When you switch to compressor it just hum, motor not running. why?

The compressor won't switch off until it either overheats or the fridge cabinet is cold enough for the thermostat to switch it off and if there is a fault preventing it cooling, it is likely to run until doomsday though it is worth mentioning an auto defrost panel if stuck switched on can cause this...

If that is the problem the compressor will be working hard and will become quite hot as will the condenser and one of the compressor pipes.

When power is supplied to the motor it is likely to hum whether it is running or not, though it is a different sort of hum if it isn't running.
If the compressor pipes aren't getting hot/cold and the condenser isn't warm/hot then either the compressor is broken, not running or the refrigerant has escaped.

In a safe manor, when the motor is humming, give one of the pulleys a push to help get the motor started. If the motor runs OK with the help push, the start capacitor is bad. If not the next thing to check is the tank check valve.

I would check the capacitor since it appears that the motor is not locked up. Here is a how to on testing capacitors;


how-to-check-capacitor

If it doesn't try to start, then it could be the pressure switch. Another suspect could be the thermal overload on the motor. The motor (or just the overload) might be overheating and shutting the unit off, then allowing it to restart when it cools.

Is there any humming, or does it blow the breaker? When ot shuts off, do you hear a brief hiss of air?

If there is humming or breaker blowing on restart, or no hiss of air at shutoff, the pressure switch unloader valve is probably bad. If the air blows off, and there is no humming, and the compressor doesn't reliably start, it is probably the pressure switch. If pressure drops too low, give the pressure switch a tap with a screwdriver. If it starts then, the pressure switch is your problem.

Are you a mechanical type person? There is a fast and easy way to test the motor but it is not recommended for anyone who doesn't do this for a living. I have replaced 4 shaded pole motors in the last 3 years that did the same thing and never had a problem with a capacitor start or capacitor start/run motor. If the motor hums, it is trying to run. Try giving the stator help getting started while the motor is humming. Use a small piece of wood or soft plastic. Any non-conductor will do. Look inside the rear of the motor and you will see the field coils around the outside surface and the stator is the round thing in the middle of the motor that rotates. Use the non-conductor to start it turning in the direction it would normally run while the motor is humming. If the motor starts and runs ontill the pressure switch shuts it off, the motor is bad. If it has a capacitor on it, try replacing that first. Please let me know how this works out for you.

Try trouble shooting it in this order
1. remove the v-belt connecting the motor and compressor
2. run the motor, if it runs well then check the compressor. try to turn the flywheel of the compressor by hand. if it turns alright then check the pressure switch.
3. check the setting of the pressure switch, it cuts off power to the motor when the preset pressure is reached, maybe it is set too low, adjust the pressure switch to a higher setting

The problem is probably a faulty motor run capacitor.
You will find it under the cover where the cable goes into the motor. Stephen.

The compressor may be fine after setting off or a day. After retyring it then if it still dosnt start either replace the run capacitor to the compressor and fan motor or have it checked to see if it is in range.

The hum your hearing could that be the contactor relay humming?

The fact that your compressor completes a cycle indicates that the motor and pump are probably working correctly, however when the pressure switch cycles off there should be a slight release of the air that remains in the airline. If this air is not released thru the unloader of the pressure switch the motor cannot overcome the back-pressure in the airline and just hum. There should be a 1/4 in. hose extending from the pressure switch to the checkvalve in the tank. There is a small tab and needle valve that releases the air. Also check that the tank checkvalve is not leaking air back into the airline as this would cause back pressure. Finally, failing motor start capacitor will cause motor to hum. Weak capacitor will start motor when cold and no pressure in tank, but when things heat-up and with pressure in tank, motor may not have enough power to overcome load and hum. Enjoy

All electric compressors share a basic design with ac power pressure switch with unloader and checkvalve between the pump and the tank. When the pressure switch powers off, it unloads the compressor by releasing the air in the discharge hose. You will notice a small tire stem pin and a lever to engage it somewhere on the pressure switch. If the compressor stops before reaching set cutoff point, the air pressure remains in the discharge hose not allowing the motor to start, thus the hum you hear. Solution is to check for the proper function of pressure switch /unloader, tank check valve (if check valve leaks air back to compressor it will load motor causing hum) and extra measure check capacitor for at least 90% of rated valve. Finally, do not run this compressor with an extension cord, use more hose instead. enjoy