"Ignoramus19393" wrote in message
.. .
I built another self starting phase converter kit, with a 7.5 HP
idler.

That's because I have a 5 HP 3 phase compressor that I want to sell,
and I want to offer a phase converter with it so that it appeals to
garage mechanic types.

Almost everything is great. The compressor runs beautifully with the
7.5 HP idler. Voltages between legs are 240, 250, and 256 volts, which
I consider well balanced.

Here's the problem. The idler motor vibrates without load. As soon as
I turn on the 5 HP load, vibration stops. I am not sure why that is.

If I remove incoming 240V by turning the contactor off, vibration
stops instantly as well as the rotors spins down.

Any thoughts as to why that could be happening?

i

I would suggest the 7.5 HP idler motor was originally designed for very high
starting torque; maybe for an air compressor. NEMA designates the design of
squirrel-cage induction motors in several classes according to their
electrical characteristics. The winding configuration of the rotor
(generally) is varied from class to class in order to modify starting
torque, running current, slip, efficiency and etc. In all liklihood, the
design of your motor was for high starting torque - such as class "C".
Leaving the start cap permanently in place (self starting RPC) could cause
excessive noise in a motor designed for high starting torque.

Bob Swinney