phase converter question

I have a phase converter with a 7.5hp idler. I never envisioned having to run a 7.5hp load, but now I want to try it. This is a question for either someone
who's done it or for one of the pointyheads who abound on this NG. If I were to first start my phase converter, THEN start e.g. a 3hp motor with no load, can I then run a 7.5hp load motor fully? My thinking is that the 3hp motor would act as an "idler booster". In this way my phase converter (I'm hoping) can be made to act as though it had a 10hp idler.
Grant Erwin Kirkland, Washington
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having to run a

someone
If I were to

load, can I

would act

can be made

I used to have a static phase converter, and to start my lathe in top speed I did exactly what you are proposing, and first started an unloaded motor on my cylindrical grinder. However I did had the luxuary of switchable capacitors to balance it.
I use a rotary now which has auto switching so far less hassle !
AWEM
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Short answer, yes.
If properly tuned it might start it without the extra three in there. This sort of depends on what kind of load you have.
Jim
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My own experience:
1. With a 10 HP rpc, I could start a 7.5 HP motor instantly, with a bang.
2. With a 7.5 HP RPC, I could start a 5 HP compressor in 1-2 seconds.
I do not think that you will have problems, but remember, a motor that is unloaded by drives a bunch of gears and other inertial things, is harder to start than a motor with shaft not connected to anything.
i
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jim rozen wrote:

I'm pretty sure I can *start* the thing (big abrasive cutoff saw) but what I want to do is to source enough current so I can run the saw fully. I always thought you could run one size down e.g. if you have a 7.5hp idler you can run a 5hp motor fully.
I am considering adding some run caps to the idler booster unit, so when I switch the unit off they will be out of the circuit. Only when the idler booster is running would the extra run caps be apparent.
GWE
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On Thu, 01 Dec 2005 14:46:57 -0800, Grant Erwin

Grant, I have a phase converter which uses a 15 hp idler motor. I can run 60 hp with it. The largest load I have run on it at one time was 32 hp. Two of the lathes connected have 10 hp DC spindles and so contribute nothing to the system like they would if they were induction motors. The manufacturer said that I can start 15 hp with it. I also have a 7.5 idler motor hp unit made by Eyelander up in Everett. I could start the 10 hp CNC lathe by using the soft start in the DC spindle drive. After the spindle was up to speed I could run the lathe flat out and take just as heavy a cut as when the lathe was connected to real 3 phase. Eric
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On Thu, 01 Dec 2005 14:46:57 -0800, Grant Erwin

First things first is to see how well it starts on the 7.5hp idler. If it starts fine then you'll be good for most of your work. Then if for some reason you want to saw something heavy you can go turn your lathe or whatever on and use it for the boost needed for the heavy work.

It's certainly the best way to do it but I wonder if it'll be overkill here.
Wayne Cook Shamrock, TX http://members.dslextreme.com/users/waynecook/index.htm
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