3 Phase Motor Operation on Single Phase Power

I live out in the sticks as well, When you look up at the power lines alond the road, are there one or more power lines? If there are more than one, all you have to do is call the power company and have them install a 3-phase transformer and meter to your place. It may be costly, but the advantages at that point are huge.

Yes, you're right. IF i had three phase running by the place I'd maybe be able to afford three phase service. The closest three phase is down by the interstate, a full mile away. I can't afford to bring that up to the barn.

We got lucky. A mill down the road had the three phase run right by us. Are single phase motors not an option?

All the motors in the shop are three phase motors. I like reversing the motors during operation and a three phase 4HP motor is not practical.

OOPS, sorry. I should rewrite my last return.

All the motors in the shop are three phase motors. I like reversing the motors during operation and a single phase 4HP motor is not practical.

Hi Bob,

I'm a little confused by your statement "After the 3HP motor is running, I can shut down the Rotophase and start and run the 4 HP motor just fine" but I think I know what your looking for.

I'll do my best to answer:

1. What in the world is happening? How does a three phase motor run on single phase? And how does it produce three phase to start other three phase motors?

Answer: Your Rotophase is somewhat a mini genset commonly known as a phase converter or inverter. In essence, the 120V, single phase motor drives the three phase generator. That is how your three phase motor "appears" to run on single phase.

2. If I operate the 4HP motor at its rated power, with the 3HP motor doing its thing, will I over heat the 3 HP motor?

Answer: Based on your statement "It has been acting bad. I think the capacitors in it are going bad. I can start a 3 HP 3 Phase motor on the Rotophase but not the 4 HP motor in the lathe" you may overheat both motors AND the Rotophase if you get enough of an overload on the Rotophase due to the Rotophase not having the capacity to run both motors.

3. If I were to replace the Rotophase with a single phase motor running a three phase motor as a generator, why did I buy the Rotophase?

Answer: It is a little more complicated that just coupling up two motors. Lets not even go there. By trying to home make something to serve your required porpose, I personally feel the safety risk is NOT worth it.

It sounds like your Rotophase may be under rated for the loads (combined?) or just worn out (you did not mention how old the unit is or its size).

As far as a three phase service drop (if available), you need to look at what your input costs will be. There will be a utility fee to install the transformer(s), a three phase service drop, there will be a small monthly fee from the utility for the transformer(s), you will need to install a three phase panel, and conduit in between.

If you are running this equipment as a hobby rather than for income, in my opinion, you are better off to purchase a new Rotophase of adequate capacity to run your equipment.

Good Luck!

KJK

I've had the Rotophase for thirty years. It is sufficient to do the job, or has been at least. The Rotophase is a single phase motor, running another single phase generator with phase 120 degrees shifted. The Rotophase has a big capacitor which shifts the phase an additional 120 degrees. I'm guessing that the cap is failing. When I shut down the rotophase, after I get the 3HP motor running no load, I can start the 4 HP lathe motor just fine. It runs as good as it ever did on the Rotophase. I'm just wondering how all this is happening. AND what I may be doing to the 3HP motor as and if I run the 4HP motor at 4 HP load, which I'm sure I never do, just a question.

Try the following link if you want to try to solve the problem yourself

www.arco-electric.com

Here's a book that has a chapter talks about using 3-phase motors with single phase input. It is fairly practical - not too technical. You might find it interesting.

http://www.amazon.ca/gp/search/ref=pd_lpo_ix_dp_go_uk_ca_en_electric.020motors.020jim.020cox_gl_book?keywords=electric%20motors%20jim%20cox&tag=lpo%5Fixdpgoukcaenelectric.020motors.020jim.020coxgl%5Fbook-20&index=blended

I will look for a single phase motor with a 3 phase generator output- call it a motor-generator set. Then connect your 3 phase motors to the out put. It will be rugged, cost effective, take care of lead, lag issues, easy to maintain. Calculate and do sizing of motor and generator properly.

Solid State Phase inverters are the solution. We don't have three phase power here, only single phase 220.

Until I converted to solid state devices, I was forced to use a Phase-o-Matic to fool the motors. The problem is that not all motors will tolerate that and there is a power loss.

Solid-State units are remarkable. I use them everywhere and have even converted some machines (my big band saw) from single phase to three to gain the advantages.

For one, by varying the output cycles you can regulate speed below and above that normally generated by 60 cps. The better inverters have a torque compensation algorithm. Band saws, lathes and milling machines respond beautifully to this technology without negative side effects.

L.J.

try this out. http://www.isomatic.co.uk/3phConverter.htm#General

get a VFD

oilcan13

Oil can. You get the door prize. Who are the rest of these idiots that are responding. VSDs are cheap and reliable and this solution has been around and back again. It works well, done it many times over the years. It is even preferred using 3 phase motors in water wells to control pressure when only single phase power is available--on the farm. Good old American can do.

