Capacitor

A motor run capacitor keeps a single-phase compressor or fan motor turning. When it fails - and they do, usually at the end of a hot season - the motor either will not start, hums and trips on overload, or runs hot and slowly until the winding fails.

The CBB65 series is the aluminium-case, bakelite-cover type with explosion-proof construction, the standard part for compressors and larger fan motors. Capacitance runs from 2 to 100 microfarad at 250, 370, 440 or 450 VAC, with tolerances of plus or minus 5, 10 or 15 per cent. The CBB61 series is the smaller plastic-case version used on fans and small motors.

Both series are made to GB/T 3667.1, which is the IEC 60252-1 standard for AC motor capacitors. The climatic categories are 40/70/21 and 40/85/21, operation class is B (10,000 hours) or C (3,000 hours), and the safety protection class is S2 - meaning the case is designed to fail without flame. Case sizes run from 40 x 60 mm to 63.5 x 125 mm.

Selection is by capacitance, voltage and case size. The voltage rating must be at least the highest voltage the capacitor will see, which on a 230 V motor is not 230 V but the back-EMF at start - fitting a 250 V capacitor to a 230 V motor that is switched frequently is a common cause of early failure.

When replacing, match the microfarad value: a larger capacitor raises the current in the auxiliary winding and shortens motor life, a smaller one reduces starting torque. Discharge the old capacitor before handling it, and fit the new one with the terminals clear of the case.