Building a rotary phase converter

      How to build a rotary phase converter. November 29, 2000

I would appreciate any information regarding building a rotary phase converter.

Forum Responses
Do you mean, like, from scratch? I don't think you mean that!

What you could do is get a motor and generator and connect them together. Is that what you mean? What capacity do you need (how many HP is your load)? Is it for just one machine or several? I assume you are going from 1 phase to 3 phase. What voltage--same on both sides?

There are 2 basic routes you can take. One is to use a small hp single phase motor to get a large 3 phase motor spinning. Once spinning, the 3 phase motor will continue to spin by feeding 220 single phase to 2 of the 3 legs. The 3rd leg will produce the 3 phase 3rd leg you need for your 3 phase machinery. Once the 3 phase motor (the one generating the 3rd leg) is spinning, you will need to disconnect it from the small hp single phase motor that started it spinning. One way people typically do this is by having the small motor on a slide base that you can move toward the larger motor, subsequently slacking the belt that connects the two.

The other method is a bit more complex, and takes more electrical skills. You can build a capacitor start system for the 3 phase motor, eliminating the need for the small hp motor to get it spinning. These capacitors will have to be matched to the 3 phase motor you will be using to generate the 3rd leg. By being capacitor started, the rotary converter will be "instant on", versus the slow "up to speed" arrangement that results from the small motor start up system.

I will emphasize this now: if you don't completely understand this brief explanation, then you probably don't have the electrical background to proceed. If that is the case, either purchase a commercially made rotary converter, or hire an industrially experienced electrician to put the converter together for you.

I have to agree with the last post. An easy way to do it is to buy a static and use a three phase motor as a slave, which is essentially what a rotary is. Also you must size all your wires as if the motors were all single phase and a sub panel should be installed after the converter. It's really too complex a subject to be dealt with in a single post.

Cheaper to buy a used one and have an electrician hook it up properly. You might end up paying more in the long run unless you have a thorough knowledge of electricity. Also your application may determine other ways you might want to go.

There is a device called an inverter which will give the ability to vary the motor speed which is useful for drill presses, shapers, and milling machines but is only good for one machine, whereas the rotary will run several.

Tell us what you are trying to do, such as how many machines you are trying to run, hp involved as well as future needs.

From the original questioner:
I am not an engineer or a licensed electrician, however I do have a good understanding of the technical issues involved and the rudimentary converter described by a post above.

I have also done a bit of homework on the subject and found a couple of articles on the web. I am interested in how to calculate the size of capacitors both for the starting circuit and to balance the legs vs. trial and error as some authors have suggested.

At this point I'm afraid you leave me behind. I bought my rotary converter used from a friend, and he bought it from a fellow who made it himself. I have replaced one of the capacitors (I think) over the years, but would not know how to guide you on construction design.

I will say, if you aren't already aware, that a rotary converter has it all over a static converter, unless quick and easy setup is your only concern. A static converter will give you about a 25% power loss on your 3 phase motor, whereas a rotary will maintain full hp rating. Balancing the legs on the rotary converter is an issue, although I think not totally critical. I have some imbalance with mine and the only problem I've experienced in nearly 10 years was that a current limit switch on my SCMI shaper would trip out for no "apparent" reason. We believe it was because of imbalance in the 3 phase legs, and resolved the problem by installing a slightly higher range limit switch.

Also, the inverters are indeed very nice. I have one on my CNC router, but as stated above it will control only one motor at a time (I suppose you might run more than 1 motor at a time if you were looking to run both at full speed, but I'm not certain on this). The only downside is that they are still, in effect, a static converter, and will result in a power loss to the motor.

I'm no electrician but have done a lot of research and had to install many phase converters and such in different applications for people. As a service tech I got to talk to many different experts, electricians, manufacturers. A three phase motor will run on 220 single phase and the static type only kicks in to supply third leg to start, then does absolutely nothing, hence only 2/3 power from the motor. Haven't done anything yet with inverters but my motor guys tell me they give full power and are not like static at all. They are solid state and have reliable info that many can actually give variable speed from a few hundred rpm to double the motors rated rpm.

