Electrical and Controls
What this domain tests
Electrical and controls questions cover the compressor circuit, safety controls, defrost, and the control sequence. You must distinguish line voltage from 24-V control voltage, recognize start components, and understand why a safety lockout occurs.
Core rules and concepts
- RLA is running load amps; LRA is locked-rotor amps. An overload protects the compressor from excessive current or temperature.
- The contactor coil is energized by the control circuit. Verify coil voltage before replacing a contactor.
- Start components include current relays, potential relays, and start capacitors. Capacitors must be discharged and checked for the correct microfarad rating and voltage.
- On three-phase compressors, rotation can be reversed by swapping any two line leads. Scroll compressors may be directional and must not run backward.
- Defrost is usually time-initiated and temperature- or pressure-terminated, with a fan delay after defrost.
- Low-pressure controls may provide pump-down or low-charge protection. High-pressure controls protect against condenser failure. Oil-pressure safety controls include a time delay.
- Thermostats and EMS controls have setpoints, differentials, and anti-short-cycle timers.
Worked example
A 230-V single-phase compressor hums and trips its overload. The contactor pulls in and 230 V is present at the compressor terminals, but the start capacitor is open. Without start assist the compressor cannot develop enough torque, so it draws locked-rotor current and the overload opens. Check the capacitor with a proper meter, replace it with the correct microfarad and voltage rating, and inspect the start relay and run capacitor.
Traps that cost marks
Assuming control voltage is present because the thermostat is calling; bypassing a high-pressure or oil-safety control; installing a higher-rated overload; checking a capacitor without discharging it; and ignoring three-phase rotation.
Key points
- Verify coil voltage before condemning a contactor.
- RLA and LRA describe different operating conditions.
- Start capacitors and relays must match the compressor.
- Three-phase rotation is changed by swapping two leads.
- Defrost termination and fan delay are separate control functions.