It's the cheapest and works okay if you just have one motor to run and its load is fairly steady. A "rotary converter" is basically another 3-phase motor with some mechanism to get it started. They'll provide more power and are more tolerant of variations in load than static converters. Both types are rated in HP.

If it was me, I'd make my own rotary converter by finding yet another almost-free used 3-phase motor and rigging up some method to start it spinning. You hook one phase of the 3-phase motor to your single-phase power source, start the motor spinning somehow, and then you can draw good 3-phase power off of its windings. It needs to be rated close to the power level of the biggest motor you'll be running off of it. Ways to start the motor spinning include a capacitor bank in series with one of other phase windings, with a push-button switch to connect it long enough to start the motor (manual method), a small single-phase auxilliary motor directly coupled to the end of the shaft of the phase-conversion motor (I built a small 1-HP unit like this and it worked well), or a rope wound around the end of the motor shaft and yanked like a chain-saw starter cord. That last method sounds pretty funky but it works. You wind the rope on the shaft, turn on the juice, and then quickly (before the windings overheat) give the rope an almighty yank. The rope has to be arranged so it will come completely off the shaft once it's unwound. You don't have to get it spinning all that fast for the electricity to take over and spin it up to operating speed. As you might have figured by now, any 3-phase motor will RUN on single-phase (albeit with less power). The trick is getting it to START.



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