Hello all, another question I wanted to know why do people put idler motors running on 3 phase power created by a rotary phase? Does this help balance out the load on other machines? Yes I still pondering my power issue in my shop when I turn on my milling machine and the neighbors lights dim out abit. I have talked to an electrician and he said 100 amp service is plenty and he thought a 25 kva transformer was sufficient.
Without an idler motor, you have no 3rd phase generated. You can run a 3-phase load motor (like your mill's) on single phase power, but it will only develop about 1/3rd the horsepower and certainly won't run as smooth.
Idler motor = 3rd leg
Static phase converters don't have an idler, they are just starting mechanisms which allow you to get your motor going but it won't be as smooth and you won't get the power it's rated for.
Carbonite, All induction electric motors act as generators when running. The opposite is also true. Induction motors spin because of a rotating magnetic field. In a three phase motor energized from the single phase line the rotating magnetic field generates current in the third, unenergized, leg. This leg is called sometimes the wild leg or the manufactured leg. Using an idler motor bigger than the motor run from the converter will enable the smaller motor to do things like instant reverse, just like the motor would do running from utility supplied three phase. When using a static converter to start a three phase motor the capacitors in the converter shift the phase so that the motor will start to spin. Once the motor is spinning the starting caps are disconnected from the circuit, leaving only the running caps connected. It is not necessary to use capacitors to start a three phase motor from single phase. If the three phase motor is first spun up to a minimum RPM the motor will continue to spin when energized. Some rotary converters use a small single phase "pony" motor to spin the large idler motor and when spinning fast enough the idler is connected to the single phase line. Once spinning under its own power the idler is disconnected from the pony motor. A real simple way to do this is to use the weight of the pony motor to tension a belt that drives the idler. Once the idler is up to speed a board is used to raise the pony motor so that the belt flips off. If the idler is spun up to to operating speed with a pony motor and then energized it will draw much less current at start. This helps prevent dimming lights. Induction motors can draw more than three times the current when starting than when running. Capacitors are used with rotary converters to either start the motor or balance the voltages from the converter. A rotary converter can have either or both. For a Bridgeport mill a simple rotary converter can be made with a 5 hp three phase motor and a 1/4 hp single phase pony motor. The three phase output won't be perfect but it will work fine for the 2 hp mill motor. ERS
Do I understand that you state a 3 HP 3 phase motor running from a single phase source will develop only 1 HP at it's shaft? Do you have any data on what would happen if a 3 HP, 3 phase motor running from a single phase source is loaded to 2.8 HP??
Your understanding is correct. I have no data, but my intuition is it would stall; i.e. stop turning and draw an immense amount of current.
Of course, I'm just citing conventional wisdom most of which I learned from reading this newsgroup consistently since about 1993. I was amazed to find that I can start and run a 7.5 hp load motor with a convertor with a 7.5 hp idler. So maybe I'm wrong. Love to be. For years I had a Phase-A-Matic (commercial flavor of a static phase converter) installation/owner's manual sheet. I believe it said that you can't ever get nameplate horsepower out of a 3 phase motor using single phase power via a static phase convertor.
I am certainly not a Phase Converter Expert. But, a couple years ago I was inspired by Fitch's work with converters and I built a small dyno for accumulating data on 3 phase motors driven with single phase. Don Foreman was polite and patient enough to help me figure out how to buld a dyno and get good data from it. The dyno that Don showed me how to construct worked well. Excel did the rest of the work. As with so many things like this, The more I learned, the more I became aware of how little I really knew.
In the process of accumulating data on 3 phase motors driven by single phase power, I learned that a 3 phase motor can deliver 98 percent of its full name plate rated HP, but at a lower than name plate specified RPM. I didnt even try to get data on heat and vibration. I was certainly surprised to see that the 3 phase motor could deliver so much power even though it was driven by single phase. I would like to se some data on the
3 phase motors that *cant* deliver nearly their name plate rated power when driven on single phase.
I remember those threads and I am a great believer in actually trying things to find out if and how they work. Did you get any data to determine how much power a 3 phase motor could develop on 3 phase power if you did not consider RPM drop and heating? Somehow in the back of my mind I seem to remember that several times nameplate horsepower can be developed for a short time.
No I didnt follow up on learning about the heat limit for the 3 phase motor as a limiting factor for Max HP while running on single phase. It would have been fairly easy to do, but it would require alot of time/interest because it meant running the motor at full load for a long time. The 3 HP 3 phase motor that I was using when I ran the HP vs "load power" sure didnt get warm while I was loading it to 3 HP several times while evaluating the effects of the Idler. There is so much thermal inertia in the motors that it might distribute the heat from the excited windings to the heat dissipating area around the unfed windings. So, to make a valid comparison, I probably have to install heat sensors within the windings. there is just too little interest in the data. I discontinued my investigation when I realized that there was so little interest in actual data.
