Selecting a motor for rotary phase transformer

I am going to build an RPC. I will go to an industrial junk yard to pick a used 3 phase motor, tomorrow. I do not want to buy a wrong motor.

Some questions, possibly very ignorant:

  1. Is TEFC better than drip proof, for me?

  1. Is there a difference in wye vs delta wound, as far as I am concerned?

  2. Do I need ball bearing or I can get away with sleeve bearing (remember, it is an idler, no load or thrust on shaft other than at startup).

  1. What HP to choose? With these motors essentially free, the cost difference between 10 and 7.5 HP is minimal. 10 HP, though, draws a lot more current at startup. My garage has a subpanel that is on a 60 amp breaker.

  2. I salvaged a 1/4 and 1/3 HP motors from two broken vacuum pumps that I had. I would buy a 5/8" ID pulley on one of them. Would they be able to spin up a 7.5 or 10 HP motor? Jim Rozen reportedly used a 1/4 HP pony to spin up a 7.5 HP motor, that makes me hopeful.

  1. How fast would the idler need to spin before I can remove the pony and apply 220V to the idler input?

  2. Is there something that I forgot to ask?

thanks

i
Reply to
Ignoramus5833
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Forgot to say, perhaps I will make a capacitor start system. For the first test, though, I may try it with an idler.

One more question, is it true that the "quality" of 3rd phase is better if the idler motor is bigger?

i
Reply to
Ignoramus5833

If you want a 220 V three phase output, just make sure the idler motor is rated to run at 220 V on the data plate. This will most likely mean a delta (mesh) connected motor. Should you want a 440 V output (you'll need a transformer for this) you'll need a motor rated for 440 V operation. Depending on the size this could mean either a star or delta connection.

Personally I like sleeve bearing motors. They're quiet and there's something nicely "old school" about them. Some of the poorer ball bearing motors don't even have grease nipples. I would check that there is a way of lubricating the bearings from the outside.

If that big transformer happens to be 220 V -> 440 V, keep it! It might be useful for a phase convertor designed for running larger motors.

I am very tempted to build a phase convertor now. It would mean I didn't have to build individual capacitor start and run systems for each machine. I built one for my power hacksaw, but then it stopped working last winter. Probably too complicated and the damp got to some of the components. I will have to dismantle it and locate the problem when I get chance.

I can get 2.5 hp and 22.5 hp motors free for a phase convertor. I reckon

22.5 hp is too big, both to carry and in terms of current demand! Any idea how big a load I could run using a 2.5 hp idler motor? By the way, it is a 940 rpm motor. Would a flywheel to store extra energy improve performance?

Best wishes,

Chris

Reply to
Christopher Tidy

OK, the idler motor in that setup is *five* hp, and it is spun up by a 1/8 hp repulsion-induction motor. It takes about one second to do this.

It should be real close, or slightly higher than nameplate speed to mimimize the inrush currents that flow when you energize it.

You should ask about balancing capacitors and phase angle correction capacitors. Also you really should have at least one contactor in the setup which will drop out (self-holding) if you lose power while the converter is running.

As a data point, my setup will easily start and plug-reverse one hp load motors, and is powered off a 15 amp 240 volt branch ckt.

Jim

Reply to
jim rozen

I like an open frame or drip proof motor, but I've used TEFC with no problems. If there's much of anything in your shop- dust, overspray, etc. for a motor to pick up the TEFC will be more convenient 'cause you won't have to blow it out.

I'll let someonne else answer that, but I think you'll find wye more common.

No big deal here, as Chris Tidy said a sleeve bearing should run quieter.

I can start a 10 hp on my 50 amp breaker, you should have no trouble there.

The closer you get it to the nominal speed the less current surge there'll be when you switch it in. 2/3 nominal will work well.

Take a good ohmeter with you to check the motor windings. The two problems you may find is a short to ground and a shorted winding. Neither makes for a good convertor;-) In my experience it's not common to find a cooked motor that's not shorted to ground in a big way but it happens, measuring resistance from the windings to the frame is probably adequate. If you have a megger all the better, but it's not absolutely necessary for checking a junkyard motor.

John

Reply to
JohnM

Thank you.

I see. Let's say that inrush current was not an issue (it could be on a bigger breaker). Then, if I spin it up only modestly, to a slow speed, would it be able to accelerate to the proper speed?

Hm, I thought that you did not use capacitors. In any case, yes, it is a great issue to ponder, but I cna do it after I get the actual motor.

Do you mean that the contactor would open a 3 phase circuit of the power is lost on the 2 phase circuit?

