Rotary Phase Converter? Anybody Built One?

[Tom LaRussa]

I find myself constantly lusting after some of the older "BIG IRON" machines I see on various used/auction machinery sites, but the problem is that most of them sun on 3-phase power. In addition, three-phase motors, (for a future cyclone system, upgrading other tools, and the like), can be had for a fraction of equivalent one-phase motors. So, I've looked into building a rotary phase converter.

(For those interested, here are a couple articles on the subject that are written in something at least resembling english.)

http://www.metalwebnews.com/howto/phase-converter/phase-converter.html

http://www.metalwebnews.com/howto/ph-conv/ph-conv.html

QUESTIONS:

Has anybody ever built and used one of these things?

If so, are there any particularly difficult safety and/or code issues to keep in mind when building one?

Is this a realistic project for a home shop?

TIA for any info, tips, etc.,

Augie

[Chris Padilla]

Augustus,

First thing to do is to check your local utility (power) company and see what it would cost to bring 3-phase into your home. Odds are good you'll both have a good laugh but it is worth a shot...it might work out.

As you probably know, you can buy rotary phase converters but they aren't cheap by any means. Check on Grizzly...they sell them. In fact, you can find them at just about any place that sells motors.

[Mac McAtee]

I built a 5 hp rotary phase converter from a scrap yard motor and a $60 static phase converter used to start it spinning.

Keep in mind that you can only run motors that are the same rpm as the phase converter motor. I am not an electronic person and don't play one on TV, so I can't tell you why. You want to run 3600 rpm motors then your phase converter needs to be based on a 3600 rpm motor, something to do with electrical frequency that is created.

You also can only run as much horsepower as the rotary unit. If the rotary unit is 5 hp you can only run up to 5 hp on the system. You try to run one more HP and you will start loosing RPM's on all the running motors.

You must have something to get the rotary converter unit spinning. Once that motor starts turning over then it will run, no problems. But it won't start spinning from a dead stop. I did that with a static converter that I found on an Internet search for "static phase converter". Static converters are rather simple things and not too expensive. You must have a way to get the rotary converter motor/generator spinning. When i first built my unit I used a piece of rope wound around the motor shaft. Pull the rope, motor starts spinning, hit the power and the rotary unit starts running. Once it starts running you have three phase power to your circuit. By the way the motor you choose to make your rotary converter must be three phase to make three phase current.

If I can help you with more information, just get in touch.
Mac

[Tyler Howell]

Wow!! After all that I've got a nose bleed.:o Intesting reading.

thanks guys.;)

[Michael Ballent]

Where's the Advil, that gave me a headache ;) I agree it is interesting

[Jack Wood]

I know that there would be a difference between a residential setup and a commercial one but a friend of mine who manages a fire protection company had to have a 3 phase put in to his shop to run a dry ice machine, very big dry ice machine, and I seem to recall him saying it ran $20,000 grand after it was all said and done!:eek:

[Ian Barley]

Now you see I read a couple of articles like those and they made my ears bleed so I bought a converter.

Well actually I bought mine rather than never quite making one because most of the three phase stuff over here is also designed to run at 400-430 volts so you also need a transformer to raise voltage. Don't you have similar voltage issues?

Good luck if you do decide to build your own - let us know how you get on.

[Rob Russell]

Augustus,

I'm building my own RPC from a 15 HP idler motor that I picked up for $25. Attached are 2 images - a scan of the original draft of my schematic. I've since modified it to correct a few minor errors and share the start and run capacitor banks. The next thing is that I need to current-limit the start, or my neighbors lights will blink when I fire up the RPC. I'm waiting on Ebay to get the right power resistors to add that into the mix. My schematic does not reflect those resistors or the additional contactor I'll need to deal with that wrinkle.

My design is "self-starting", where a voltage relay drops the start caps out when the RPC gets up to sync speed (when it will keep itself going). It also has a 3-phase voltage monitor, so if something happens and I lose an incoming phase, the run cap bank goes south, the run cap contactor trips open for some reason - the RPC output to my 3-phase breaker panel will open to prevent any load motors from single-phasing. Because of the currents, I've got some pretty hefty contactors for the whole thing.

I've also attached a pic of the current state of the RPC. I have an old welding control panel I also got for $25 that I've cleaned out and am laying out the components for my RPC. I got a deal on 100 mf run caps, so I'm actually using a bunch of those as my start caps.

I suggest you read through the posts on this forum - Practical Machinest RPC and VFD forum (http://www.practicalmachinist.com/cgi-bin/ubbcgi/forumdisplay.cgi?action=topics&forum=Phase+Converters+and+VFD&number=3&DaysPrune=1000&LastLogin=). There is a wealth of information here. Scroll through the posts. There are some extremely knowledgeable and helpful folks on that forum.

I haven't finished redrawing my new schematic yet, partly because I'm putting it on the computer and I need to finish Frank Pella's DC layout first. Haven't had the time at home.




Keep in mind that you can only run motors that are the same rpm as the phase converter motor. I am not an electronic person and don't play one on TV, so I can't tell you why. You want to run 3600 rpm motors then your phase converter needs to be based on a 3600 rpm motor, something to do with electrical frequency that is created.



