need large 1ph motor

Charles A. Sherwood · 2004-07-22T14:46:01+00:00

I am looking for a large (7.5-10HP) 240V single phase motor to drivea 15KW 3PH alternator to make real 3PH power for my workshop.Anyone know a source in the chicago area?I already have 3 phase converters in the shop and I like them forspecific applications but I need more 3ph power for grinders andthings that don't need VS. I already have the alternator so I mightas well use it for something useful.chuck

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Jerry Martes 22 years ago

Bob

I've actually made 3 phase power sources with a 60 Hz motor turning an automotive alternator. That works very well. But, the 3 phase frequency sure isnt 60Hz. I like Charles's plan to build a motor-alternator if there is need for 3 phase power that varies in frequency while the load varies.. .

For Charles, I was just proposing as a *low cost* way of spinning an alternator when only single phase voltage is available. And, any 3 phase motor can be reconnected to make it into a single phase motor with a fair starting torque. I've made single phase motors from 3 phase motors by disassembling the motor and locating where the 3 windings are connected at the center "Y" in the stator windings. One of the three windings is disconnected, while the other two windings remain connected together. The winding that got disconnected is used as a *Start* winding. The other two windings are the *Run* winding in the single phase motor.

Jerry

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Old Nick 22 years ago

On Thu, 22 Jul 2004 12:14:25 -0500, "Robert Swinney" vaguely proposed a theory ......and in reply I say!:

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Is that how inefficient it would be? 50-65%?

I was considering a similar idea to Charles's. I assumed it would have less problems and better efficiency than a rotary converter (which from reading, I gather do have their own inefficiencies and problems)

***************************************************** It's not the milk and honey we hate. It's having it rammed down our throats.

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Charles A. Sherwood 22 years ago

I think it depends on a lot of variables. In my case, I am using a relatively large alternator (15kw) to get a small amount of power. I think the dominating loss here are loads like the alternator fan and excitor which will probably pull 2HP. So I lose 2HP right off. With 5HP in, power out will be 2-3HP. With 7.5HP in, I should get

4-5HP out. I don't have any machines that are over 2HP, so it should be fine.

Why do this? I think it will have much better phase balance than a rotary phase converter and I won't need to mess with balancing caps or change the caps for different loads. The efficiently will be low, but the initial cost is only a motor because I got the alternator.

Does a rotary phase converter make more sense? Maybe, but I don't always feel the need to be completely logical. Its fun to experiment.

chuck

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Jerry Martes 22 years ago

Chuck

Unless the 3 phase load is special and needs to be "real 3 phase", it would be alot easier to use single phase to feed it. 3 phase motors run very well when fed single phase unless the motor is loaded to near its name plate HP rating. I'd expect there to be some very good advice from this news group on "how to feed your 3 phase load". Maybe you can identify your exact needs to them.

It is *my opinion* that balancing of a rotary converter has little value when the 3 phase load is a motor. It is *my opinion* that the biggest idler is the best idler. It is *my opinion* that a pushbutton started idler (or a pony started) is adequet for the majority of home-hobby shops.

So, if the 3 phase load is a couple of 2 HP motors that dont get loaded heavily for extended time, or started while loaded, I'd suggest the use of a simple, untuned, idler.

A major benefit from using "real 3 phase" for driving 3 phase motors is the resultant start up torque, and the smoothness of delivered torque when heavily loaded.

Jerry

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Old Nick 22 years ago

On 26 Jul 2004 14:25:24 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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Ok. I see why you could get inefficiencies if the gear chews up a bit just by turning.

Your thoughts are like mine. I figured that using an electric motor (or matched motors) to run a 3Ph alternator would be (a) cheap) (b) give better 3Ph than convertes.

But then everybody uses converters. There has to be a reason.

***************************************************** It's not the milk and honey we hate. It's having it rammed down our throats.

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Charles A. Sherwood 22 years ago

Well the bottom line is it is not cheap. I found a used 10HP 1ph motor to drive the alternator for 350. Now I need some pulleys and belts; add another 100. Now I need a contactor to start the 10HP motor; I can buy 2 VFDs for less money.

Cost; You can build a rotary phase converter pretty cheap. VFDs are also getting pretty cheap. Motor generatos are big, heavy and expensive. Makes me think of the dynomotors used in old mobile radios. Low tech and pretty interesting by todays standards, but still very inefficient, big etc.

gee, sounds like I just talked myself out of this project.

chuck

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