Rotary Phase Converter for Hobby Machining.

[Zigeuner]

In an earlier post, I mentioned that I bought a Webb Mill a while back. It has a three hp., 3 phase induction motor that powers it with a variable belt system. When I got it in mid-2009, I was vaguely aware that I needed some form of power conversion. I knew from talking to people I know at the local electrical utility that real three phase power would be difficult to get from the utility. Therefore, I ruled that out very quickly.

My intention here is not to instruct anyone how exactly to build a Rotary Phase Converter since many of you may already know how this is done or have done it yourselves. I mainly want to show what I did.

I know a bit about electricity, enough to have wired a home and barn, but that was all single phase equipment. I do have a 100 amp subpanel in my barn workshop that I wired in some 32 years ago when the barn was put up so there was plenty of single phase 120 and 240 VAC available.

So, not knowing exactly what was required I bought a 5 hp rotary phase converter from the Phoenix company on eBay. I hooked that up to a single phase 240 VAC line and ran it for about a year. It worked fairly well but it had some annoying characteristics. It had a 3,600 rpm idler which made an annoying whine most of the time when it was running as well as the fact that it was underpowered. The mill would bog down on a heavy cut. I finally checked the motor and discovered that it really only drew enough current for a 3 hp motor. Apparently the motor was overrated. At least it seemed that way.

With that as a background I started studying the designs available on the internet for RPC's. I knew that I wanted a real 5hp 3 ph idler with no more than 1,750 rpm. I felt that this would have sufficient power for the mill and would also be quieter. Turns out that I was right. With this design, it will actually start the mill even if, for test purposes, the mill switch is left on while the RPC is started. That's something that the Phoenix unit wouldn't do. I also wanted an RPC that was automatic startup.

I located a very nice 5 hp 3 ph motor with 1,740 rpm on eBay and had it shipped in. It was reasonable siince it was a flange mount and had no base. I had to turn a large ring and drill four holes to mount it but it worked out nicely.

This design (schematic shown below) uses dual 250 µf of start capacitors in parallel in conjunction with a Steveco 90-660 potential relay. The start capacitors will start the motor when power is turned on and when the third leg of the three phase begins to generate from the idler, the relay will remove the start caps from the circuit.

During operation, output of the phases is balanced with two 50 µf run caps in parallel across the two legs L1 to L3 , and L2 to L3. It seems to be OK since voltage on all three phases is very close to 240 VAC when the mill is under load.

I initially used one leg of the 240 VAC to obtain 120 VAC to power the contactor to switch the voltage to the circuits. After some study, I changed this to incorporate a 240 to 120 VAC control transformer. This give me what is in effect a separately-derived circuit with a neutral which powers the relay coil in the definite purpose contactor. Taking one side of 240 VAC to make 120 will work but it's unsafe since you don't want to run any power into the 240 VAC ground. The transformer also gives me 120 VAC for a red power light courtesy of Radio Shack. LOL.

Anyway, the nice thing about a rotary phase converter is that they are relatively inexpensive and they are extremely reliable. I have about $200 total in this one. The frame was built from a surpuls steel coffee table and some bed frame angle iron.

I chose the RPC over a VFD since I only have one three-phase machine and it's a variable speed with belts and it's fine for what I do, mainly small pieces. My other machine, the Enco 13 X 40 lathe has a nice Chinese 2 hp single phase motor that's been running nicely for 15 years now and I'll just keep that as is. 8 speeds is plenty for me.

I understand that VFD's are quite nice but I don't know enough about them to be able to deal with issues that might come up. With the RPC, there's literally nothing that I can't fix on it since there are no solid state or digital features.

I hope that this will encourage people to build a similar unit. An RPC is a neat way to enable them to use some of those larger old machines with three-phase power that they might otherwise not want to buy.

The unit sits to the right of the mill under the table where I can reach it. It's fairly quiet so I just leave it idling while I do setup and so forth. Thanks for listening.





Here's a picture of the insides. It looks sort of intimidating but it's really quite simple.



This is the schematic that I found on the internet. It's a good starting point. With a little swapping of capacitor values, it's possible to balance out the three legs of the RPC output so that they are very close when under load.

 

[Zigeuner]

[quote author=RandyC link=topic=731.msg3835#msg3835 date=1296793600]
Many thanks for that well-written description of a project that can be totally confusing to most of us.

Your installation is neat, professional and considerably less expensive than 5 horsepower units that I've seen advertised ! Maybe you can elaborate on balancing the phases (by capacitor substitution) to obtain maximum efficiency ? Of particular interest would be a description of how to do this using measurement instrumentation that is commonly available to the HSM.

