Ignoramus5833 wrote:


4. What HP to choose? With these motors essentially free, the cost
difference between 10 and 7.5 HP is minimal. 10 HP, though, draws a
lot more current at startup. My garage has a subpanel that is on a 60
amp breaker.

I can start a 10 hp on my 50 amp breaker, you should have no trouble there.


Sounds good. Do you start it from a low RPM?

That was a capacitor start unit, so it was starting from zero.



5. I salvaged a 1/4 and 1/3 HP motors from two broken vacuum pumps
that I had. I would buy a 5/8" ID pulley on one of them. Would they be
able to spin up a 7.5 or 10 HP motor? Jim Rozen reportedly used a 1/4
HP pony to spin up a 7.5 HP motor, that makes me hopeful.

6. How fast would the idler need to spin before I can remove the pony
and apply 220V to the idler input?

The closer you get it to the nominal speed the less current surge
there'll be when you switch it in. 2/3 nominal will work well.


What I am kind of thinking about is, what if I spin it to 1/10 or some
such slow speed. Would it be able to spin up further, inrush current
be damned? I am curious about it, practically speaking, a 1/3 HP
should be able to speed it up.

They'll spin up from a fairly slow speed but a 1/3 hp motor should be
able to bring a 10 hp convertor to pretty near full speed anyway.



7. Is there something that I forgot to ask?

Take a good ohmeter with you to check the motor windings. The two
problems you may find is a short to ground and a shorted winding.
Neither makes for a good convertor;-) In my experience it's not common
to find a cooked motor that's not shorted to ground in a big way but it
happens, measuring resistance from the windings to the frame is probably
adequate. If you have a megger all the better, but it's not absolutely
necessary for checking a junkyard motor.


I have a megger, although I never used one. I will take my regular
multimeter and will check the motor for shorts to frame.

Best of luck with it, hoping to hear of success soon..

i