In the presence of a burnt engine of a compressor, maintenance technicians generally consider that the cause must be sought in the engine itself or in the electrical components: This is not true, although it is sometimes difficult to prove. Short circuits are generally caused by engine or od failure to the compressor caused by irregularities in the system, such as those described in the previous chapters. Therefore, before incriminating the engine, one must carefully analyze all the possible causes that may have caused the fault. If the safety devices work perfectly, it is very difficult that an electrical accident can occur, except for external causes.
The electrical faults in the compressors are for the most part caused by: Power failure on one of the phases of a three-phase motor, too low voltage, defective winding, abnormal overheating, rotor friction against the stator or adjustment problems. When a motor fails, it is recommended first of all to check the state of the windings; the look of these will generally lead to the apparent cause of the fault.

Total Short Circuit

The total short-circuit of a motor occurs mainly at start-up; in fact, when windings are energized, stronger electrical and physical stresses are produced. If at this moment, the voltage is weak or the compressor stops mechanically, the motor it burns, unless the safety devices quickly enter into action. When an engine burns at start-up, soot and combustion products are confined to the intake side of the system. This finding may be useful for the diagnosis.
Without adequate safety devices, an engine can also burn due to lack of cooling, caused by insufficient or no refrigerant flow rate. If this type of fault occurs during the compressor operation, the combustion products are found inside the exhaust pipe of the system.

Causes of Total Short Circuit

When all the phases of a motor are burned out, it must be checked whether the compressor is blocked. If the compressor is completely or partially blocked, it can be assumed that the root cause of the fault is in fact mechanical origin. We must then look further to find out the cause of this mechanical failure. If, on the other hand, the compressor is not locked and seems to work properly from the mechanical side, then it is possible to assume an electrical origin of the fault. The electrical check should be started with a check of the voltage and phase balance. The supply voltage should be within a tolerance of +/- 10% compared to the plate value and phase imbalance should not exceed 2%.

Another cause of failure may be a reduced flow rate of the sucked fluid, or a zero flow. It must check the condition of the contactors: if the contacts are welded it is possible that the compressor has led the system to vacuum and could not stop: prolonged operation without refrigerant flow rate on motor can cause overheating and possibly a short circuit.
Even the short-cycle operation, in the case of reduced demand, such as during the end of the week, can cause the engine to overheat. The repeated starts, with peaks of intensity that
they result, associated to a scarce refrigerant flow on the engine during the short operating cycles they can also cause real engine destruction over time.
The lack of compressor cooling can be detected by the numerous grooves in the bell mouth
of the piston, without any other visible damage to the connecting rods or bearings.
Since the refrigerant contributes to the cooling of other moving parts, as well as of the motor, a reduced or almost no flow rate causes a heating of the piston-cylinder assembly, and given that the expansion coefficient of the aluminum is greater of the material of the cylinders, the pistons tend to seize, thus causing scratches. After verifying that the motor has failed, always check the condition of the contactors, since the point of intensity that accompanies a short circuit often ruins or welds the contacts. In the event that the system has to operate at minimum for prolonged periods, or if it is subject to load fluctuations that cause frequent cycles, it is necessary to install a timer that limits the number of starts at 4-5 per hour.