Cap Start/Cap no-run motor

[Open Neutral]

Got handed this:
Single phase cap start/cap run 1.5HP motor driving a compressor. It stalls and clears the line breaker when the tank is up to ~3Bar.

My first guess is the run cap is failing or too small. It has been literally decades since I've seen a cap start/cap run motor. Is there something else I should investigate for stalling out at increasing load.?



(It's a Marathon Motor, a line now owned by Regal-Rexnord but they can't even decide what division handles such ...)

 

[mtnelect]

Capacitors are cheap, in our Maintenance Contracts we change them yearly. HVAC compressors get worked harder than any other type of motor. You don't have only the heat to deal with, but the refrigerant pressure that the compressors have to start under. Our combination capacitors cost us $11.00, and we charge the customer $75.00. We do this because we don't like getting service calls in 110-degree heat.

 

[mtnelect]

Also, we always make sure that the HVAC system has a time delay installed for at least 5 minutes. To give the HVAC system time for the High & Low side time to equalize before the next call for service.

 

[Eddie702]

This does not sound like air conditioning. He said a 1.5 hp motor driving a compressor...probably an air compressor.

Get a meter that can check a capacitor and check them. It also probably has a relay to disconnect the cap when the motor come up to speed. That could be the problem as well.

 

[ActionDave]

Uncouple the motor from the load and run the motor. Listen for any noise coming from the bearings. Check the starting switch and make sure it's working. Put an amp meter around one of the leads going to the starting cap and see if amps drop out.

 

[Jraef]

This sounds like maybe the centrifugal switch is not opening, ie it’s welded shut. When that happens, the start cap and aux winding are left in the circuit, so once the motor gets to speed, they ROB the motor of torque, so the motor has to work harder and that causes things to trip.