Electrical and Control Systems
Core concepts
Expect single and three-phase fundamentals, Ohm's law, power, power factor, motor starting and protection, contactors, relays, overloads, capacitors and thermistors. Understand control circuit versus power circuit, ladder logic reading, series and parallel behaviour, and the role of safeties in series: high pressure, low pressure, oil pressure, flow and overload switches. Know thermostat and pressure switch types, defrost timers and termination, variable speed drives, electronic expansion valves and sensor types including NTC and PT1000. Fault diagnosis relies on systematic measurement: volts, amps, ohms, insulation resistance, and verifying supply at each point. Traps include assuming a clicking contactor is energised, misreading normally-closed safeties, ignoring control transformer failure, and diagnosing a capacitor by appearance rather than capacitance test. Understand electrical isolation, proving dead, and testing before re-energising.
Traps
Questions may describe a compressor that hums and trips: consider start capacitor, run capacitor, windings, supply phase loss and mechanical seizure in a structured order rather than guessing.
Worked example
A condensing unit will not start; the contactor coil reads 0 V. Trace back: the high-pressure switch is open. Check why head pressure is high, for example a failed condenser fan. The safety did its job; the fault is elsewhere.
Rule of thumb
Measure, do not assume: prove supply, prove coil voltage, prove contact continuity, then condemn the component.