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Generator size for rotary phase converter

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timcki

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Jan 5, 2013
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#1
I have been going from site to site trying to figure out the math... My head is going to explode.
1 site says my setup will work , another say no it wont..

please help me with a formula..
I have a 15 HP rotary phase converter and I am looking to use a 9,500 watt generator to power a 7.5HP Lathe


I am looking at a military MEP 803-A generator 10,000 watt (brush less) and a Miller Bobcat 250 welder/generator

dedicated power VS. dual purpose power...
When I put it on paper I think I am answering my own question...

Yes I have to use a generator there is no other option..
 

cathead

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Feb 7, 2013
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#2
In my experience, generators are notorious for not being able so start a heavy load unless way oversized.
There are two obstacles in the way. First, will the 15hp rotary phase converter spool up?
Second, will the lathe bog down the generator and or converter when it gets put in line? Personally,
I think the chance of it working is questionable. It would be an expensive learning process if you buy
a generator to find that won't get the job done. Starting loads are one thing and running loads are considerably
less. Maybe take a look at a 3 phase generator( a big one) for your application as there would only be obstacle #2 in the
way for that. It's good to see you are doing your homework. Be sure to factor in starting loads, conversion loss,
and line loss in your math calculations. I wish you the best of luck.
 

Blackjackjacques

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#3
If you are asking if you can use a 9,500 watt generator to power a 7.5 hp motor:
9,500 watts I assume to be the continuous capacity of your generator. If you did a 1-1 conversion at 746 watt/hp, then your generator should be able to handle a 12.73 hp motor, or at 240 V - about 40 A (at single phase). However, inrush or locked-rotor current for that motor will be about 5 times or about 200 A. This inrush current is only momentary and although may likely trip a 9,500 watt generator at 12 hp, you probably can startup and run a 7.5 hp motor (~23 A cont. running current).

Because you are using a RPC, I assume that your motor is 3-phase and your power source is single phase. In this case, your current demand requirements are even lower (by a factor of 1.73) and puts you in better circumstance. If you are dealing with 440V in lieu of 240 V, then that is even better. Given the above, you should be able to "spool up" your RPC, however, I would spool it up first without load, and then, once at speed, connect your 7.5 hp lathe motor. In theory, it appears that you should be able to make it work. Provide the voltages you are dealing with and we could tell you better.
 

Blackjackjacques

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#5
Fine, the calcs in my post are based on 240 V - so you should be good to go.
 

Karl_T

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#6
A suggestion, get a soft start for your lathe. It will greatly reduce the starting load.

If you have trouble spooling up your converter, get a small motor and belt it LOOSELY to the 15hp motor. Bring it up to speed with the pony motor before kicking the converter in. Then the start load will be almost nothing.
 
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