Real 5hp compressor, ~10 years old with a newer single phase motor, 5
nameplate hp. 'occasionally' stalls on startup. Loud hum, no rotation,
kicks breaker but not heater on starter.

whaddaya think, starter cap?

Is the unloader valve releasing the pressure from the discharge line?
Some compressors won't start against that pressure.

Dn

wrote:

Is the unloader valve releasing the pressure from the discharge line?
Some compressors won't start against that pressure.


If it is an unloader problem, the motor will attempt to
start, but may not make a full revolution of the compressor.
But, it will usually move some. If it is a bad starting cap,
it may or may not behave as above. If it is a bad centrifugal
switch, it will not move at all when the switch contacts fail
to close. Visual inspection of the contacts will generally tell
you what you need to know in that case, ie. massively burned.

Jon