Refrigerator That Hums, Clicks and Never Cools
A 190 L single-door domestic refrigerator with a hermetic compressor and R600a refrigerant is five years old. The owner reports that it stopped cooling two days ago. The compressor can be heard to hum for 6–8 seconds and then click off; the cycle repeats about every two minutes. The condenser grille at the back stays at room temperature. The socket voltage measures 196 V while a 1.5 TR window AC on the same line is running, and 224 V when the AC is switched off. With the PTC relay removed, resistance readings at the three compressor pins are: 12 Ω between A and B, 26 Ω between B and C, and 38 Ω between A and C. Insulation from any pin to the compressor shell is above 100 MΩ. The PTC relay rattles when shaken. The overload protector shows continuity at room temperature. No frost pattern appears anywhere on the evaporator.
1. Identify the common, run and start pins and the faulty component, showing the reasoning behind each conclusion.
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The readings satisfy 12 + 26 = 38, so pin B is common: A–B is one winding (12 Ω), B–C is the other (26 Ω), and A–C measures the two in series. The lower value is the run winding (A, 12 Ω) and the higher is the start winding (C, 26 Ω) - a start/run ratio of about 2.2, normal for a PSC compressor.
Both windings are therefore intact and the start/run ratio is correct. Insulation to the shell is above 100 MΩ, so the motor is neither earthed nor open. The PTC relay rattles when shaken: the ceramic pill has fractured, the start winding is never connected, and the compressor cannot develop starting torque. It hums on the run winding, current stays at locked-rotor level, and the overload protector opens after 6–8 s and resets - exactly the reported cycle. A condenser at room temperature confirms that no refrigeration took place at all.
Diagnosis: failed PTC start relay. A mechanically seized compressor is the alternative and is ruled out by fitting a correct replacement relay and re-testing before condemning the compressor.
2. Explain the significance of the 196 V reading and the advice you would give, using numbers.
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Induction motor starting torque is roughly proportional to V². Against 224 V, a supply of 196 V gives (196/224)² = 0.77, i.e. about 23% less starting torque, while current for the same load rises, so the overload trips sooner. That matters practically: a new relay may still fail to start the compressor during AC peak hours, and the customer will blame the repair.
Advice: give the refrigerator its own circuit from the DB, check terminal joints and cable size for avoidable drop, and measure voltage at the compressor terminals while it attempts to start, not only at the socket. If the supply stays near 196 V for hours, recommend a correctly rated voltage stabiliser, or ask the utility/landlord to check the incoming connection and balance the loads. Record the voltage on the job sheet: a compressor repeatedly failing to start can be damaged, and this evidence protects both the customer and the shop.
3. The owner asks you to 'top up the gas'. Explain why not, and list the safety rules for working on this unit.
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Refuse and explain: a domestic refrigerator is a sealed circuit - refrigerant is not consumed, so if it is short there is a leak that must be found, repaired, evacuated and recharged by weight. Topping up without leak detection overcharges the system, raises head pressure and current, and cannot restore cooling when the compressor is not even running. On this unit the fault is electrical; no gas diagnosis is possible until the compressor starts.
Safety, because the refrigerant is R600a (isobutane, flammable): no smoking or open flame; never drill into the circuit; work in a ventilated area; use only refrigerant-rated gauges and a spark-free vacuum pump; never vent refrigerant to atmosphere; braze only after evacuation and nitrogen purging. Discharge the run capacitor before touching terminals, fit the correct PTC relay rated for this compressor, check the overload protector, and never substitute a relay 'to make it run'.