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Localized Short Circuit

Metal fragments resulting from a mechanical accident can fit into the motor windings and spoil the insulation of the same. A localized short circuit like this can occur even after several weeks, or even months, after the restarting of the compressor which has been repaired. Metal particles can remain in the engine without causing any damage for a time, and then moving to a critical point and causing a failure. It is therefore recommended, after each mechanical repair carried out on a compressor, to inspect the motor and its shaft very carefully.

A localized short circuit can also be caused by the relative movement of the individual turns of a winding. When the engine starts, the ends of the windings flex slightly and this causes a certain friction between the turns; over time this friction can cause deterioration insulation and therefore a localized short circuit. The heat released causes the insulation of the adjacent turns to melt, which gives rise to a short circuit between the phases, or between phase and ground.
If the short circuit originated at the point where the ends of the windings enter in the slots of the magnetic circuit, there may have been a pressure, or a metal fragment may have inserted between the coils thus speeding up the wear process insulation. A similar short circuit can occur, for the same reasons, in the slots of the magnetic circuit. After a localized short circuit has occurred, the motor must therefore always be removed and inspected carefully its shaft and windings, looking for any metal fragments; these obviously must be completely removed before replacing an engine.

Causes of Localized Short Circuits

When a valve breaks, a piece of it or the spring can be sucked in from the low side compressor pressure and go into the motor windings, thus causing a short circuit. Even physical stress on the engine can, over time, cause the same kind of electrical failure. If no metal particles are found in the windings or in the motor shaft, you can reasonably expect that the failure was caused by these stresses.

Phase imbalance Causes

The lack of power on one of the phases of a three-phase motor causes a considerable imbalance and it forces the other two phases to absorb excessive current. If the safety devices do not react fairly quickly, the two overloaded phases will burn out. The windings of two phases are burned out, while the four poles of the remaining phase are intact. There arrangement is as follows: two burnt windings - one intact, two burnt - one intact, etc. When this kind of arrangement appears, the power supply has been certainly interrupted on one of the phases.

This situation is typical of a power imbalance; however, cases can arise where, with the same unbalance conditions, one phase heats up quicker than the second, therefore is the only one to burn. In this case, it is advisable to carefully inspect the other two phases: if one of these has traces of abnormal heating, it can be assumed that there was an imbalance at the origin of the fault feeding.


Author, Riccardo Tigani  Linkedin Page
General Manager at Linea3C Srl

9th February 2020

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