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One of the common problems found while troubleshooting reciprocating compressors is that they simply fail to start. This could be the result of many different causes, both electrical and mechanical. One common mechanical cause is a stuck piston. Mechanically the piston cannot overcome an internal resistance to move.

Normally when this is found to be the problem, the compressor will need to be replaced. However there are some tricks of the trade, which a technician can use to try to free a stuck piston. If any of these tricks solve the problem, it may be only a temporary fix as the defect within the compressor can cause it to lock up once again.

HARD-START KIT

On single-phase compressors with a permanent split capacitor motor, a hard‑start kit can be added to the compressor’s motor. This kit consists of a starting relay and start capacitor. This will give the motor additional torque at startup and may allow the compressor to start. It is normally best to contact the compressor manufacturer to determine the correct size start relay and start capacitor to use. There are universal hard-start kits available that can be used on multiple size and style compressors. Check with the manufacturer or local supplier of these universal hard‑start kits for the right size to use.

REVERSE START AND RUN LEADS

Another trick which can be used on single-phase compressors is to reverse the start and run leads of the compressor and momentarily energize the compressor. This may also help to free the piston. Only do this for a few seconds and then switch both the run and start leads back to their original position. Applying continuous voltage to a compressor in this fashion will damage it.

This process can also be used on three-phase reciprocating compressors. Switching any two leads may help to free up the piston or pistons. On a three-phase compressor you do not need to switch the leads back. They can be left reversed. Do not use this technique on scroll compressors, as they are not designed to operate in this fashion.

ENERGIZE THE COMPRESSOR

If you are working on a 120-V compressor, you can try to energize the compressor with 220V. This may help free the piston. Do this for only one or two seconds as it could further damage the compressor’s motor.

THE HAMMER METHOD

One other method to try is to simply strike the top of the compressor with a hammer while starting the compressor. If you are lucky, maybe it will start. More than likely it will not, but it may be worth a try.

Trying to free a stuck compressor usually does not yield positive results, but occasionally it will allow a technician to get a compressor started if only for a short period of time. Again, if any of these tricks do solve the problem, it may only be a temporary fix at best, as more than likely the defect will once again cause the compressor to lock up.

For more information, visit www.rhvactools.com.

Publication Date: 08/03/2009