VERY nice job; I hope that my own will look (and function) as nicely as yours if/when I get around to it, LOL. I'm currently operating a European horizontal mill from a static converter. It is a temporary expediency and I've been able to work around the limitations but it is definitely not an optimal configuration !

Thanks again,
Randy C

P.S. Glad to see you here !
[/quote]

Thanks for the kind words. I'm in a recovery mode from a couple of other websites. LOL.

Longish response.

I must confess that I didn't do much balancing. There is some luck involved if you wish to avoid buying all manner of different capacitors.

I've read extensively on the Internet about rotary phase converters. From what I've been able to gather, there are many people who use no balance capacitors whatsoever. The idler will run perfectly without any run capacitors since we are talking about a three phase induction-driven motor. Once started, they will run in a close to synchronous mode. There is some "slip" and inefficiency but that's for a more technical study. Suffice it to say, balancing isn't all that hard.

Some background: When I was running the former Phoenix unit, it failed to start one day. The idler wouldn't come up to speed and would just roll over slowly. Being a good customer, I called Phoenix for some help. I wanted a schematic and they refused to give me one. They did tell me that they thought that it had blown a start capacitor. That tuned out to be the case. Unfortunately, all of the caps were glued in with clear silicone. I then decided that I would teach myself the basics of RPC's so that I would never have to call anyone again.

I literally tore the control box apart and basically reverse-engineered my own schematic. I was able to replace the bad capacitor with a larger, better one and I also replaced the smallish box with a larger plastic brand (available from Home Depot in various sizes, made by "Cantex".) Once I got the schematic figured out, I went looking for schematics on the Internet. I found the one above on a site called Electo-Tech, which I also joined. It similar to the one that Phoenix uses. Thanks to a fellow there (Tcmtech) who posted that schematic. It's the best one I've seen and he also posted notes sufficient to balance the caps.

http://www.electro-tech-online.com/re-projects/100563-3-phase-converter-schematic-miller-system.html

Once your basic components are laid out and fitted into the box, you will need a clamp meter. I bought a Craftsman 82369, which is a digital clamp meter that will read to 400 amps AC or DC. It will read capacitance but only to about 40 µf so it's limited for direct reading, I got around that limitation by testing unknown good capacitors against known good ones in series. The formula for series capacitance is available anywhere and is outside of the scope of this post but it will give a much lower reading than parallel capacitance so it brings the total capacitance into the range of the meter. In any case, you will probably be working with new caps that are good so you probably won't have an issue. Remember that a clamp meter will only read one wire at a time!

The Clamp meter did reveal the mystery of my former Phoenix RPC, however. When running at idle, the meter showed that it was only drawing about 6 amps per line. There was no motor plate so there was no way to tell otherwise. This indicated that the motor was overrated to 5 hp since 6 amps is more like 3 hp. Mystery solved! (For the record, I sold this unit to a fellow with full disclosure. He wanted it for a one hp Bridgeport so it was fine for him).

When I finished the motor and mount, I discovered that a full 5 hp motor won't start on 250 µf like the smaller Phoenix. I coupled two of them in parallel for a total of something just over 500 µf and the motor started right up.

I didn't do anything fancy on the balancing act. I just went with two 50 µf run caps in parallel as shown in the schematic. The result was, IIRC, L1-L2 (Line Voltage) 240, L1-L3 of 246 VAC, L1-L2 of 245 VAC at idle. Under load the voltages drop somewhat to 240's or so and the amps per line are about 12, which is correct for a real 5 hp motor.

According to the schematic, increasing or decreasing the capacitance on the run caps will cause an inversely proportional change in run voltage. That's a matter for experimentation but two 50 µf caps for a total of 100 µf would be a good starting point for a 5 hp idler based on my experience.

As to the other components, I highly recommend the Steveco Potential Relay, P/N 90-66. I was lucky and found two NOS units on eBay. They seldom show up there. If you have to buy a new one, it would be worth it since they are really good. (They are normally closed and will shut off the start cap instantly when L3 voltage comes up close to level)

The so-called "definite purpose contactor" (really just a large 240 VAC relay) should be something like 40 amps per contact. Mine is a two pole 240 VAC with a 120 VAC coil. You can use 24 VAC also but will need a corresponding transformer. I used an Acme control transformer to get 120 VAC and a separate neutral and it's just fine. It draws less than 1 amp and I fused it to 5 amps on each line.

I use a 20 amp breaker for a switch to turn the unit on. There are also heaters in the mill motor, a 3 phase 20 amp breaker on the mill and a 30 amp 240 VAC line coming in so it's well protected.

