Phase converter success

For some time I have had a Pollard Corona 13 AY Drill installation on my round -2-it list. It is a 8 speed floor standing beastie from the 50s and was the prototype for all brick sh*t house designs.

The 8 speeds are obtained with a two ratio gearbox and a 4 speed pole switching 3/4 hp motor with 2, 4 6 and 12 poles. The speed range is 2945 to 411 rpm. I only have a single phase supply and the motor is effectively 4 sets of star wound 415 volt motor windings in one casing. There is no chance of opening up the motor to dig the star points out without risking cracking 60 year old insulation. I would normally use a step up inverter to run single speed 3 phase motors but the electronics really don't like any switchgear between the inverter and the motor. So a converter was decided upon. These rely on the motor inductance and capacitors to create the third phase so this would mean 4 sets of capacitors one for each winding and interfering with the delicate wiring. A rotary converter would solve this problem. The pilot motor acts to balance up the third phase and reduce the impact of different load motor inductances. I bought a small static converter on flea bay for a song which gave me a transformer and a case plus other redundant bits. I used the Fitch-Williams design guide here.

paper is very well written an guides the reader through the design steps by practical experimentation. I used a 1.5hp 1450rpm pilot motor and the resultant converter works really well. It won't start the motor on top speed from stationary but it is easy to run it up in 4 pole mode and switch to 2 pole. The final stage of power factor correction is well worth it as it drops the transformer standing current considerably and keeps it within spec. On test the unit ran for an hour today with minimal temperature rise.

For anyone thinking of "rolling their own" rotary converter, I can thoroughly recommend the F-W method.

Bob