Each year, the compressor manufacturer’s receive numerous field returned compressors that apparently have failed. A significant number of these, believe it or not, are still in good working order. A high percentage of these good compressors are single phase compressors condemned for locked rotor.
Single phase compressors have 3 windings, COMMON, START, AND RUN. All of these play an important role is getting the compressor to start and pump. Aside from possible refrigerant issues like stuck closed TXV’s or other problems, it is important to understand how to properly check out a compressor that doesn’t seem to want to “get up and go!”
Many times these compressors are returned because the technician measured locked rotor current (amps) through the compressor COMMON lead and condemns the compressor based on that measurement alone. There is another current measurement that should be made.
I was always taught that the current (amps) be measured in the START AND RUN leads and not just the COMMON lead. Almost all single phase compressors have permanent capacitor motors inside the can. This means the compressor will not start without a run capacitor (A) in series with the start winding (B). (Refer to figure 1 below to follow the description). The start lead current (amps) measurement, give a good indication of the condition of the capacitor.
With the contactor (C) engaged, there should be some amount of current flow through the start winding (B) of the compressor. If there is a START KIT (D) in the circuit, the current will be momentarily high. As the compressor comes up to speed, the START RELAY (CSR-1) CONTACTS will remove the start capacitor from the circuit. The start leg current should then drop to a lesser value. If instead, the current drops to zero amps, we can suspect an open run capacitor (A).
If there is no start kit in the circuit, the current surge through the start winding will be considerably less and will remain at a low value while running. Without a start kit, the system pressures need to be equalized or close to it. If the compressor is cycling against a high pressure difference, the start kit may be required. `
If there is current flow through the RUN winding, but none through the Start winding, either the motor start winding is open or there is a problem with the starting circuit or wiring or connections. Check for an open start winding, open capacitor, weak capacitor (see posts on compressor or capacitor diagnostics). Remember, on most single phase compressors, the internal overload is in the common winding. An open between common and start or common and run is an indication of an open overload. An open reading between run and start is a bad compressor.
The last measurement which should be made is a voltage check. First, check the voltage without a load to see that it is within the range specified on the units nameplate. Then, close the contactor and measure the voltage on the LOAD side. The voltage under load should not drop more than 10%. If it does, check for loose or corroded or dirty electrical connections. Also check for pitted or dirty contact on the contactor.
If all the checks mentioned in the post and in my other posts prove OK and the compressor is still drawing LOCKED ROTOR AMPS — now you can replace the compressor.
Last note — keep in mind that if the compressor has not failed electrically, it is having a mechanical problem. The most common cause for compressors seizing is liquid refrigerant in the compressor. If you replace the compressor and don’t correct this problem and make sure there is no liquid in the compressor, you will be replacing another compressor down the road.