That's nice to know. I was hoping to be able to run 3 HP motors with this, maybe one day I will buy a nice mill. They will auction great Sharp 3 HP and Bridgeport 2 HP mills from a defunct factory soon.

i
Reply to
Ignoramus5833

good point.

No, I am only looking at 220V.

Okay, I will count your post as a vote for sleeve bearing motors.

No, it is a POS from a big UPS.

Yes, I rather like the simplicity of the concept of an RPC.

22.5 HP is kinda hard to "carry". :) i
Reply to
Ignoramus5833

Well, I use metal grinder, cutoff wheels etc in my garage.

Thanks.

Sounds good. Do you start it from a low RPM?

What I am kind of thinking about is, what if I spin it to 1/10 or some such slow speed. Would it be able to spin up further, inrush current be damned? I am curious about it, practically speaking, a 1/3 HP should be able to speed it up.

I have a megger, although I never used one. I will take my regular multimeter and will check the motor for shorts to frame.

i
Reply to
Ignoramus5833

I live in naperville. Please tell me where you are buying motors so I can get one too.

thanks chuck

Reply to
Chuck Sherwood

I do not want to recommend a place that I have not visited. Maybe they are not nice people. They are in Addison, the place is called Pioneer Industrial Services. I will post a report tomorrow, keep an eye on this newsgroup...

i

PS I used to work at Lucent too

Reply to
Ignoramus5833

One other thing to look out for is the speed; I built mine with a 10HP TEFC motor, but it's 2 pole so it runs at 2800RPM (on 50Hz, would be 3400RPM on

60Hz) and it's damn noisy; you will be better off with a 4 pole 1700RPM motor. Another point to watch for is the orientation, I happened to get one that had bearings rated only for use with the output shaft pointing downwards so I've had to build a frame that mounts it that way up; the positive side is it takes up less floor space and there's no way I'm likely to get caught up on the spinning shaft. Martin
Reply to
Martin Whybrow

I would love to find an idler like that, pointing downwards. Thanks for noting an advantage of lower speed.

i
Reply to
Ignoramus5833

This is different than start capacitors. Call it fine-tuning the output.

Nitpic: It's center-tapped single phase, not 2-phase (which requires 4 wires and has the sinusoids at 90 deg to each other). It just looks like two phases because we measure from the center tap.

The contactor is to keep the motor from trying to restart on its own without the pony motor. A lot of people put them on saws, lathes, and milling machines for the same reason (so sudden starts under load). This is regardless of phase.

Reply to
glyford

Correct. Most purists consider this to be an abomination. :^)

No. It drops out the incoming 240 volt single phase power. Consider the following failure mode: You are running stuff in your shop with the converter spun up. You go upstairs to get lunch and leave it running. While you are up there, the power goes out and the idler motor spins down to a stop.

In the heat of the moment you forget what state the thing is in.

The power now comes back up.

The idler motor is now 'locked rotor' and will draw a large fault current. If for some reason the breaker or fuses do not pop, you have a problem on your hands. A drop-out contactor prevents this problem and is a 'must-have.'

Jim

Reply to
jim rozen

My hope that with a large idler and small motors to be powered, capacitors are not necessary.

I got it. I have a 110v coil 90 A contactor that I could not make work, no matter what I tried. I may spend more time with it.

i
Reply to
Ignoramus5833

That was a capacitor start unit, so it was starting from zero.

They'll spin up from a fairly slow speed but a 1/3 hp motor should be able to bring a 10 hp convertor to pretty near full speed anyway.

Best of luck with it, hoping to hear of success soon..

Reply to
JohnM

Thanks.

I re-read the phase converter webpage more thoroughly.

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I am, at this point, fairly certain that I agree with Jerry and I will build a simple self starting phase converter using run capacitor(s).

i
Reply to
Ignoramus5833

You've got that air compressor now. Get an impact, weld a nut on the end of the motor's shaft, and spin it up before you switch it on. That's the manly solution anyway! Just make sure you go the right way and don't overdo things. (: Impacts can spin awfully fast.

Reply to
B.B.

I used to own a 3600 RPM diesel generator that I would speed up this way, with a cordless 18V drill and the compression release open. Each time was a harrowing experience. I was afraid that tha drill would be caught up and spun by the generator.

Reply to
Ignoramus5833

Yes. What's more it's an old-fashioned cast iron motor made by Metropolitan Vickers. Probably from the 1930s or '40s. A real beast of a machine, but I don't think I can manage it. Would be a laugh to try recovering it though. It needs to be hauled up some steps from a pump room in a factory.

Chris

Reply to
Christopher Tidy

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