Mac,

I politely beg to differ with that statement. It's my understanding that it doesn't make much difference at all to the output waveform whether you're using a 1800 or 3600 rpm motor. The biggest thing about a 3600 rpm motor is it will likely be a bit louder and it'll take longer to achieve sync speed during startup. My 15 HP idler motor is a 3600 rpm motor. FYI, frequency is driven by the fact that the power coming from the power company is 60 hertz. You can use a VFD to generate power at different frequencies, and a 3-phase motor will speed up / slow down proportionally , but that's degressing to another topic.

Rob

[Tom LaRussa]

Now you see I read a couple of articles like those and they made my ears bleed so I bought a converter.
I don't blame you. I'm afraid this stuff is quite a bit beyond my current knowledge -- which consists of little bits from electricity merit badge in Scouts, Phyics for NON-majors, and watching Mr. Wizard's World with my nephew. :o




Well actually I bought mine rather than never quite making one because most of the three phase stuff over here is also designed to run at 400-430 volts so you also need a transformer to raise voltage. Don't you have similar voltage issues?
I don't think so, but I can't say for sure. Most of the stuff I've seen is marked "3/60/230/460", which I take to mean 3 phase, 60 hertz, and either 230 or 460 volts. (Perhaps one of the electrical wizzes here at the Creek can let me know if that's correct?)



Good luck if you do decide to build your own - let us know how you get on.
Thanks, I will. :)

[Mac McAtee]

"I am not an electronic person and don't play one on TV"

Rob,
You left this out of the quotation. I only know what I have read. If that is not correct then I was wrong and whom ever wrote what I read was wrong.

You obviously have forgotten more than I know about electricity. The RPC that I built and ran was rather simple. A static converter made of a start and run capacitor with a momentary switch for the starting circuit. Then a couple of run capacitors to balance the load between the three legs of the three phase current. It took about an hour to wire it up and get it running. I then ran #10 3 w/ground wire to my table saw and on to my dust collector. It worked and I ran my table saw, 3HP and my dust collector, 2HP on it.

I used it for about 2 years and then had a good deal come up on a single phase motor for the dust collector and a good deal on a VFD for the saw. You are right about the noise. That was the biggest drawback to my RPC, the 5 hp motor noise running all the time in the shop. Had I not gone to the VFD my future plan was to move that cotton picken RPC motor outside some where.

[Rob Russell]

Mac,

There's actually not that difference between your (up and running, functional) and my (in-progress) RPC.

They both use capacitance to start the idler motor.
They both have run caps to balance the phases. (I do hope you have bleeder resistors for when the RPC shuts down, otherwise you could get a deadly chsock from a 370vac rated rub cap.)
You used a momentary switch, I could use a switch to pull in a contactor for the start caps (can't just use a switch because I'll have about 1800 mf of start caps and that would fuse any switch I could find). I just chose to use a potential relay because I then didn't have to worry about when the idler had reached sync speed (more acurately, when the voltage on the 3rd leg gets high enough so it means the idler is at sync speed).


Yeah, I've got some other bells and whistles in my design. The single largest difference is that I'm forced to use contactors for everything because of the currents associated with using a 15 HP idler. There have been times when I wished I were using a smaller motor. Because of some issues I'm running into with balancing the RPC at multiple load points, I may be forced to VFD's or a Phase Perfect anyway.
Rob

[Rob Russell]

Most of the stuff I've seen is marked "3/60/230/460", which I take to mean 3 phase, 60 hertz, and either 230 or 460 volts.



That means the motors are 3-phase, 60 Hertz, dual voltage 230/460. The dual voltage is just like a single phase motor that can be wired for 120/240.

[Ian Barley]

Okay - while we are in the area - surely there is some performance difference if the motor is wired 230 or 460?

[Scott E. Cokley]

Have you thought about using a VFD (Variable Frequency Drive)? It will convert your single phase to 3 phase and give you variable speed to boot! You can pick one up for around $350.00 for a 2 to 3 HP motor. Just search the net for VFD and start shoping.

[Jim Becker]

Okay - while we are in the area - surely there is some performance difference if the motor is wired 230 or 460?My understanding is no. The "higher" voltage allows for "lower" amperage on each wire...the net effect is the same output.

[Rob Russell]

Have you thought about using a VFD (Variable Frequency Drive)? It will convert your single phase to 3 phase and give you variable speed to boot! You can pick one up for around $350.00 for a 2 to 3 HP motor. Just search the net for VFD and start shoping.



VFDs are a good alternative to a RPC, depending on yoru application. Generally speaking, you can only run 1 machine off of a VFD at a time. I've read that there are ways to run multiple machines off of 1 VFD but, as I remember, it was in some pretty specific circumstances that didn't seem likely to occur in my home shop.

Dealer's electric at Dealers Electric (http://www.dealerselectric.com) has good prices on VFDs. You can get a 3HP TECO-Westinghouse for $295 + Shipping, and that's 3HP output from single-phase input.

FYI, if you need to run a larger 3-phase motor and want a VFD, you can run many VFD's off of single-phase output but need to derate the output by about 50%. To run a 7.5HP 3-phase motor, you'd need a VFD rated at about 15HP. You have to play with the programming paramaters and tell the VFD to ignore "low buss voltage", and this is where you have to be sure that the specific VFD you want to buy will let you do that. Still, a VFD to run a 7.5HP motor would be $900+, at which point RPCs become attractive.



My understanding is no. The "higher" voltage allows for "lower" amperage on each wire...the net effect is the same output.

My understanding too - similar to wiring a motor for 120v vs. 240v. The motor draws same amount of volt-amps if it's drawing 20 amps at 230v vs 10 amps at 460v.