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Generator size for rotary phase converter

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timcki

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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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#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.
 

MikeInOr

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#7
Your are going to use a single phase generator to start your rotary phase converter to run your lathe?

I would look around for a surplus 3ph generator... they pop up from time to time at good prices because people don't realize they can use 2 of the 3 legs from a 3ph generator to run single phase equipment. This seems like it would be more efficient and work quite a bit easier. The starting current required by your lathe should be considerably lower with a 3ph gen than a 1ph gen and a rotary converter.
 

rgray

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#8
Your 15hp motor is a generator if turned by an engine.
Expensive electricity when using gas engine to make it though.
 

Tony Wells

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#10
I am trying to make a deal with PR to help them out with their power crisis and have several 3 ph 480 power plants from 60KW up to near 1,000. All diesel. I also have, but am reluctant to sell, a LPG 16KW 3 ph I sort of have plans for. If you need a large one, I can help. Oh, you will need a tractor trailer or at least a tractor truck to move a couple of them, they are CAT self contained trailer mounted units. If I can't deal with PR gov, I will probably market them domestically. (Shameless plug!) Right now, they are reserved for PR, but after they do whatever they are going to do.....who knows? I may just lease them for a year. It is going to take them at least that long to get back online completely there.


On the other hand, have you considered a VFD? Yes, I know the larger ones get expensive, but so does experimenting. IMO, they are more efficient in the long run.
 

Keith Foor

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#11
OK, if you are looking for at a generator, I assume that you don't already have one. So why not get a 3 phase generator and skip the rotary phase converter all together? IF the generator is 208 3 phase, each phase to ground/neutral is 120 volts so you have your 120 power source covered. And depending on the generator some can be wired 240 3 phase. These are typically 12 wire generators and can by wired in any configuration up to 480. But unless you have equipment that is specifically 240 volts, then I would skip the rotary converter all together, get a 3 phase generator and be done with it.
 

timcki

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#12
The idea of a three phase generator crossed my mind too.. (Great minds)..... I have been actively looking for a surplus military generator..
MEP- 803A this is a 10 kw single or three phase generator ... this I believe, will be the answer to all my problems....

unless you can recommend a better three phase generator...
 

Chipper5783

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#13
Can you rent a generator similar in size to what you are thinking of purchasing? Try it out and see how well it works.
 

mksj

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#14
Not familiar with these gen sets, as opposed to the more common brand names. Certainly sounds like these military units should work well. My question on the MEP-803A is it does not show that it can be switched to 240 three phase. I shows 120/208 three phase with the ability to adjust the voltage to 220V which may be enough? This also effects the amp draw, from what I have seen elsewhere you would need a minimum of a 15KVA+ generator to start a 7.5Hp motor. See reference.
http://www.nationalpump.com.au/calculators/guide-to-choosing-generator-to-motor-size/
Gen Spec.jpg
 
Last edited:

timcki

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#15
like said above by chipper5783, before i spend any money i will be testing out the setup with a rental i am hoping....
well now with second thought that may be too close to the upper end of the three phase......
I think my balloon just popped....
 

Keith Foor

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#16
It's really going to depend on what you are doing. If it's a temporary setup then any 3 phase generator should be fine. If you are looking to do this 8 hours a day for months at a time you want a prime power rated generator. This means the gen set is rated to be run as a primary power source and not just a backup generator for a few hours a year. They are different animals. You will not find a prime rate d unit with anything close to a lawnmower engine. And the engine sizing is typically double. My prime rated RV gen set is 4000 watts and has a 16 hp Onan twin on it. A typical 4000 watt gen set has a 6 or 8 to Briggs on it. Hope that helps
 

timcki

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#17
My little shop is in a storage area. I have two spaces giving me about a 600 square foot shop.. NO 220/240 is available.... not gonna run 8 hrs a day, but if I want to make apart would like to be able to fire up the lathe...
7.5 HP biggest piece I have everything else is max 2HP...
think maybe a smaller lathe might be the only way to go....
 

rick9345

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#18
Home Depot and others sell wire,run anther hot lead from the house. Even pro done would probably be cheaper/more reliable than gen with RPC.
Whole lot less noise.
 

gi_984

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#19
timcki,
I have a MEP-003A milsurp diesel generator. Got it for the winters up here in Wisconsin. It is a 10kw and have it professionally wired as a whole house back up power source. It easily runs the entire house (A/C, lights, etc all on or running) plus the shop (AR 7.5 HP RPC, lights, single phase machines, and multiple 3 phase machines all running. Added all the loads up on paper and it looked like it would handle anything I had. I was curious how well it would handle it. One by one turned on everything I could. No real difference in the sound from idle to full load.
In a power outage I shut off the main breaker for the house, start the generator, flip an interlock switch at the generator then another on the main panel. Then the generator power is fed into the main panel and sub-panel for the shop.

The MEP-003 will also produce 3 phase directly. I've never done it though. Have to pull out the manual to see what the voltage is in 3 phase.
 

JimDawson

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#21
A 10KW military generator will easily start a 7.5 HP motor. They are all very conservatively rated. I'm sure you could pull 20KW for a short period of time with no problem. The surge load capacity is at least double the rated load.
 
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