phase converter and transformer question

I latched onto an older horizontal mill that runs on 3hp 440V 3ph power. My plan is to hook up a 220V rotary phase converter to a 3KVa 3ph transformer
220/440 and then to the mill. Does anyone see anything obviously wrong with my plan? I suppose I could try and find a 220/440 single phase transformer and then a 440V single to 3ph rotary converter but I think this way might be much more expensive. Thanks, al.
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Most agree that the lower voltage converter, and then a step up transformer for the resulting three-phase is the better way to go. I seem to recall that an open delta transformer can be acceptable for this to keep costs down. The practicalmachinist board has good discussions about this in the VFD section.
Jim
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Al MacDonald wrote:

Are you certain your motor can't be rewired to 220? Worth checking ..
You can convert 220/1ph->220/3ph->440/3ph using a 3ph transformer or you can convert 220/1ph->440/1ph->440/3ph using a 1ph transformer. 3kVA is a little light for a 3ph motor -- ignore the numbers, this is what you learn from the guys who do this all the time. You can probably get away with it but it might fail to start your motor and wouldn't that be a bummer. Which way you convert will depend on what transformer you can get your hands on. A 3ph transformer is more expensive, heavier and larger than its 1ph equal.
The other thing is you might want 220/3ph for running other tools. Then again, almost everything *can* run at 440 and if you can do that, it's better because the current is half so you spend less in wiring and much less in motor controllers.
GWE
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Before you proceed make sure the mill's motor can't be optioned for 220V, 3-phase operation. It is a pretty good bet the motor can be *strapped* (just inside the junction box) for 220 V operation. If the motor can be optioned for 220V it will say so on the name plate.
1) A 220V rotary phase converter connected to 3KVA, 3-phase transformer will work fine. Be aware the transformer will pass everything from the RPC so a *reasonably* well balanced RPC is essential. Three KVA is about 4 horsepower so there will be a 33% "reserve" power capability in the transformer when / if it is loaded to a full 3 HP. Some design practice says there should be a 100% reserve power capability in a transformer, but it is unlikely you will ever work the mill near its full 3 HP rating. So I think you will be alright with the 3KVA transformer.
2) If you have a 220/440 single phase transformer (at least rated for 3KVA) and a 440V, 3-Phase motor of at least 4 HP, you can make a 440V RPC to run the mill directly. Four hundred forty volt RPCs are easier to adjust for voltage balance than 220V ones. The recommended values of balancing capacitance goes down to around 7.5 microfarad per HP. If your capacitors are for 220V service, they can be connected in series to achieve the higher voltage rating. The total value of 2 capacitors in series is found by the formula, C total = (C1 x C2) / (C1 + C2).
Bob Swinney
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wrote:

Sure the mill cannot be rewired for 220? If it has a multi speed motor, probably not. I do have a few single phase transformers big enough, if you want to come over and pick one up.
My Hardinge TFB is 440, with a 3ph transformer mounted on the back side, run by a 220 RPC. Works fine.
Gunner
"Gunner, you are the same ridiculous liberal f--k you ever where." Scipio < snipped-for-privacy@actd.net
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wrote:

Check the motor nameplate in the mill, see if it's straight 480V or 240V/480V dual voltage (which is more common) and also check if it is "inverter duty" rated.
Stepping voltage up for big tools is a problem, as you may not have enough power feed at your house to drive it after you figure in transformer losses and start surges - especially if your house only has a 100A 120/240V 1PH panel. "Watts is watts", and you double the current going in to double the voltage going out.
Instead of a phase converter, you may be better off with an inverter drive that will run the 3PH motor off 1PH power, and can 'soft start' the motor to reduce the start current surge to something your power service can handle.
--<< Bruce >>--
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This will work fine. That is how I run my shop. I have a 240 3 ph distribution panel after the rotary phase converter, and a 480 distribution panel after the 3 ph. transformer. Then whatever new machine that needs to be hooked up can be wired to the proper voltage. That saves a lot of fussing with rewiring motors, control transformers, changing heaters, etc.
Another benifit is that hard to start motors, in my case the 15HP IR air compressor, starts a bit better on the higher voltage (480) as compared to the 240.
Your milage may vary....
Bob
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Another possible solution than what has been mentioned:
Use a 220/440V motor as the phase converter. Wire for 440, and drive ONE HALF of the line connected phase. The converter will be working as a rotary step up, eliminating the need for the seperate transformer. I have a machine that uses this trick to avoid the need for a step down. Not all dual voltage motors can be used in this way.
Al MacDonald wrote:

power. My

transformer
wrong with

transformer
might be

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enl snipped-for-privacy@yahoo.com wrote:

Can you elucidate a little on "drive one half of the line connected phase", please? This is written in English and I know what every word means, and I once had quite a bit of electrical engineering knowledge, but I unfortunately have no idea what you're talking about.
Grant Erwin

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I once had quite a bit of electrical engineering knowledge, but I unfortunately

Neither does he.
Bob Swinney
wrote:

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Neither does he.

