Ignoramus8285 wrote:

I had a hard time with something and I do not fully understand why it
did not work.

The story was that I was trying to test a 20 HP compressor that is in
my truck.

My phase converter has two idlers with the total horsepower of 17.5.

I plugged in this compressor into a running phase converter, using
approximately 25 feet of 12 gauge cable.

The motor would spin with difficulty, at about 60 RPM. (one rev per
second).

Then I tried two things.

1) I applied compressed air from my home compressor, to the unloaders
of this compressor.
2) I ran my own compressor, and disabled its unloaders, so that my
compressor would run unloaded. My hope was that after subtracting 2 or
so HP needed to run the compressor unloaded, I would still add perhaps
8 HP to my phase converter, bumping the total HP to 35 or so HP.

Despite doing both of the above things, I could not spin up the 20 HP
Quincy beying 1-2 revolutions per second.

NOTE: ** The motor's nameplate data has been erased. **

Another hypothesis that I have, is that the 30 feet of 12 gauge cable
is just not enough. According to some web based calculator, voltage
drop from conducting 100 amps, would amount to 9.6 volts. This does
not seem like much to me, given that my voltage is 245 volts, give or
take.

So, then, what is it that I am missing?

Have you disconnected the compressor from the problem motor to test just
the motor? Have you inspected the motor for voltage strapping, opens,
shorts, etc? Have you checked the current draw on each phase when
testing? How about sounds from all those idlers, if the test motor was
near stall you would expect it to be loading things down quite a bit,
voltage drop in the wires or not.