need large 1ph motor

I am looking for a large (7.5-10HP) 240V single phase motor to drive a 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 for specific applications but I need more 3ph power for grinders and things that don't need VS. I already have the alternator so I might as well use it for something useful.
chuck
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Charles
Have you determined the alternator's shaft RPM required for 60 Hz?
And, since I have never seen a used (affordable) single phase motor bigger than 10 HP, I wonder if you could consider getting a hugh (maybe 30 HP) old (cheap) 3 phase motor and drive it with single phase.
Jerry

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How about an engine?

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Been there done that. All I need is a enough 3PH power to run a 2HP motor. A 5HP drive motor might be enough but I would rather have a 7.5
chuck
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5 hp should be plenty assuming you are not trying to accelerate a huge alternator rotor on start up.
Bear in mine that the alternator output freq. is going to be less than 60 Hz by the same percentage that the motor runs below synchronous speed. This should not be a problem for tool motors except that they will be down on power by roughly the same percentage that they are down on speed.
Randy
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On Thu, 22 Jul 2004 17:16:46 GMT, "Randal O'Brian"

Belt drive 'em and build the "slip" into the ratio. To accellerate the heavy armature rotor on startup you want the old repulsion start motor (the one with brushes that lift when the motor comes up to speed)
I have a nice one sitting in my shed here in Ontario - not sure what HP but it weighs about 200 lbs. (Century Electric)

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Clare
Is there a marking on the alternator that indicates the frequency out if it with some standard RPM?
The torque to rotate an alternator is less than that required to start up an air compressor. I suspect that almost any induction motor would spin-up that 200 pound alternator if it was powerfull enough to overcome the total "windage" and bearing losses. Granted, it might take 20 or 30 seconds for a marginally small motor to get up to operating RPM, but the torque required to rotate an alternator is small compared to things like air compressors.
If frequency control of the 3 phase voltage is important, a synchronous motor can be used to drive the alternator. That way, the output RPM doesnt vary as much with changing loads.
Jerry
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Jerry,
What you wrote seems to say an alternator makes free power! I think you know better than that! An alternator's input will require at least 1-1/2 times the power it is putting out.
Bob Swinney
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Bob
I couldnt find the place where I suggested the alternator power was free. I meant to imply that alternators can be spun up with a common induction motor. I would think that an old (maybe free) 3 phase 10 HP motor could easily spin up an alternator and keep it running when the alternator was loaded by 2 motors 2 HP each. I think that if I had plans to spin a 15 KW alternator with a single phase motor, I look around for a used 30 HP 3 phase motor and run it on single phase. I occasionally see 30 HP motors in the trash but have never had need for something that big.
Jerry
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Right on, Jerry. I didn't mean to insult your intelligence with my statement - I just wanted to clear the point of any possible confusion. Yeah. I heartily concur that a large 3-phase motor would be the way to go. No doubt about that. It would be much easier to find a large 3-phase motor than a single-phase one of the same rating. Make that -- practically impossible to get a single-phase motor of greater than 10 HP.
The original poster seemed to be afraid of rotary phase converters which is why this thread got started. He will almost have to build a rotary phase converter (better if he did) in order to start and run a large 3-phase motor on single-phase power.
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Those (single phase motors above a couple of hp) are available.
Problem is, they're not common so they won't go for free. They probably will command a premium even when bought new because they sell so few.
Three phase motor, of, say, 7 hp, with some capacitors and a potential relay to start it?
Jim
================================================= please reply to: JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com =================================================
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Jim sez: "Problem is, they're not common so they won't go for free. They probably will command a premium even when bought new because they sell so few. Three phase motor, of, say, 7 hp, with some capacitors and a potential relay to start it?"
Yeah, Jim, that'd work. Or for the less sophisticated, some sort of a jury-rigged pony motor. <G>
Jerry has the solution, though, when he says go for a surplus 3-phase motor and run it on single-phase. That theme will work right up there to where you can't get heavy enough 240 volt service to start and run such a beast. I'm looking for a single-phase 300 HP motor from a streetcar or diesel loco, myself. Of course, that would probably be a universal (commutated) motor; hey! while I'm at it, I could get a genset also.
Bob Swinney
Robert Swinney says...

go.
motor
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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
Robert Swinney says...

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On Thu, 22 Jul 2004 22:37:57 -0500, "Robert Swinney"
......and in reply I say!:
remove ns from my header address to reply via email

Spun up is different from powered. It's not the milk and honey we hate. It's having it rammed down our throats.
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Charles sez: "I already have the alternator so I might as well use it for something useful."
Not likely! A 7.5 HP single-phase motor would take full load current of aprox. 31 amps at 240 volts. That 7.5 HP motor could realistically be expected to drive the alternator to generate only about 3.5 to 5 HP which is far shy of the 15 KW (20 HP) output capability of your alternator. Larger 240-volt, single-phase motors are hard to find because of the size of the branch circuit required. A 7.5 HP single-phase motor is about the practical maximum.
Bob Swinney

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Yes it is not possible to drive the alternator to full power,but I would be satified with enough power to run a couple 2HP motors.
This alternator has a 100lb rotor so I expect it will provide adequate surge power to start or instant reverse any motor I would use on a machine tool.
chuck
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You should get a gas engine and run your alternator with that. It's unlikely normal house electrical power can supply what you're thinking. I figure the same thing for compressed air, by the way. If I ever have to do sandblasting or jackhammering I'm just planning to rent a huge compressor and be done with it.
Grant
Charles A. Sherwood wrote:

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Hey Chuck,
I betcha you'll have a hard time finding a single phase 220VAC motor over 5HP. Have you considered a multi-motor drive? Say three motors at 5HP each, belt driven on a common input shaft of the 3 phase alternator, such that if you run one only, and need more power, that you cut-in another one or two. Slipping V-belts should work. I think I'd try one on a direct drive to the alternator, and the other two mounted like a reverse pyramid stack above the first, but belt drive.
Take care.
Brian Lawson, Bothwell, Ontario. XXXXXXXXXXXXXXXXXXXXXXXXX

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No, it would not be possible to drive a 15 KW alternator to anywhere near full power with only a 7.5 HP motor.
Bob Swinney

is
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I never said it was. I said it is NOT possible.
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