Fitch tried it ... didn't work.
If the motor was a wound rotor machine, it might have worked.
As an induction machine, as Fitch tried, no chance.
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On 18 Dec 2004 06:20:08 GMT, snipped-for-privacy@aol.comminch (Peter H.) wrote:

With the right machine it can work but starting is difficult and it needs a vastly oversized efficient machine because you are only applying power to one sixth of the windings and this has to handle the whole of the output power.
Both voltage regulation and efficiency suffer because of the uneconomic utilisation of the windings.
Jim
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(Peter

Jim
I dont know how to analyze this 3 phase motor that is using only 1/2 its windings (poles) to spin it. But, it surprizes me that an induction motor can still run when the number of poles is reduced to 1/2.
I wonder if a 4 pole motor can be re-connected to be a 2 pole motor. But, this gets too complex for me to analyze. I'd have guessed that the re-connected motor would produce a torque that contained ripple.
I'd sure like to know more about how a 3 phase motor can be made to spin when 1/2 itys poles are removed (not connected).
Jerry
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    [ ... ]

    But the number of poles may *not* be reduced to 1/2 the normal number. If the split windings are on the same physical poles, and only electrically isolated, there should be no problem. I just got through typing a fairly long response (which you will not yet have had a chance to see) in another branch of this thread.

    It depends on the design. Two-speed motors are connected just this way. I have even had a three-speed (2-pole, 4-pole, 8-pole) as the capstan motor on a reel-to-reel tape deck. It also was designed as an "inverted rotor" one, so the rotor doubled as a flywheel. Actually, it had three independent sets of windings. It was a cap-run motor, and fairly low torque. I spent some time learning to re-wind motors when that one burned up. And then, once I got that right, the bearings gave out. :-) It *did* come from a hamfest, and the whole deck cost me something like $2.50. :-) I later got an Ampex 440B to replace it. And the 440B has a two-speed (by switched windings) Hysteresis-synchronous motor -- with a separate flywheel. The motor was made by Bodine.

    That's the reason for the flywheel. :-)

    Because, in reality, all of the *poles* are energized -- just separate coils sharing each pole. This is apparently a function of who made the motor. Some have shared windings and can act as an autotransformer, some have separate windings, and can't.
    Enjoy,         DoN.
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DoN
My opolgies - I was assuming the windings, like the ones indicated on the name plate of the motor were always located on seperate poles. That is - all the motors I've seen dont have the poles share stator locations. I just have to study more and post less.
Jerry
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    No need for aplogies. Certainly not to me.

    I don't think that I've ever seen a motor nameplate which really made it clear whether the windings shared poles or were on opposite poles. They were just concerned with how to set up the interconnections for normal use at the two voltages it was designed for. Why document for non-standard use? :-)
    Enjoy,         DoN.
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Right you are, Don! The subject matter of this thread is so far afield from ordinary design practice any attempt to reconcile the original poster's "concept" is ludicrous. Far be it from me to belittle an idea containing even a shred of usefulness in the home shop. This thread, however, goes beyond ridiculous. As our English friend pointed out, it may be possible with a certain type of (rare) motor and then only if one is able to put up with gross inefficiency and start-up problems. All in all, an interesting concept albeit one that will see very, very limited use in the home shop. IMO, the original poster broached an enticing idea, but one that he either lied about or had no practical experience with. He generated a thread that would bring immense pride to any other troll.
Bob Swinney
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Well, I *asked* if he had actually done that. His lack of response speaks for itself - I'll take that as a "no."
Jim
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On Sun, 19 Dec 2004 17:55:57 GMT, "Jerry Martes"

Jerry
The right sort of machine needs to be one that is wound with two independent sets of 220v three phase windings. Either winding is a complete set and capable of being used on its own from a 220v supply.
This means that each slot contains two bunches of conductors - one for each set - series connected for 440v, parallel connected for 220v. Because the bunches of coductors are closely coupled, one leg can behave as an auto transformer.
It's also possible to use the distribution that you've assumed - a 4 pole distribution of the windings which is arranged for 220v supply to one pair of poles and 220v to the remaining pair. The two pairs are series connected for 440v or parallel for 220v operation.
With this arrangement the two sets of conductors are widely separated and too poorly coupled to act as an effective autotransformer.
An off the shelf 4 pole motor cannot be reconnected to make it behave a 2 two pole motor because the winding distribution is wrong - each phase is designed to produce four field maxima 90 deg apart. Efficient 2 pole operation needs two maxima 180 deg apart. The best that reconnection of a standard motor an do is to produce a distorted two pole field that has dip at the all important 180 deg point. Two speed motors have a different winding distribution to overcome this problem.
Jim
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Thanks Jim
I do need to learn more about these motors. I guess that, since I'de never seen a 3 phase motor with poles that contain both windings, I assumed they were either nonexistant or *very* rare.
I'd like to gather up enough energy to reconnect a 220/440 3 phase motor and try the method enl_public describes. I'll try to locate a motor that might allow reconnection so the 440 '3 phase' can be gotten from a single phase 220 power source
Jerry
(Peter

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