[TheForge] Motor Problems

[David E. Smucker]

While not a direct blacksmithing question this is shop related.

Today my 7 1/2 hp compressor motor quit.  It is a single phase, 1.15 service 
factor, 3450 rpm, 230 volt motor.  Capacitor start, capacitor run.  The unit 
is not very old, as if that makes any difference now days.  At most the 
compressor has several hundred hours on it.

I have been using it on a regular bases recently and was doing some spray 
paint work at the time.  It kicked in for a few seconds and quit.

I have check the reset on the motor starter and this is not a issue.  The 
pressure switch is closed and when power is supplied to the motor starter 
the coil closes and power is present at the output of the motor starter, 
full 240 volts and no sign of a problem with the overload heater.

At this point I an thinking it is a failed capacitor or a problem with 
starting switch / winding.  I get no smell by the way at the motor 
indicating a burned something and the motor was cool, showing no sign of 
having been working hard.  The compressor might have run two cycles today as 
I was just starting and the spraying I am doing is not very demanding of 
air.

Also I get no heavy hum, typical of what I have seen in the past with failed 
starting switches on much smaller motors.  I have removed the belt and could 
not start the motor by spinning up the motor pulley, but I many not have 
gotten it moving fast enough.

OK after all of that, is there an easy way to test the capacitors, or should 
I just order new ones and do the swap out testing.  I know there is a way to 
test capacitors with an VOM meter, but I only have a digital volt ohm meter. 
It has been a long time but I think with a "meter" type VOM you would 
measure the resistance across the capacitor and it would first be high and 
then drop down as the capacitor charged.  Is there a way to do this with a 
digital meter or some other method.  Second, as I type this I am thinking 
that for any test I want to do I need to disconnect the capacitor from the 
motor, is this correct?

Also what I assume to be the start capacitors (there are two) each have a 
small resistor across the inputs, what is this for?  to bleed off the charge 
as the motor starts?

Also any way to test the starting winding / switch?

Thanks in advance for any help that is out there.

Dave Smucker