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Problem: The compressor hums for a few seconds and then trips the overload.

On smaller single phase compressors, starting relays and capacitors are used. Two windings in the compressor, start and run windings, work together to move the rotor and piston. The start winding moves the rotor in one direction and the run winding keeps the rotor in motion. On small compressors, the start winding is powered only during the first few seconds, then power is switched to the run winding. When this switching relay fails, the compressor will just hum for a few seconds and then click off on the overload. The compressor will also due this if it has seized or from low voltage at start up or a blown capacitor. With a failed relay the compressor rotor may start to move and pump for a few seconds or it could just remain stationary. In either case the windings draw high amps and trip off on the external or internal overload after a few seconds. Due to the design and location of these starting relays, it can be difficult to determine if the problem is the starting relay or the compressor without actually replacing the relay, but replace a relay is much easier and less costly then a compressor replacement. In many cases it's cheaper to buy new equipment then a compressor replacement.

Look for obvious indications of a problem first. Is the compressor being powered by the controls. A single phase compressor has 2 wires which feed power from the control systems. This will be 120VAC or 208-240VAC. If the power is not getting t the compressor the problem is in the controls or power supply. If you can hear the compressor hum while it is starting and trip or is warm to the touch it is likely getting the power and the problem is in the starting components or the compressor.

Turn the power off and remove the compressor terminals cover. Small compressors usually have the starting relay and over load plugged directly onto the compressor terminals. Larger compressors will have the starting components in a electrical box beside the compressor and just the 3 wires will be under the terminal cover. Check the relay and wiring for signs of over heating or melting. If the relay is a blob of melted plastic and copper then you know the relay is shot, but the compressor could be OK still. If the wires on the relay connections are burnt then repair them. This is a common problem that is easily fixed with new connectors.

External overloads are used on small compressors are connected to the C terminal of the compressor. Found under the compressor terminal cover and clip to the side of the compressor. These are a normally closed switch which open if the unit draws high amps or gets too hot. Use an ohm meter to check for continuity. It should be either open or closed, when closed the resistances should be zero, even a few ohms is problem.

Removed the starting relay or wires to expose the compressor terminals, don't forget to mark which wire goes where. The connections to the compressor terminals should be tight but the relay should pull away from the compressor with a minimum amount of force. Use an Ohm meter to test the compressor windings for continuity and shorts to ground. Place one lead on the copper piping and the other on any of the 3 terminals and set the meter to to read the highest setting - IE 20Meg ohm- The path to ground should be infinite but maybe as low as 5 meg ohms and the compressor would still run.

There are 3 terminals, C=Common, S=Start, and R=Run. The start winding is measured between S & C and the run winding is between R & C. On most compressors there is an internal overload on the C terminal. If you get a continuity reading between S & R but show open between S & C and R & C then the internal overload is open. If the compressor is hot and the overload is open then wait for it to cool down and test again. The overload should be closed if the compressor is cool to the touch. If the overload does not close you can try hitting the compressor to knock it closed else replace the compressor.

If you have the compressor specifications, you can look up what the winding resistance and then measure the winding to compare them but in most cases this information is not readily available. As long as you under stand how the windings are configured. There are only 3 possible readings you can take on the 3 terminals. Run winding will be the lowest ohm reading, the start winding next higher, and both windings combined will be the highest and should be equal to the sum of the other two.

I.E. Run winding C to R =2 ohms, S to C =6 ohms, and S to R = 8 ohms.

If the reading on S to R is less than the sum, then an electrical short exist between the run and start windings. If one winding reads open or infinite then winding is damaged and the compressor needs to be replaced. If the readings from S to R is 8 Ohms and C to R is open and C to S is open, the internal overload is open. If this all checks out then the compressor is likely OK electrically

Low voltage can also cause the compressor to fail. This can come from a faulty thermostat, safety control, building wiring, unit wiring, weak connections or under sized wiring. In order to test for low voltage, the voltage should be checked at the compressor while it is starting. If the voltage is dropping more than 10% at start-up then the compressor can take to long to start and over heat. A typical example of this would be a freezer plugged into an extension cord.

Faulty start and/or a run capacitor can also stop the compressor from starting. Capacitors can hold an electrical charge even after the power is off. Make sure the power is off and short the contacts of the capacitor with a insulated screw driver before you touch it to remove any charge held. The capacitor often have bleed resistors to remove the charge after the power is off, but not all fo them. Visually inspect the capacitor for damage or oil residue. If the casing appears to be bloated or expanded or oil appears to be leaking from the capacitor, then it has failed. If its casing appears ok then use a capacitor tester to check capacitance. Even an Ohm meter can give provide some information if you do not have a capacitance tester. When using and ohm meter the reading should go to 0 and then ramp up to infinite as the capacitor discharges, it doesn't read the capacitance but is a likely indicator the cap is OK.

If all this checks out then a new starting relay might do the trick, but you still don't know if the compressor is seized. You can only prove this by trying to start the unit. This would take a new relay or a compressor testing cable.

There are 100's of different starting relays and overloads, you must refer to the manufacturer compressor data to find the correct one or check the relay for the part number. Solid state starting relays which can be used on a multitude of compressors work well in but may not fit under the compressor terminal cover.

If the compressor has seized a booster or hard start kit which can sometimes free a struck piston and squeeze a little more life out of the compressor. The compressor will be running on borrowed time and will fail again, but it can add another year or more life to a compressor. .