Oilcan13:

Please tell me what a VFD is. Does VFD mean variable frequency drive? If so where can I find out more about them?

Bob

Yes. VFD = variable frequency drive. A VFD is the solid-state version of your rotary converter, with the extra feature that you can adjust the output frequency, and therefore a motor's speed. It's what you want and need.

http://en.wikipedia.org/wiki/Variable-frequency_drive

VFDs take either single-phase or 3-phase 50 or 60 Hz power as input, then convert that to variable-frequency 3-phase output, allowing you to control the speed of machine tools with 3-phase motors.

You can find them on eBay, for starters. That should get you going.

James

You need three phase supply.

Rebuilt: Thanks. Finally I got an answer to at least one of my questions. I already know it works because I have done it. The only remaining question is: Will a 3HP motor, acting as the "generator" be enough to reliably start and run a 4HP motor? The 4HP motor is on the lathe, and the lathe has a lot of starting load due to the stuff in the gear box , big 4 jaw chuck, etc., especially when I'm starting the lathe in a high speed gear, lots of inertia. I'm wondering if a short time "overload" on the 3HP "generator" will over heat it etc. I'm sure you'll agree I'm not puting a 4HP load on the lathe motor all the time, just during startup. I'm also not doing heavy cutting with it.

Thanks again,

Bob

Hamish -- rebuilt is giving you good information.

Your 3 hp "generator" will probably do OK starting the 4 HP lathe motor as long as neither one is facing more than a momentary high load from starting the 4HP motor against it's high mechanical inertia. Remember it is the heat from too much current in the windings and its cumulative effect that eventually "kills" induction motors. Seldom except in optimized production applications do we load lathe motors up to their maximum with continuous heavy cuts. Therefore the undersized 3hp "idler" motor is not likely to be stressed by excessive current losses under most conditions where a single operator in a shop is likely to be operating only one manual machine at a time under significant loads.

I'm no expert on this subject. I'm just reading from the "book" (actually a mid 1980's self published 32 page set of typewritten notes and drawings by Robert C. Leber, "Running Three Phase Motors On Single Phase Units and Systems" that received some distribution in the San Francisco Bay area with the help of a San Jose surplus machinery dealer). Who is Robert C. Leber? ...... try

http://www.phaseconverter.com/

This guy knew what he was talking about 35 plus years ago. In 1988 I followed his instructions given in Chapter 2, page 7-8, Figure 5 and set up a 2 hp 3 phase TEFC idler motor fed through a heavy duty switch by both legs of a 230 volt single phase circuit into two of the three motor connections. The third motor connection was fed from one of the first two with 4-20 microfarad 370 volt oil filled capacitors in series mounted securely in an old steel fishing tackle box. Feeds were taken off of each of these three wires and fed through a second switch to the 1-1/2 hp 2 speed 3 phase motor of Jet JVM 840 vertical milling machine.

This system has been working great for 23 years in my shop with no indication of distress to either idled motor or the mill motor.

What's going on here? Well, we know at this point that an idler motor running with small load has the ability to work with a load carrying motor when wired together to shift the phase ("angle", if you like) of the odd leg to a useful phase relationship to the other two phases. ....... once they are running at speed. But getting them started with that setup requires some help. One approach is to provide some external rotation from a mechanical source. But another is to electrically shift that third phase so it is closer to where it should be. Capacitors can do that. I suppose inductors can do that too. But the mechanical engineer in me knows that capacitors are cheaper. I'll leave it up to the sparkies to explain any other considerations.

There are other ways to do the capacitor thing involving starting and running capacitor combinations and the requisite relays or push buttons. Again; leave that to the experts to explain. But one thing sticks in my mind here. That is the reason for using 370 volt rated caps. 230 volts is actually an RMS value. The peak is up around 330 volts. Also there is some power loss in the capacitors that results in heating. For that reason electrolytic capacitors will over heat and explode when used as run capacitors. (Have I got that right?)

Another interesting comment about my mill and its homebrew rotary phase converter. When I first switch on the current to the idler motor alone it growls quite unpleasantly. As soon as I switch the mill motor on the whole thing quiets down. This suggests that the phase relationships in the idler motor are far from ideal when it is running by itself with capacitors on one leg. I wonder if this is also a similar nuisance problem for idler motors that are mechanically started such as by a small motor or a rope pull.

I've read that where people set up large idler motors to run an entire shop they often mount them on foundations outside the building walls because of this noise.

Ed Weldon

Thanks you, Ed Weldon.

I think you have concluded this thread. You have answered all the questions I have had. You have done an excellent job in your response.

Thanks again,

Bob Clark

http://stores.ebay.com/Norman-Machine-Tool-Ltd__W0QQsubmitZSearchQQ_nkwZphaseQ20converterQQ_sidZ2797170

you can use frequency inverter with single phase input & three phase out put. I am using for 2 hp motor. Inverter make is Mitsubishi