Electrical controls, motors, starters, wiring diagrams and PCB/electronic controls
What this domain tests
Compressor motors and their starting circuits, capacitors, relays, overload and safety controls, starters, wiring diagrams and the electronic controls (PCBs, thermistors, inverters) used in modern air conditioners.
Core rules
- Single-phase compressor circuits: PSC (run capacitor only), CSR (current relay with start capacitor), CSCR (potential relay with start and run capacitors), and split-phase with centrifugal switch. The start winding has higher resistance and thinner wire than the run winding; a winding check compares resistances and tests each winding to earth.
- Capacitors: the run capacitor is continuous duty (typically 1.5–3 µF, 440 V); the start capacitor is short-time rated (higher µF, 250–330 V) and is switched out by the relay. Replace a swollen or leaking capacitor, never repair it, and discharge it before testing.
- Protection: internal line-break overload (klixon), external thermal overload relay, HP and LP cut-outs, discharge temperature protector, phase-failure and low-voltage protection on three-phase units.
- Starters: DOL (simple, high starting current), star-delta (starting current reduced to one third), auto-transformer, soft starter and VFD/inverter drive.
- Wiring: keep live, neutral and earth colours correct. The PCB works on low-voltage DC, so reversed polarity or a wrong sensor can destroy it. An error code points to a circuit, not always to the component.
- Inverter ACs rectify to DC and drive a brushless compressor through an IPM module; the DC bus capacitor stays charged after power off.
Worked example
A 415 V, 10 A motor takes 6 times full load on DOL, that is 60 A line current at start. In star-delta the motor starts in star, so each winding receives 415 / √3 = 240 V and the line current is one third of the DOL value: 60 / 3 = 20 A. Starting torque also falls to one third, which is why star-delta suits compressors that start unloaded.
Traps
Fitting a run capacitor where a start capacitor belongs, so it fails within hours; testing a capacitor on the wrong meter range; resetting a tripped overload repeatedly without finding the cause; touching inverter DC bus terminals; misreading PCB terminal numbering.