Domestic refrigerator compressor trips overload after a generic PTC starter was fitted
A domestic frost-free refrigerator rated 230 V, 50 Hz, 1.1 A has stopped cooling. The interior lamp and condenser fan work. The compressor hums for about 15 s, draws 9 A, then the external overload trips; after 10 minutes it repeats. The owner has already replaced the PTC starter with a generic part and checked that the supply is 231 V. You measure the compressor terminals with the plug removed: C–S 12 Ω, C–R 28 Ω, S–R 40 Ω, and insulation to earth 50 MΩ. The run capacitor marked 4 µF measures 3.8 µF. The condenser coil is clean and the refrigerator is in a 28 °C room. The customer wants to know whether to replace the compressor or the starter, and asks if a 6 µF capacitor would give more starting torque.
1. Interpret the winding resistance and insulation readings. Do they prove the compressor is mechanically seized?
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The readings show continuity through both windings and a consistent relationship: C–S plus C–R = 12 + 28 = 40 Ω, which equals S–R. That is the expected pattern for a split-winding single-phase compressor and rules out an open winding or a simple internal break. The 50 MΩ insulation reading is not a dead short to earth, so it does not prove a ground fault, although a higher value would be preferable and a megger test at the correct voltage should be repeated if contamination is suspected.
These readings do not prove mechanical seizure. They cannot detect a shorted turn or a mechanical fault inside the shell. The 9 A current is about eight times the 1.1 A rating, which is locked-rotor current, so the rotor is probably not turning or the start winding is not being energised correctly. The generic PTC starter is a strong suspect because its resistance and temperature behaviour may not match the original. Check the PTC cold and hot resistance, the overload continuity, the capacitor after discharging it, and the voltage at the compressor terminals during the start attempt. If voltage stays near 231 V and current remains 9 A with the correct start device, a mechanically seized compressor becomes likely.
2. Evaluate the proposal to fit a 6 µF capacitor. What safe tests would you do before condemning the compressor?
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Do not fit a 6 µF capacitor in place of a 4 µF one. Increasing capacitance can raise starting torque, but it also raises start-winding current and can overheat or damage the winding, overload the PTC, and mask the real fault. The capacitor must match the manufacturer's rating and voltage; the measured 3.8 µF is close enough to 4 µF that it is not the obvious cause.
Safe tests, with the unit unplugged before resistance checks, are: confirm the correct PTC or current relay is fitted; measure PTC resistance cold and compare with specification; check overload continuity; discharge and test the capacitor; measure voltage at the compressor terminals while attempting a start; use a clamp meter to compare common, start and run currents; and check whether the compressor equalises when off. If the start device and capacitor are correct, voltage is stable, and locked-rotor current persists, replace the compressor. Recover the refrigerant properly, do not vent it, and fit a new filter drier.