Aircon Compressor Windings: The Motor in the Refrigerant

A compressor motor is copper wire wound into coils and packed into a steel core. Those coils are the windings, and current through them produces the turning force. The pump sits on the same shaft.

Nothing separates one turn of that wire from the next except a film cured onto the copper. The wire arrives at the winding machine already coated, and nothing is wrapped around it afterwards. Two thicknesses of film stand between neighbouring turns.

The same film keeps the windings clear of the metal around them. The core they sit in is steel, fixed to the compressor body, which is earthed through the unit. A coil pressed into a metal slot with a coating between the two is the whole arrangement.

Everything here follows from two facts: the insulation is thin, and nothing else shares that job. Almost every electrical death of a compressor is that film letting go at one point.

Putting the motor inside solves a sealing problem. A motor outside would drive the pump through a shaft crossing the shell wall, and a shaft crossing a wall must pass through something that wears. Welding the machine shut removes that joint along with the leak it becomes.

The arrangement also feeds the windings their cooling. Gas returning from the room enters the shell first and washes over the motor before it reaches the pump. Heat coming off the wire is picked up there and leaves with the stream.

That ties the motor to the circuit. What cools the wire is the same refrigerant that cools the room, so less arriving means less heat carried away, and the motor runs hot for reasons sitting nowhere near it.

So a low charge is not only a cooling complaint. A system short of gas returns warmer vapour to the compressor, and warmer vapour cannot pull the same heat out of the windings. A circuit quietly undercharged has held its motor above design temperature for just as long.

Restriction reaches the same place by another road. Anything narrowing the path back to the compressor reduces what arrives at the windings, while a choked outdoor coil raises the temperature the machine discharges into.

Heat is the ordinary one. Every run puts current through the wire, current makes heat, and insulation ages faster the hotter it is held. A motor kept warm season after season arrives there on its own.

Moisture is worth understanding because it never attacks the coating directly. Water left inside a sealed circuit finds the refrigerant and oil to work on, and acid comes out of that reaction; refrigeration guidance ties those acids to winding insulation deterioration. That is why a system is dried before charging, then given a drier to hold it dry.

The drier is a wear part rather than a fitting, and what a filter drier does is written up on its own page. Its condition and the wire's condition are one story told at two places on the circuit.

Starting is where the wire takes its heaviest load. A motor at standstill draws far more current than the same motor turning, and anything making the break-away harder stretches that moment out. Tired starting components belong here, as does a restart attempted before pressures have settled, which compressor lockout holds a unit back to prevent.

Every electrical verdict on a compressor lands on one of three findings: the copper path is broken, two things that should stay apart are touching, or a winding is touching the body. Each says something different.

An open winding means the path along the copper is broken somewhere: nothing flows, so nothing turns. The outdoor unit stays silent while the rest of the system behaves normally and the indoor fan carries on.

A short means the film gave way between two conductors meant to stay apart. Current then takes a shorter route than the motor was built around. A machine in this state often still runs: it pulls more than it should, makes heat where it should not, and cools less.

A winding touching the body is the third, and it ends the discussion fastest. The steel shell is earthed through the unit, so a winding in contact with it sends current to earth. Earth-leakage protection exists to catch exactly that, and these usually trip the ELCB the moment the unit is asked to run.

What separates the three in the field is what the machine does while power is still on it. Silent outdoors with everything else alive points one way, running badly another, and cutting the supply the instant it starts points at the third.

Almost everything else about a sealed compressor is inferred. Pressures, currents and temperatures describe behaviour, and a technician reasons backwards toward what must be happening inside. The windings are different: the wire has ends that come out through the shell.

Two things get read there. One is resistance along the windings, which says whether the copper path is intact and whether the two windings still stand in the right relationship. The other is resistance between a winding and the steel body, which says how well the film holds current away from earth.

That second figure is what people usually mean by the compressor being tested. It measures the coating rather than the copper, and it is the only direct evidence about the insulation; everything else on a sealed part is argument from behaviour.

One figure on one day is not a verdict, and the manufacturers say so. Copeland's published position is that a single poor reading should not condemn a hermetic compressor; contaminated oil and refrigerant both move the number.

Temperature moves it as well. A compressor standing cold with liquid refrigerant in the shell reads lower than the same compressor warm and running, because what surrounds the windings has changed. Two honest technicians can measure one machine hours apart and disagree.

None of that makes the measurement worthless. It makes it evidence rather than a verdict, a distinction worth holding once a large number is attached to the answer.