Also, Baldor makes a neat one for three phase motors 1 hp or less that will do this as well as work on 110 volt single phase. Pretty amazing. They have one set up on a small drill press. Haven't seen it personally but plan to get over there soon to see for myself. Been quoted a price of around $250. This has a lot of possibilities.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor A:
Just a few comments on some of the previous posts:

"I will say, if you aren't already aware, that a rotary converter has it all over a static converter, unless quick and easy setup is your only concern. A static converter will give you about a 25% power loss on your 3 phase motor, whereas a rotary will maintain full hp rating. Balancing the legs on the rotary converter is an issue, although I think not totally critical."

I know a fellow who runs 3 CNC lathes (about 15HP each) on one 100HP rotary converter with no line filtering at all. Pretty amazing. I used a digital AC meter to check his 3 phase and that wild leg was really WILD, but his setup works.

"Haven't done anything yet with inverters but my motor guys tell me they give full power and are not like static at all. They are solid state and have reliable info that many can actually give variable speed from a few hundred rpm to double the motors rated rpm."

Be real careful here. It's true that these devices can vary the speed of AC motors, but common AC type motors have a cooling fan that is engineered to cool the motor at rated speed. If you slow the motor down, the motor fan can't move enough air to keep the motor cool and it will overheat the windings. NOT GOOD. The answer for this setup is a separate blower that runs at a constant speed to keep the motor cool.

Comment from contributor B:
Building a rotary converter is actually quite simple. As far as the capacitance needed, it's approx. 100mf of electrolitic per hp of the idler motor for starting, and approx. 15mf of oil-filled for the run circuit. The trick is to drop out the start caps once the idler motor is up to full speed. I have found that 4 pole motors connected delta work best for the idlers. I build converters that can be loaded to 300% of the rated hp of the idler motor.

Comment from contributor C:
I'm a licensed electrician. I've successfully built many phase converters over the years, however with the new wave of electronics out there, there is a much better solution to your problem. The devices are referred to as frequency drives. The original intent of these was to vary the speed of an AC induction motor, but over the years they have been perfected. The nice thing is that they not only vary the speed of the motor, which is great for a wood lathe, but they also convert 220 OR 115vac single phase to three phase. A 5 horsepower drive will run approximately $200. If you use one you will fall in love with it.

Comment from contributor D:
I just finished building a 5hp rotary converter and it works great. I am going to use it to run a vertical mill as well as a lathe and drill press. I have already hooked the drill press to it and it works flawlessy. You do not have to be a rocket scientist to build one. I do machine work and making this converter was a much cheaper alternative than converting all of my machines to single phase or buying a manufactured unit. It is not very expensive and pretty easy to make a converter if you have good basic knowledge of electrical components. Personally, I am not sure what the other postings are concerned with.

Comment from contributor L:
I have built several RPCs. I use a 15 HP three phase motor as the distributor and a 1 HP single phase to run it up. Mine has been in use 20 years and is still going strong. It runs my metal lathe and compressor. The input is 480v single phase which is common in the country of Australia.

Comment from contributor M:
If you want to keep it real simple to start with: find a 3-phase motor that has more hp than the motor (or motors combined) that you want to run. For instance I have a 3 hp Delta Rockwell tablesaw so I bought a 5 hp 3 phase electric motor. I ran my supply 220 to the three phase rotary motor and then I wired my saw motor to 5 hp 3 phase motor. Then I wrap a cord around the shaft of the 5 hp motor and pull it to start it spinning. Next I throw the breaker for the rotary (converter) 5 hp motor and it starts up. Once I get that 5 hp 3 phase motor running it is generating 3 phase power. I can then turn on my table saw. I bought the 5 hp 3 phase motor used and rebuilt for $80.00. The 60 amp shut off box cost $30.00 used.

Comment from contributor E:
I've been using a static 3 phase converter 7 1/5 HP for over 40 years. I use a 7 1/5 hp idler motor. I run an Agie EDM , a Brown and Sharpe grinder, a Bridgeport milling machine, and a CNC Lathe. I usually run 2 machines together. However when my idler motor was down for bearings I would turn on my other lathe and use it for an idler motor.

When performing instant reversing on the lathe or the milling machine, sometimes the idler motor would reverse. I fixed that by installing a 30 lb. inertia wheel on the idler motor. My phase converter uses a potential relay to switch out the starting capacitor which is in parallel with the running capacitor on the third leg. When I connected the CNC and the EDM machines I make sure the third leg is connected to the motor and not the electronics. I have at one time used a Lincoln Heliarc electric welder with it and havenít had any problems.

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