Figure 1.

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electrical schematic diagram v a time clock regulating a defrost circuit and also a refrigeration circuit.
Many organization technicians troubleshoot refrigeration and also air conditioning systems on a everyday basis. Finding out the most efficient methods the troubleshooting not just saves the company technician’s time, but likewise the customer’s money. Good, methodical troubleshooting methods offer a win-win situation for both customer and also technician.

This write-up presents an example of a systematic troubleshooting technique incorporating the symptom-cause method.

Figure 1 illustrates an electrical schematic diagram showing a time clock controlling a defrost circuit and also a refrigeration circuit. Notice in the refrigeration circuit the the compressor’s operation winding has been opened up from a motor overheating problem.

The business call is a “no-cooling” call for a low-temperature walk-in cooler. As soon as the technician easily looks the system over and listens for any type of clues that what the trouble may be, he have to study the electrical schematic, if available. Understanding the logic or sequencing of the circuits prior to diving head over heels into the problem is that the utmost importance.

In this scenario, an open run winding gives details symptoms that will not exist because that other feasible system problems. For example, the technician listened to and also examined the refrigeration system, then studied the electrical schematic drawing. He then provided the symptoms.


Compressor motor hums and will not turn.

Compressor engine draws locked rotor amps (LRA).

Compressor motor’s overload trips shortly after illustration LRA; resets ~ 2 min.

The technician then turns power off to the refrigeration device to allow the motor cool down. After researching the electrical diagram again, the technician list some possible causes that will correlate come every symptom listed. If a possible cause does no correlate to every symptom listed, it can not be a feasible cause.

POSSIBLE reasons

open up start winding;

open up run winding;

open up run capacitor;

open up start capacitor;

Compressor mechanically stuck; or

Potential relay contacts between 1 and also 2 stuck open.

Notice the every possible cause detailed correlated to all of the symptoms. Every the technician needs to do is examine only this six feasible causes to find out i m sorry one is resulting in the symptoms, rather of thoughtlessly checking out the entire system.

With the strength off, the technician take away a wire off that the start winding and ohms the winding. The technician finds the it has actually 4 ohms, definition that that is no open. The technician currently takes a cable off that the operation winding and ohms that winding. The technician finds that it is an open winding due to the fact that of the infinity analysis on the ohmmeter. The compressor needs to be replaced. With either winding open, the compressor has actually no phase transition for beginning and will lock that is rotor, drawing LRA till the overload trips.If one of two people capacitor was bad, the motor may not have actually had sufficient phase transition to start. This is specifically true if the motor was under a high load. In details cases, the engine may progressively turn. If the compressor was mechanically stuck (for example, if something was wedged in between the piston and also cylinder), the motor would certainly lock that rotor and draw LRA.

If the contacts in between terminals 1 and 2 that the potential relay to be stuck open up for part reason, the start capacitor would be the end of the circuit. This again probably will not reason enough phase shift to begin the motor turning. The motor would again draw LRA.

Notice the in every case, every symptoms were met.


What about an open overload, or an open up potential relay coil between terminals 2 and also 5 that the potential relay? If the overload were opened at the beginning, perhaps from also high of a compressor amp draw, the compressor would not hum or attract LRA. This would certainly not correlate with all of the symptoms listed and might not be a possible cause.

If the coil of the potential relay to be open, the contacts between 1 and also 2 that the potential relay would remain in their normally closed position and also would no open. This would cause the begin capacitor to it is in in the circuit every the time. The motor would turn, drawing higher-than-normal amperage, and also eventually open up the overload. These symptoms carry out not correlate v the original symptoms listed, thus they can not be a feasible cause.

Once the technician has actually replaced the compressor and also the mechanism is up and also running, it is crucial to operation a system inspect to check out what resulted in the compressor overheating that opened the winding. Evaporator superheat, complete superheat, condenser subcooling — together with suction pressure and also head pressure — must be taken for the device check.

In this case, the technician took a device check and also found the evaporator superheat come be an extremely high at 40 degrees F and also the full superheat to be a an extremely high 90 degrees. Condenser subcooling to be fine at 12 degrees. Both suction and head pressures were low (see Table 1).

THE systems

The problem that led to the overheating to be a faulty thermostatic development valve. The valve would certainly not open enough and the entire low side of the mechanism was gift starved. The compressor was refrigerant-cooled. This brought about the compressor to overheat and cycle top top its overload till the run winding ultimately opened.

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Without this last system check, the new compressor for sure would have failed in a short time.

Tomczyk is a professor that hvac at Ferris State University, huge Rapids, MI, and the writer of Troubleshooting and Servicing contemporary Air conditioning & Refrigeration Systems, published by ESCO Press. Come order, call 800-726-9696. Tomczyk have the right to be reached at tomczykj