phase converter and transformer question

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

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

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

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

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.

-->--

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

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 MacD> I latched onto an older horizontal mill that runs on 3hp 440V 3ph power. My

transformer

transformer

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

I *believe* that fitch tried this at one point, and that it did not work.

Jim

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

Neither does he.

Bob Sw>> Another possible solution than what has been mentioned:

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.

How is this possible? Unless you have full supplied single phase 220V and a start cap for the third leg, you wouldn't even be able to start the idler motor. Right?

transformer. I

Many 220/440 indution motors (I have several) are converted between voltages by connecting the coils either in series or in parallel, similarly to selection of the windings on dual voltage transformers (such as the fairly comon 220/440 input on many welders) If the connection is made for 440, then there are two series connected windings on a common core and if ONE of these windings (half of the series connected pair) is powered with 220, then 440 will be seen across the pair (it is acting as an autotransformer in boost configuration)

The same behavior is exhibited by the windings on many 220/440 three phase motor-- each phase is two windings, either series connected, or parallel connected. The motor must be designed for delta connection.

A motor designed for 220/440, why connected, would again need all three phases series connected, but the line connection would be from the center tie of one phase to the center tie of another, the 480 exhibited at the outside of the three phases.

Correct. A starting means is still needed. With a delta connected motor as phase converter, the third leg of the starting circuit is connected to the center of the phase that shares the corner with the primary power connection. With a why connected motor, the third leg of the starting circuit goes to the center of the phase not used for the primary power.

You got any 'scope pictures of that?

Bob Swinney

>

enl_ Does that indicate that a 220/440 3 phase motor will spin properly if only

1/2 of windings are connected?? If I wasnt so lazy, I'd go out and try setting up an experiment. But, it sounds like you already know that the motor will spin properly when only 1/2 its windings are energized. The fact that the motor spins at all with only the one set of windings surprizes me. Jerry

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

Have you done this?

Jim

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