Electrical and Electronic Controls for AC
The circuit
A domestic split is single-phase: phase, neutral and protective earth, with the earth sized and never switched. The compressor is fed through a contactor or relay, protected by an overload, with a run capacitor and sometimes a start capacitor with a PTC. The motor has common, start and run terminals: resistance C–S is the highest, and C–R plus R–S approximately equals C–S. Fans have their own capacitor and speed taps.
Electronics you will meet
Inverter units rectify to DC, switch through an IPM (intelligent power module) and vary frequency to match load. Sensors are thermistors - a 10 kΩ bead at 25 °C is typical - and the PCB displays error codes. Communication between indoor and outdoor boards is low voltage; never probe it with a low-impedance lamp.
Worked example
A compressor will not start. Nameplate current 7.2 A, capacitor 30 µF. Measured: the capacitor reads 11 µF and C–S is open. There are two faults. The open winding condemns the compressor; the low capacitor alone would cause a hum and an overload trip. Replace both, and check the run capacitor before starting the new compressor.
Traps
Fitting a 25 µF capacitor where 30 µF is specified; testing a live board; forgetting to discharge capacitors before handling; and ignoring earth continuity and insulation resistance, which should be at least 1 MΩ.