I can send you the Excel data and plots I made while testing the 3 HP 3 phase motor with and without the 5 HP idler. The material is not well organized but it is pretty accurate. It sure was repeatable. That was accurate enough for me.
The 3 HP 3 phase motor RPM dropped to 1730 when it was loaded to 2.9 HP. The same motor dropped to 1740 RPM when loaded to 3 HP when the 5 HP idler was inserted.
One thing I found very interesting was that any/all the tests I made (and there were many) showed that, if the 3 phase motor is loaded to about 1/2 its name plate rating, it didnt matter (for HP /RPM) if there was *any* idler in the system. Any given 3 phase motor I tested would drop to the same RPM when it was loaded to about 1/2 its nameplate rated HP, no matter which idler was used, from zero idler to 15 HP idler. So, if the home/hobby user had a 3 phase motor with a nameplate rating of about twice whatever could be anticipated as the normal / max load HP, there is little need for an idler *or* tuning.
I'd really like a copy of that Excel data. Just to look at, if you don't mind. Please send it to
I've noted here a number of times, that I use a home-made auto-start
5HP RPC and distribute its output via single phase 220 "laundry" sockets and plugs located in various places around my shop. My machinery includes a 2.5HP geared-head lathe (which is NEVER asked to "struggle"), and I have a second 5HP motor on wheels I can bring out and plug into one of the sockets when I want to run my 5HP lathe. This second motor has a starter contactor with the coil across the "generated leg", so that this "idler" (your term---I like the term "additional output") will not attempt to start unless the prime RPC is already operating.
Thanks for the info, Jerry. I have a home-built 5HP tuned converter and have only used it so far to run a 5HP Kearney and Trecker mill and a 3HP Thompson surfaace grinder. It starts the mill quite rapidly. The mill has a clutch which reduces the starting load quite a bit. I also expect to use the converter for powering some electronic equipment.
I've not seen any quantified data but the theory is that the idle motor acts as a generator to stabilise the 3 phase supply. As a generator needs to be driven, we must ask what mechanical input exists. There is no direct power driver so it's only source of energy is it's own inertia. Adding to this energy store with a flywheel will therefore increase the amount of electrical energy it can produce. The exact electrical effect will depend on the generating efficiency of your idler motor. Exact mechanical figures can be calculated from energy stored in a flywheel. Google flywheel energy without quotes and there are lots of sites. One point to be careful of is to make sure your flywheel is capable of withstanding the centrifugal forces developed. When flywheels let go, the results are dramatic and can be fatal. I take no responsibility for any failures I'm afraid. Again, Google for details.
The flywheel will smooth the mains supply by generating the peak starting loads of motors. It will not make a 100A supply work like a
200A one so if the lights dim and stay dimmed, it won't help. If the dimming is a temporay problem, it will help if not eliminate it. The mechanical equivalent is the engine flywheel. If an engine has a light flywheel, the speed variation per revolution is quite high either side of the power stroke. In extreme cases, the engine stops as the flywheel isn't going fast enough to push the piston up the compression stroke. Anyone who's tried starting a single cylinder diesel engine by hand knows the problem. The greater the flywheel inertia, the easier it is to start and the smoother the power. It also allows them to run slower. ISTR the flywheel on a model T Ford weighs about 80 pounds allowing the engine to run slower. Flywheel energy storage is now taking over from batteries in UPS's. Admittedly their delivery time is quite long and there are loads of electronics involved which allow almost complete energy recovery from the flywheel. We're not looking for whole minutes of power delivery and we want a cheap solution. Bolting an automotive flywheel onto the slave motor will help smooth the electrical power by keeping it spinning at the best speed longer.
Do you have any empirical data to support your theory, or is it just that it "makes sense"? I thought so too! Until I made the first one. This same discussion was making the rounds here and on Modeleng-list back then, and some of the other guys I respect were writing that with "their" theory it made little or no difference, but that you do end up with a metal mass spinning, a loss of some space it occupies, and at least some small hazard in it being a rotating surface providing something else to get "snagged" on. So, I made mine with the 30 pound motor coupling (special use motor) still in place, set-up the "starter/run circuit and had it running OK. Used it like that for a month or so. Then I cut-off the shaft flush with the end of the bearing housing. As mentioned above, the only difference I can see is flywheel versus no flywheel on the same motor there is/was no difference once it was up to run speed and in use (lathe/mill/grinder). What the flywheel DID do, was slow the start-time perceptively. The timer to cause the start caps to drop out was almost 3 seconds with a flywheel, and only a little over 1 second without the flywheel.
And Jerry Martes empirical tests data supports this. The greatest difference affecting running, is the HP of the "idler" versus the HP of the "load" motor.