building rotary phase converter

The common type of "static converter" has a current sensitive relay (or perhaps a voltage-sensitive one, depending on connections) which switches the start caps in only until it is up to a self-sustaining speed. If you load it down enough, it will switch back in, and if you keep that much load on it, it can quickly burn out the start caps. This describes the relatively inexpensive ones such as those normally sold by Phase-o-Matic and others.

From some earlier postings on this newsgroup, apparently there are some which use tapped inductances to achieve the phase shift, and maybe continue to feed that even when the load is up to speed.

But you could achieve the same effect as the normal "static phase converter" with a momentary contact switch and the caps, and just hold it in long enough to spin up the motor. Some rotary converters are set up like this to start the idler. This is a royal pain without the idler, for a motor which you are starting and stopping frequently, as some of these days you (or someone else) will lean on that start button just a bit too long, and there goes the start cap. :-)

I suspect that the torque available from single-phase operation may be a function of both the number of poles in the motor's design, and the amount of iron in both the rotor and stator. And it may be possible to get (for some combination of the above) more starting torque (as the earlier article suggested) than the rated running torque. This is in part because the motor is drawing a lot more current when starting. And IIRC, the cap array being used to develop starting torque was tuned for optimum starting behaviour. That cap will probably be a very poor value for normal running.

So -- for normal operation, I think that *any* reasonable source of three phase (power company, VFD or rotary converter) will produce more power from the motor at peak than the same motor running from a so-called "static" converter.

Enjoy, DoN.

Eric

You might be overemphasizing the amount of degradation in power delivering ability of a 3 phase motor when running from single phase. The power and torque of a 5 HP 3 phase motor will be very little different for either single phase input or 3 phase input when the load is 3 HP or less. A major difference in the single phase compared to 3 phase is the smoothness of torque throughout each revolution. The instantaneous torque from a single phase motor (or a 3 phase motor spinning without the rotary) will go to zero within each revolution. That doesnt happen with the 3 phase power.

Rotary converters certainly make a 3 phase motor run smoother compared to running the same motor without the idler, especially when loaded heavily. When a 3 phase motor is loaded to less than 1/2 its name plate rated power, the power to the "tool" is very close to the same, with or without a rotary converter.

Big rotary converters are very noticeable in smoothing the 3 phase motor running from single phase power when the load motor is at or near full name plate rated load.

Jerry

One way you could optimize each machine is to put caps on each machine (on the machine side of the machine's contactor). This way each one could have a different set of run caps. I haven't bothered to do this of course...

Steve Smith

Grant Erw> What is your definition of C1 and C2? Won't things be wrong if you run

Jerry, The motor behaved virtually the same at low loads. Instany reversing, such as when power tapping, was slower. But when taking heavy cuts the bridgeport would bog down when run off the static converter but be just fine with the rotary. I machined lots of stainless in the past and would take the machine to it's limits on a regular basis. The rotary converter saved me lots of money because of the time saved taking heavier cuts. Eric

Eric

The data I recorded indicated that there will be virtually no difference in performance of a 3 phase motor loaded to 1/2 its name plate rated HP, when run from single phase, with or without the idler. Is that what you experience?

I also notice that, when the 3 phase motor is loaded to its full name plate rated HP, without an idler, the RPM drops 15 or 20 RPM below the RPM of the same motor *with* an idler. And, if the motor is loaded to 'over name plate rating', it will stall completely without an idler. But, with a big idler, the motor will deliver more than its name plate rating without stalling. Is that about what you experience?

Jerry

Jerry

Any difference in smoothness between single-phase and generated 3-phase? - GWE

Grant

I cant distinguish any difference of 'smoothness' *with* or *without* an idler when the 3 phase motor runs from single phase but loaded to 1/2 its name plate rated HP. When the 3 phase motor is loaded to its full name plate rated HP, the idler really helps smooth the Torque per Revolution. The bigger the idler, the smoother the motor runs, at high load. At low load, the motor runs the same *with* or *without* an idler.

I tried to build a sensor to monitor and record the "unsmoothness", but got distracted from that project when it got difficult.

