phase converter questions

Are the motors truly in parallel? As in; all three legs of one motor connected to all three legs of the other motor? If so, you should see some

3rd leg current between the motors. Is there a separate switch on the 1/2 HP motor? If they are in parallel you should be able to switch on and start the 1/2 HP motor *after* the 1.5 HP motor is up to speed. The larger motor will be generating some unbalanced 3-phase power by transformer action. It is acting like an idler motor in a rotary phase converter configuration.

On 25 Aug 2004 19:18:36 GMT, snipped-for-privacy@w-sherwood.ih.lucent.com (Charles A. Sherwood) vaguely proposed a theory ......and in reply I say!:

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Thanks for all the replies. I am interested because I see 3PH stuff for peanuts, and single phase stuff is often limited in power.

These were my thoughts, too. Alternators are available, and many have themselves set up to generate power as bought. There certainly are engines with governors that maintin a very good constant speed.

However, I have supply, but not 3PH. I was looking at using an electric motor to drive the alternator to get 3PH.

The reason I see the alternator as better is that RPCs and VFDs seem to have a rep for having trouble with load balance etc. The idea and construction is simple, but they seem to be "picky". I have never used one, but read a lot of stuff here about the problems getting them to behave under various load conditions.

They also do not seem all that efficient, and seem to need a far larger motor to act as the supply than the load.

***************************************************** I have decided that I should not be offended by anybody's behaviour but my own......the theory's good, anyway.

On 25 Aug 2004 19:07:03 GMT, snipped-for-privacy@w-sherwood.ih.lucent.com (Charles A. Sherwood) vaguely proposed a theory ......and in reply I say!:

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AH. Thank you. That was my impression. See my other reply, posted before I saw this one. Thanks.

***************************************************** I have decided that I should not be offended by anybody's behaviour but my own......the theory's good, anyway.

On 25 Aug 2004 13:18:31 -0700, jim rozen vaguely proposed a theory ......and in reply I say!:

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Ahah! OK. So simply throw grunt at the thing and avoid the fiddly bits? That was something I had seen, but not understood (absorbed into my soggy brain) the reason for, I think

I agree about the lack of cheap/free 3PH alternators in the lower ranges. They usually have a motor (fuel) attached. I did have the opportunity yo buy one reasonably cheap. Did it work???? shrug. I never went there, because I was not sure if my idea was the right way to go.

snip

***************************************************** I have decided that I should not be offended by anybody's behaviour but my own......the theory's good, anyway.

On 25 Aug 2004 21:45:26 GMT, snipped-for-privacy@w-sherwood.ih.lucent.com (Charles A. Sherwood) vaguely proposed a theory ......and in reply I say!:

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Well if you're going to start fighting for glory...then so did I

Seriously, I did, and have experienced a lot of the same pricing troubles and problems with the RPC idea that you have and so I still want to keep looking at the motor-gen idea.

***************************************************** I have decided that I should not be offended by anybody's behaviour but my own......the theory's good, anyway.

Ah, the classic "dynamotor" of ww2 fame. My first radio had the B+ supplied by one of those - with the 'primary' run off a train transformer!

I wasn't sure if it was actually going to work, it seemed like a whacky idea with this little thing screaming away at speed. Then I touched the output terminals and got one heck of a shock. Yep, it works!

Jim

On 25 Aug 2004 15:01:18 -0700, jim rozen vaguely proposed a theory ......and in reply I say!:

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I win! I win!

AHereas I have to admit I do _not_ have the stuff available.

Yes. My question also was not rhetorical, but is certainly theoretical at present......some day...(sigh) :-<

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On 25 Aug 2004 13:18:31 -0700, jim rozen vaguely proposed a theory ......and in reply I say!:

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I did some reading. I remember now what my reservation was. While you can throw plenty into the RPC, they are not good at allowing the driven motor to produce full power. I am wondering what happens to welders etc. Welding is one area where I see real bargains for very powerful MIGs etc. I have seen a MIG that gets 350A from 240 volt, but it was a new model, brand new with a VERy "new" price!

***************************************************** I have decided that I should not be offended by anybody's behaviour but my own......the theory's good, anyway.

Got a wiring diagram? Ive got a 20hp 3600rpm Western Electric waiting to be turned into a RPC. Pony motors Ive got by the score.

