"DoN. Nichols" wrote in message
...
On 2008-02-13, wrote:
On Feb 12, 1:56 pm, "Jerry" wrote:

[ ... ]

It isnt clear that the OP is considering a Rotary phase converter. A
converter is often a capacitor that is temporarily connected to the 'not
connected leg' of the 3 phase motor. Once the 3 phase motor is
spinning, it
will run well with single phase input power while the third leg of the
motor
disconnected.

[ ... ]

My brother is planing to use one of these of suitable size.
http://www.grizzly.com/products/Stat...-to-7-HP/G5843
Thanks everyone.

What he is planning to use, then, is the equivalent of a
capacitor temporarily connected to to the "not connected" leg of the
motor. This gets it spinning in the right direction and then
disconnects.

The sure indicator here is the range of horsepowers (instead of
"up to X horsepower). This is because the capacitor needs to be sized to
the horsepower of the motor which it is being used with. Too large a
motor or too small a motor will not start properly. (Of course, the
word "Static" in the name is another clue.

You can perhaps get nearly full horsepower from the motor with
this, but at the cost of running one winding at a much higher current
than it was designed for. It would be more likely to burn out if run at
this level for quite a while, though you can get away with it for short
spurts.

And this is not what *I* would call an "electronic converter".

If you want to do it right, the better way to go (other than
making a rotary converter) is to get a VFD (Variable Frequency Drive), a
truly electronic device which converts the incoming power (single phase
or three phase) to a high DC voltage, and then converts that to true
three phase -- with the added boon of generating the three phase at a
frequency of your choice, so you can run the motor significantly slower
or significantly faster.

When running significantly slower (say about 25% of full speed
or so) you do have the problem that the internal fan in the motor won't
keep it cool enough, but if you wish to run it at such slow speeds for
long periods, you can add an external fan to keep it cool enough. Just
mount it to blow into one end (in the same direction that the internal
fan blows) and you are fine.

I forget what kind of machine tool this was to be used with, but
quite a few benefit from the ability to tune the speed to the task at
hand. For example metal lathes can produce variations in finish quality
as you face from outside towards center or vice versa. The reason for
this is that at certain surface speeds you get buildup of metal on the
cutting tool which will eventually break clear and give good finish for
a short while before building up again. If you can turn the spindle
speed up as you face in towards the center, or down as you face out from
the center to the outside, you can tune away from the speeds which give
the problem. And this is something which you can't do with a step
pulley, since stopping to change belt steps would change the finish at
the point of the stop. (Though if you have a variable speed pulley, you
can do quite well with this.)

I personally would not use a "Static" phase converter, and would
choose either a rotary converter (home shop made) or a VFD (commercially
made). Since I have three VFDs and no Static phase converters, you can
see that I live by that.

Enjoy,
DoN.

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Hi Don

Why do you write that a static converter with more capacity will have
difficulty starting a smaller motor?
I refer to your statement -
This is because the capacitor needs to be sized to
the horsepower of the motor which it is being used with. Too large a
motor or too small a motor will not start properly.

Jerry