Jerry

Not an exact answer and maybe not even a close answer but when I changed the crap single phase motor on my mill drill to a leeson three phase motor and VFD, I could see a much smoother surface finish when milling.

chuck

The torque of a 1 ph motor (or a 3 ph motor run on 1 ph) goes to zero 120 times a second (100 times a second in Europe). The torque of a 3 ph motor run on 3 ph never goes to zero. So the 3 ph will always be smoother.

As others have noted, a static converter of the sort typically used is just a motor starter. It does not supply the motor with 3 ph power when the motor is running. However, a rotary converter does produce actual 3 ph power, so the load motor behaves as if it were operating on utility 3 ph power.

Gary

Chuck

Yeah, thats about the same subject. The theory indicates that single phase motors of this type do have a Torque per Rotor angular rotation that has zeros. So, the motor actually produces zero torque at some position, or positions, of the rotor shaft. Three phase motors's torque doesnt go to zero at any angular position, so they are much smoother.

Big idlers really improve the smothness of a 3 phase motor runing from single phase. But, that improvement is realized only when the tool motor is loaded well above1/2 the tool motor's name plate rated HP.

Jerry

Jerry

An interesting experiment - maybe it's one way of rescueing a motor with a single dud winding.

Jim

Firstly my comments only related to the setup of static converters which have facilities to switch from a large single start capacitor to smaller capacitors chosen to optimise the run performance. These static systems have no rotary idler motor so optimum choice of the run capacitor arrangement is vital.

Setups that that also include an idler motor are much more more forgiving because the idler is always running on no load close to synchronous speed and this reduces the variabilty of the operating conditions. With a sufficiently large idler it is possible to operate as you have done with a variable number and rating of load motors switched in to circuit without change of the run(tuning) capacitors. This also makes them more tolerant to the capacitor setup and they can survive with capacitor setups that would severely degrade a purely static arrangement.

For a purely static setup each motor would need to be set up at least with it's own separately chosen run capacitor and,unless one motor is always running, each will need it's own starting circuit.

To answer your questions.

C1 and C2 are generally referred to as "run" or"tuning" capacitors and are permanently in circuit once the motor has reached operating speed. C1 is connected from the live side of the supply to the phantom phase motor terminal. C2 is connected from supply neutral to the same terminal. Circuit diagrams usually show them bridging the motor terminals which is of course technically accurate. I prefer to show them as two capacitors series connected directly across the supply with a wire joining the central connection to the motor phantom phase terminal. This is electrically identical to the usual depiction but shows clearly that the capacitors are acting as a voltage divider. It also makes it obvious that the capacitors make partial power factor correction i.e. in addition to their primary function, C1+C2/C1xC2 - their series connected capacitance value appears as power factor correction across the supply.

The load motor(s) will run in the direction controlled by the sequence in which its' windings are connected.The back EMF's at the motor terminals will be just the same for either direction so the load seen by the converter will be unaffected.

Your comment that it's difficult to discover how to fine tune a system with multiple loads and maybe it's academic is entirely valid. If you've a reasonably large idler and are not trying to extract the last oz of torque from the load motors, accurate tuning is not going to make that much difference.

Jim

I'm still confused. Most of us run 3 phase motors at 220VAC, right? At my house 220 power is between L1 and L2, in other words you have 2 hot leads with respect to earth ground. Your comments would make sense if referred to 110VAC wiring, but who uses machinery motors at that voltage?

Grant

Jim

That would work but, with a dud winding, it'd have to be spin up with a pony (or rope).

I went inside the 3 phase motor and disconnected one the *Y* windings and reconnected it to be the start winding for single phase use. I rate this "rewire" of a 3 phase motor for use on single phase as an interesting project with very little usefulness. And, all 3 windings would have to be useable.

Jerry

Sorry - I slipped into a UK specific habit. Our 240 AC domestic supplies arrive as a three wire system - Live and Neutral and Earth (Ground). For your supply read "live" as L1 and "neutral" as L2

As far as the motor is concerned it doesn't care which way round the supply wires are connected - reversing the connections to the supply makes no difference.

Jim

Thanks again for a post that gives me a better insight. You are one of the people that post on RCM that I make a point of reading. Do you teach electrical engineering?

Dan

Glad to hear you find the posts useful. I've learnt a lot from this group and I try to make some return.

In answer to your question - no - just a retired electronic engineer with a fairly wide range of interests.

Jim