Gunner, who was given a Hardinge DV-59 today. Disassembled but complete....sigh

Anyone need any Federal air gauges? Got about a dozen of them

Schaublin Lever action tailstock with built in collet holder in the nose of the ram?

Scored today....

Reasonable enough -- as you really running it purely from single phase after it spins up to speed. The available torque goes through zero twice per cycle (that is, at 1/120 of a second intervals in the USA).

Real three phase -- or synthesized three phase from either a rotary converter or a VFD does not, as when the torque from one phase is approaching zero, another phase is approaching peak, and a third is just past peak.

People who replace a three phase motor on a surface grinder with a single phase motor notice a difference (degradation) in the resulting finish.

Enjoy, DoN.

This is a problem with a "static" phase converter, but for a reasonably sized rotary converter -- especially one which has been tuned and balanced by tweaking capacitance on the windings -- you should be able to get full power out of the driven motor.

No experience with this, but I can't see any serious reason to expect problems running it from a RPC. Here, I am not sure what will happen from a VFD -- given that you have a switching regulator synthesizing the output of the VFD, and possibly another one converting the input to the welder into DC, so they might interfere with each other. (I would also expect similar problems running a typical computer from a VFD, too.)

But welders (and computers) don't need the variable frequency, so I see little reason for using a VFD for this.

Enjoy, DoN.

*What* wiring diagram? Switch power to two windings of the idler motor, and to the single-phase pony motor to spin it up, then switch off the pony after it has thrown its belt. (Based on what I remember of Jim's setup.) Wire the three windings of the idler to the three inputs of the target tool.

You just need a larger idler than with a well tuned one, and since you are not tuning for the power factor, you may blow circuit breakers more often than you would like, depending on the size of the idler motor.

You also might have problems running electronics from the generated power if it is across the wild (generated) leg. If you can be sure that all electronics are connected to the primary two input lines from the power company, you should be fine.

Note, also, that this has *no* safety circuit to disconnect the idler when the power fails, so when it comes back on, the motor will sit there humming until it burns up, or trips the breaker. So *always* turn it off when you walk away.

If *I* were going for a rotary, I would want the safety circuits, self starting, and properly tuned to reduce the reactive current drawn from the wall.

Enjoy, DoN.

Yep. Somewhere around, I have something even closer to what is being discussed. It is a DC input (28 VDC) 120 V 400 Hz three-phase output (to drive old aircraft instruments). It screams, but it works.

Enjoy, DoN.

Hey Gunner,

I was going to use a 20HP motor too. I kicked it into gear with a pony motor, and it was drawing 80 plus amps. I've settled for a

7-1/2HP, which is still drawing 20 or so.

Take care.

Brian Laws>>

Gunner sez: "Got a wiring diagram? Ive got a 20hp 3600rpm Western Electric waiting to be turned into a RPC. Pony motors Ive got by the score."

Wiring, schmiring, Gunner! Get after it, dude. I think it could be started with about 1400 microFarad of motor start caps, but that wouldn't be code, now would it?

Bob Swinney

Not to worry, Brian. That 20 amps you see with a clamp on meter is highly reactive - meaning the real draw (voltage and current in phase) is much less. Even a perfectly "balanced" 7-1/2 HP rotary will show 20 amps, or so, reactive current in the 3rd leg when running unloaded. It's the nature of the beast, so to speak. Same as with the 80 amps you saw on the 20 HP motor - it was largely reactive current and had little bad effect, other than heating up the wiring if you didn't take that into account.

As did the transmitters of early 2-way radios. Ever wonder how they got

400+ volts of B-plus from a 6 volt car battery? Dynamotor that came > >

They are both on the same contactor and start at the same time. However they are connected together before the overload controls. One contactor suppling power to seperate "heaters" for each motor. The third wire of the static converter connects to the third wire of the motors through the heaters. Seems like the logical way to wire it (at least to me).

I could connect the third legs together before the heaters, but I really don't think that would make any difference.

I see no current in the third leg with a clamp on ampmeter after the motors have started. The voltage in the third leg is 208 and 224 with respect to the other two legs. I guess there is no current because the voltage is low.

Probably could, but the motors are hard wired together.

I found mine in the newspaper. I think he got them at an auction when an importer was scrapping a bunch of stuff.

chuck