My Rotary Phase Converter Project w/ pics

As mentioned in an earlier thread I purchased a used Ronk Roto-Con/Mark II rotary phase converter with the following specs.
Converter size - 20 KVA (Baldor motor)
Largest single 3 phase motor - 10hp
Total 3 phase motors - 20 hp
Single phase input - 96 A
After given some confidence by members here that the system will meet my limited needs with possibly some modifications I decided to forge ahead. As can be seen in the pics, 3 of the 8 capacitors (bank to the right) are disconnected or shorted out. Apparently that was done by a previous owner tailoring it to his needs. After watching a video by sbirdranch I thought it might be a good guide for my project. I believe a member here provided most of the guidance in his project. So thanks in advance to both. The first thing I want to tackle is to make sure the Baldor motor works and toward that end noticed the Ronk panel does not have a start capacitor. My plan is to wire or re-wire the panel based on what sbirdranch did but I don't know what size start capacitor to purchase. Any advice would be appreciated.

The capacitors in the box may be the equivalent of a start capacitor. Better to draw out the circuit diagram and post the picture.

Yes, the unit seems to use the "start/run" capacitor setup. I have an Arco that has that as well. motor used is generally a special with no output shaft, and which has a rotor type that is easier to start than the usual. Often that is a "dual cage" type rotor. It is a bit less easy to modify, because there still has to be enough capacitance to start the motor. So reductions are less easy. But, probably it can be done even in your case. First, do you need to do anything? Check the unloaded voltage and see if it is too high. That is pretty much the only thing that can be "wrong", an RPC is a simple device, and inherently (without run caps) produces a voltage that is typically a bit lower than the incoming line. The "balance" or "run" capacitors are added to kick up the voltage a bit. Measure and report! Just see what the smallest motor you will want to have connected is, and verify that the voltage you get is sufficiently low when that motor is connected. Often that motor will be relatively small, such as a coolant pump. That can cause a problem, but I suspect you can find a start/run cap value that will work fine.

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Unfortunately most of the wiring in the panel was either modified or cut off when removed from previous service. So it's almost like starting from scratch. As mentioned before I would like to get the motor running first before tackling the rest of the converter setup just to make sure I'm not beating a dead horse. I checked the contactor today and it makes a horrible buzzing sound. Apparently that's a sign that it should be replaced. I also tried to check a few of the capacitors and failed in that attempt. I'm not sure what I'm doing wrong but I get no sign of life on my multimeter. If they're bad those would have to be replaced as well.

beeser said:

Unfortunately most of the wiring in the panel was either modified or cut off when removed from previous service. So it's almost like starting from scratch. As mentioned before I would like to get the motor running first before tackling the rest of the converter setup just to make sure I'm not beating a dead horse. I checked the contactor today and it makes a horrible buzzing sound. Apparently that's a sign that it should be replaced. I also tried to check a few of the capacitors and failed in that attempt. I'm not sure what I'm doing wrong but I get no sign of life on my multimeter. If they're bad those would have to be replaced as well.

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If you have a doorbell transformer or something that can output ac voltage around 12-24V you can put the capacitors across that and measure the current.

If you let me know what voltage you have (measure it with the capacitor hooked up for most accurate results) and know the capacitance, I or many other people on this forum can tell you what the amperage should be.

Also, looking at typical dual rated start/run capacitors I see that the start capacitor is typically 5 times the capacitance of the run capacitor. That should be close enough to start an idler.

before getting excited about the supposedly "missing" start capacitor, you need to get the thing running as it was intended, which appears to be a start/run setup with no specific start capacitor.. Then check the no-load voltage. It may be fine. I would then also check the voltage with the smallest motor load you expect to use. A short test will not hurt a motor, and you can start with a larger motor to get a feeling for what is going on, if the smallest is very small, such as a coolant pump. This is assuming that the "motor" being used as an idler is the one the panel as made for. A clue there is whether it has a shaft sticking out. If so, it is likely not the original, if that was a commercial RPC.

As for checking capacitors. put meter on ohms, put probes on the two capacitor terminals (make sure the cap is discharged first). You may see a short indication of some resistance. Now reverse the probes, and you should definitely see a short indication of resistance, which will increase until it shows an "open". If your meter has resistance ranges, set it to one of the higher ranges, somewhere around 100k ohms or so. (the site will not allow edits at the moment, so I have added this as a post)

I tried checking the capacitors the way suggested but get no response at all from them. Maybe this is a project that should take a pass for the following reasons.

I just found out that 2 gauge wire should be used to feed the converter. The location planned for it is a good distance (80 ft.) from the power source, which will cost a lot more than the 6 gauge wire originally budgeted.

My needs are a lot less than the 20hp motor will provide, a nice thing to have but at considerable expense. As mentioned before I only have a 3hp, 2 speed lathe and a Bridgeport mill that requires 3 phase power. I may be able to buy a 7 1/2hp or 10hp rotary phase converter from one of the online vendors for about the same cost as what the 20hp used converter will end up costing the direction it seems to be taking.

And lastly, I may be over my head with the project. I enjoy the process of learning something new but this project is beginning to make me uncomfortable.

All you need is a 5Hp Motor and remove a few capacitors. No big deal. Get a used 5Hp motor and sell the 20hp.

I start a 10hp RPC no issue on 8 gauge wire. If you stay below amperage draw requirements I have little doubt you can start a 20hp motor with 6 gauge.