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Phase converters and what NOT to do

Keith Foor

Active Member
Active Member
SO I just finished rebuilding a commercially sold phase converter. I will not name the manufacture to protect the fact someone else may have been in the unit at some point.
This is a 3 HP RPC with a shaftless motor running one start cap, one run cap, a potential relay and a toggle switch controlled 2 position contactor to power the unit up.

It had been taken apart by the previous owner and connected to a 5 HP motor so again, someone didn't know what they were doing.
The run cap was wired from L1 to L3 (generated leg) and was 78uF. Now based on the 10 to 12 uF per horsepower, this was too big even for the 5 HP but on the 3 HP it made the no load generated leg voltage 275 volts. This is what happens when you guess at what you are doing. Had this been connected to sensitive equipment it would have sent the expensive smoke rolling out of the cabinet for sure.

Once I got done I got the following voltages.
L1 to L3 was 236 volts
L2 to L3 was 221 volts
with an input voltage of 224

The 77Uf cap was replaced with a 30uF cap and everything was right with the world.

I still don't like potential relays, but that's what this one has and I left it in place.


Active Member
Active Member
Eventually ill be building my own 5 hp RPC, I've purchased several run caps to tune mine. It will have push button switches , 220 v contactor with a fuse block inside. Even using rubber mounts to help with vibration and noise. Think ill even get a load center and power all the machines through that. Better safe and done right. My biggest motor is 3HP so I'm safe even with all my three phase running ill be ok.


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I'm starting to straighten out the RPC that came with the new mill. "Straighten out" is not really the right term, it looks like I will be salvaging the box, the fuse block, and the two push buttons. It currently has four caps in parallel between T1 and T2 (T1 is the generated leg!!). The voltage tested pretty good, but that is likely because three of the caps were off-line due to a burned out connector. I'm trashing the four, and starting over.

It's a 3hp idler. Based on what you said here, it seems a 35MFD cap would be a good start. The question is, one or two? You mention only one here, but in the knowledge base article you mention two. Which is correct? Or which is more better? Also, one of the caps has three banks of terminals on it. Could this be used for both

And if you want examples of what NOT to do, I'll get some pictures. :)

Keith Foor

Active Member
Active Member
If you want a fully balanced RPC then run 2 one from L1 to L3 and one from L2 to L3 of course L3 being the generated leg.
35uF is a good starting point for a 3 HP motor. THose numbers should get you really close.

The one I have I didn't build, it was a commercially manufactured one. It only had one run cap and one start cap. With that resign you give up a little in capacity and voltage regulation and balance but it's still very usable.