After all the years of setting up machinery I thought I knew a lot about 3-phase converters. Well, I didn't know as much as I thought.

In putting in a converter for the 2-bag, 5 hp, 3-phase converter I picked up used for my single phase retirement shop I discovered a few important things I never knew.

1) A rotary converter is nothing more than a static converter with the idler motor added. You can add an appropriately sized motor to any static converter to make it into a rotary converter. It is not a difficult change to make.

2) A static converter for a larger motor than you are using can be easily altered to be sized for the smaller motor. It's just a matter of changing the capacitor(s) to the correct size. As an example I found an 8 - 12 hp static converter to use on my 5 hp dust collector. I needed an 80 - 140 mfd capacitor (if I'm recalling that size correctly) for my motor. Coincidentally the 8 - 12 hp static converter had three of these in parallel. I simply snipped the wires to remove the 2nd and 3rd capacitors from the circuit. All other internal components were the same or oversized for the drop down in size. Voila......the correct size static converter for my 5 hp motor.

BH Davis

I knew #1 but lesson #2 is really handy to learn. Thank you for sharing your new-found knowledge.

My rotary phase converter will run up to a 7.5 hp motor or a smaller hp motor. I forget what the total load can be at one time but it is higher than 7.5. It may be 15 hp total. You can not run another 7.5 hp at the same time but you can run a 5 and let’s say a 2 hp while running the 7.5. If you add the three together it comes to 14.5. Some companies that make converters list the total as the maximum hp but that doesn’t always mean of a single motor but total of a few machines combined.
If you sized your converter for a single machine it sounds like you may not be able to use that converter for several other machines.

On are related surprising note I ran the 15 hp main motor (plus belt drive motor) on my original 37x60 Timesaver with a 10hp rotary converter. I ran it that way for many years with never a problem. I suppose it worked because a wide belt sander has a relatively low start up load as it's just getting the sanding belt moving.

BH Davis

You can make a rotary converter out of any three phase motor. Run your two 110V leads (from your 220V service) to the motor, the third motor lead generates the other leg. You then would use all three legs on your tool. The trick is getting the motor started. You can use capacitors and switching, when started the capacitors drop out of the circuit. You can also use another motor with a pulley and belt to get the "donor" motor started. Once started the start motor is cut from power and just idles. The other way is to use a pulley and wrap a chord like starting a lawn mower. You would pull it to get the "donor" motor RPM up before energizing power. My kids have home made converters in their garages because the equipment gleaned from my shop is three phase.

I built mine out of a 20 hp motor I got free for the removal. I ended up with a small 1.5 hp motor to get it up to speed using a timer then power kicks on. You have to balance the voltages between each leg as close as possible. I'm not aware of any formula to do so as each motor has it's own characteristics. With the help of my dad who was a retired electrical engineer we played with various sized capacitors until we got it close. Warning, if you measure one leg against ground you'll get frightening results as it will be be high. My dad assured me it was fine. Then we checked each individual machine while it was running and on one or two we added some caps to get them smoothed out. That was over 20 years ago and it still works perfect with no issues. One note of warning, if you are not experienced with working in a high voltage/amp environment and know and follow safety protocol, go find someone who is. This is serious s$&t not to be trifled with.

One other warning just came to me about the differing voltages if you measure one leg to ground. The two phases you measure off of the grid coming in will be pretty close to what you'd expect, it's the third phase you generate that will be high. If you're running a machine that has electronic controls such as my wide belt, then you need to be certain that the one phase that the electronics are using is connected to one of those two legs that does read normal to ground as in that case it does matter.

I always mark the manufactured leg with red tape. Then it's easy to make sure that leg doesn't get connected to any electronics.......only to the motor itself.

BH Davis

measuring three phase power between the leg and ground doesn't work. you need to measure between two of the hot legs which usually is anywhere from 220 volts to 240 volts depending on what your power company supplies. the wild leg measured between either of the two other legs will be the same. I have a Delta bank in my shop now which is 120 volts on two legs and the wild leg is about 175 volts. measured between the two is 240 volts =/-. as long as your magnetic starters are 240 volts then it won't matter. the other three phase power you can get from the power company is 208 volts in which all three legs are balanced. some machine motors are 208v and some are 240v. if I want 110v or 220v single phase from my power source I need to make sure not to use the wild leg.