Interestingly, a good friend is a professional machinist who works out of his ranch down the road from me. He has real three phase in his shop. He tells me that sometimes, he has measured as much as 280 VAC on one of the lines coming in. He also runs a CNC Milling station and a Mazax CNC lathe so it must not make too much difference how well-balanced the lines are, but that's only a guess on my part. LOL.

If you want to avoid using a relay, you could put a small pulley on your idler and start it with a rope. Or you could use a small 120 VAC pony motor with a pulley and a belt that can be loosened once the unit starts.

The best thing that you can do, however, is to get a really good motor for your idler. I wouldn't build anything smaller than a 5 hp since you might want to expand later. If you need more hp, a 10, 15, or even a 20 could be done but then you are gong to have to consider rope or pony start to limit your startup current. You would also need a larger feed line to your idler, of course.

I like RPC's since as I said above, I like to be able to fix anything I use. I checked on a VFD from a dealer and asked him how they could be repaired if they quit. I got the standard "deer in the headlights" response so that was enough to tell me that I would go with an RPC.

VFD's are wonderful and I expect someone to now come in and praise them. That's fine. I'll just turn on my RPC and smile.

Hope this helps.

 

[Zigeuner]

[size=12pt][quote author=RandyC link=topic=731.msg3854#msg3854 date=1296851572]
Very helpful and thanks for taking the time to compose that post. I was hoping that the answer to balancing might be a little simpler - for example simply monitoring the A.C. voltage across each winding and substituting capacitors until the voltage is roughly equal. (Actually, a better method might be monitoring the phase of each winding but that requires test equipment the average HSM likely doesn't own.)

Interesting point about the horsepower rating of the motor - I've heard that expressed as "Chinese horsepower" although it's more fundamentally an issue of truth in advertising, at least in my opinion, which is definitely not limited to a single country !

Thanks again for that information, if/when I get around to building the thing, I will read your experiences over several times before commencing. Also, as previously noted, it's good to see you here. (This is a well-behaved forum and I hope that it will stay that way as it grows.)

Cheers,
Randy C
[/quote]

Well, I think that's all that you have to do. Get a good clamp meter that reads AC voltage and amperage and once the idler is running, monitor each wire for voltage: L1 to L2 line voltage for reference, L1 to L3 and L2 to L3. Once those readings are taken, you could start with two 50 µf caps in parallel as shown. That's a good starting point. Then you could raise or lower the capacitance to balance. You can also test voltage and amperage under load with the mill running. The schematic talks about the inverse relationship between capacitance and output voltage. Amperage should vary along with the voltage based on Ohm's law.

I pointed out that Phoenix sells their RPC's with no motor plate. That doesn't tell you anything. Also, they cut off the output shaft for some reason I can't figure out. I left the motor in mine with the shaft on and made a cover for the rotating shaft. I'm not about to butcher a good motor. I got a motor plate too.

It's important to understand that there's nothing original about the information that I've passed on here. It's all available in various forms on the internet, particularly Google Groups. While I do have some background in electrical matters, I knew very little about RPC's as recently as two years ago. I more or less rooted out all of the information. You and others here can do the same.

The nice thing is you can make use of the wonderful three phase motors that are fitted to these older machines when otherwise we might think that we are limited to single phase units.

Happy Trails1 [size=12pt]

 

[Zigeuner]

Hi Mumbles.

If you have a 3,450 rpm motor that runs quiet, I'm sure it would be fine. The one that I had from Phoenix sounded like a banshee with an internal ailment. It had an annoying high-pitched whine that was sickening. It had to go.

There may be some difference electrically in various types of motors that makes them better or worse for rotary converter use. I really don't know. I was very lucky to get the one that I did. It wa a NOS Boston gear with flange mount and it's very nice indeed. When I saw in on eBay, I jumped and hit the button - $130 delivered!

As to the shaft, I was not about to do any cutting on my unit. I simply got some ABS pipe fittings and bored the hole in my home-brewed flange mount so that they would tap in with a mallet and cover the shaft. I'm the only one who uses the unit so no problem anyway. I can understand that there could be some liability for a shaft sticking out and I guess it would cost more to cover the shaft than it would to cut it off. See the picture below.

As to balancing, I had, for some unknown reason, no problem. The three legs are quite similar, My mill motor is three hp and it will start when I start the RPC if I forget and leave the mill switch on. Can't complain.

There's nothing wrong with a pull start or even foot start LOL. I just followed the schematic and added the auto start circuit since I'm not pulling much current. I've got about all I need as far as an RPC. If I had a larger motor like you have, I'd do a rope start or a pony start.

Nice-looking setup you have. I love those old motors too!