Compressor motor struggles against no resistance

[rcx132]

Something weird going on with my 4HP compressor. The motor acts like it's struggling against heavy resistance, tries moving, makes very slow start, voltage drops (I can tell as the backup compressor slows down) and then the built in breaker kicks in. However I can turn the pulley wheel by hand showing that there's no resistance in the pump or the motor itself. I checked that there's no pressure in the system by disconnecting the thing depressurising tube that goes to the switch. Any ideas?

 

[anto44]

Sounds like a start capacitor problem, that's if it has one fitted. Pics of the motor will help.

 

[voipio]

Shorted windings, or a failed run start capacitor, possibly.

 

[rcx132]

Sounds like a start capacitor problem, that's if it has one fitted. Pics of the motor will help.

It does have a very large pair of capacitors. Any way to test them? My multimeter has a oscilloscope built in, I think that can be used to test capacitor circuits.

 

[voipio]

Does the multimeter have a capacitance measuring function? If it does, check the measured value against the value printed on the cap itself. The cap(s) will need to have at least one lead disconnected to test properly. A capacitance meter cannot do its job whilst the cap is in circuit.

 

[rcx132]

Does the multimeter have a capacitance measuring function? If it does, check the measured value against the value printed on the cap itself. The cap(s) will need to have at least one lead disconnected to test properly. A capacitance meter cannot do its job whilst the cap is in circuit.

Yes it does measure capacitance, never used these features, so glad there's an opportunity. I should be able to check tomorrow.

 

[anto44]

You can test them with a multimeter, there's a good few videos on YouTube showing how to test them.

 

[carbon]

The value of capacitance measured needs to be with the tolerance that is marked on the capacitor. So if the capacitor is marked for example 60uf with a 5% tolerance then the minimum capacitance needs to be above 57uf, anything below that needs replacing.

Another option could be that the centrifugal switch isn’t working correctly (if fitted) and is not switching the start capacitor in when the motor attempts to start

 

[mtt.tr]

Have you tried to compressor with no air in the receiver and the outlet valve wide open?

 

[rcx132]

What does the multimeter capacitance tester actually do? Does it actually test the capacitor or just measure the capacitance that it should do?

@mtt.tr yes I switch on the compressor with the cylinder empty, before switching to my backup compressor to carry on my work.

 

[carbon]

What does the multimeter capacitance tester actually do? Does it actually test the capacitor or just measure the capacitance that it should do?

It will measure the actual capacitance of the capacitor, but at least one lead of the capacitor needs to be disconnected for correct measurement

 

[the snooper]

Short the capacitor connections before measuring so there is no stored power in it

 

[rcx132]

Only just got round to fixing the compressor, as have been using the backup instead. It was indeed the capacitor, thank you everyone for helping on this.