[TheForge] 3 Phase Converter DIY

Sun Dec 8 17:18:00 2002

Ed F wrote:
> Hi Steve,
> 
> Thanks.  I have a few.
> 
> 1). Everyone says that the generator motor has to be rated for the HP you
> will drive plus some, except a friend who is sending me something out of a
> text book on it.  He says that only one set of windings are doing much work
> so you can use a motor about 1/3 the rating of your consumption.

There are two issues in sizing your idler. The first is starting your 
load motor, the second is running it at the highest HP you can without 
overloading anything in your system. As mentioned by Darrell, you need 
some minimum size idler to start a given load motor. If the load motor 
starts with "no load" (i.e. the machine has a clutch between the motor 
and other parts), a good rule of thumb seems to be same size. So I don't 
think the small motor your friend is proposing will start other motors, 
especially if they start under some load.

> By that, a 1 HP generator would have the current running through one set of
> windings that your 3 HP driven motor uses.  He argued that the windings are
> rated for heat not current, and the one working leg is wound together with
> the 2 low current legs so it's fine.

This is an interesting idea. Let us know if it works.

3 phase motors are (or should be) cheap and readily available as 
industrial surplus/scrap. You might as well get a big enough idler.

> It's not really a big deal for my press motor which takes about 5hp, but if
> I make one for my welder at 1 to 1 I would need a 30 HP motor!  Quite a
> difference from 10.
I think a transformer welder would be a different situation (not that I 
know how different...). This begs for experimenting. Your welder will 
only draw 30HP equivalent at max load. Hook it up to your 5HP converter 
and see if it works at all at a low welding current. Measure the 
currents (in each leg) of your idler motor. This should give you a good 
idea of how big a motor it will take. You can just switch in additional 
5HP motors to share the current.
> 
> 2). Did you do anything for fusing?
What is generally used is called a motor starter or a magnetic starter. 
All they are are big contactors with "heaters" that will shut down the 
contactor when too much current is flowing. I really recommend these; I 
have two converters, and the one that does not have a motor starter on 
it caused a motor to catch fire when its bearing siezed. Do as I say, 
not as I do--I still need to install the starter on that system. I find 
mine in the scrapyard.
> 
> 3). What precautions did you take with your capacitors to keep from getting
> zapped?
Put them in a closed box.
> 
> 4). Did you have to play much with your capacitors to get the motor
> balanced?  Did you balance the current or voltage?
The rule of thumb (12uF/HP) works pretty nicely. Fitch on 
rec.crafts.metalworking has delved into this quite a bit, and he says 
that balancing the voltage works fine. I think his recommendation is to 
make the third leg voltage 10% high with no load (just the idler) and 
you'll be pretty close.
> 
> 5). Did you start the generator with a capacitor?  I have heard of some
> people using a small single phase motor or even a pull start.
I've done pull start on a 7.5HP motor. I recommend you do this for 
awhile, then set up for capacitive start. You'll be amazed at how 
wonderful pushbutton start is. I haven't tried the pony motor.

As mentioned above, you can just add in idlers as you work on a larger 
machine. Setup a 5HP, capacitor start converter. Use it to start the 
next idler and so on. Makes for more start buttons, but less capacitance 
and less inrush current.

Steve
> 
> Thanks again,
> 
> Ed
> 
> 
> ----- Original Message -----
> From: "Steve Smith" <[email protected]>
> To: <[email protected]>
> Sent: Friday, December 06, 2002 7:52 PM
> Subject: Re: [TheForge] 3 Phase Converter DIY
> 
> 
> 
>>I've built several converters, I'll be happy to help if you have
>>questions (as will several others, I'm sure).
>>
>>Steve Smith
>>
>>Ed F wrote:
>>
>>>I want to look into making a 3 Phase converter to run a 3 phase forging
>>>press motor off of 220 single phase.  I can get away with a 5 hp but I
>>
> will
> 
>>>probably go to a 7.5 hp.  I ordered the plans from ABANA way back  and
>>
> lost
> 
>>>them.  I remember them  being about 3 pages and on the minimal side.
>>
> I'm
> 
>>>thinking that by now someone probably has better directions on the web
>>>somewhere, or maybe someone here is willing to share experience?
>>>
>>>Ed
>>>
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>>
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> 
> 
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