Electrical Systems, Motors, and Controls
Circuits and motors
Ohm's law (E = I × R) and the power relationships (watts = volts × amps × power factor for single phase; ÷ 1.732 for three phase) underpin every troubleshooting question. Motors come as PSC (permanent split capacitor), ECM (electronically commutated, constant airflow and more efficient), and shaded-pole or three-phase varieties. Nameplate values matter: RLA, LRA, FLA, MCA (minimum circuit ampacity) and MOP (maximum overcurrent protection) are not interchangeable - wire and breaker are sized from MCA/MOP, and equipment must be protected at or below MOP.
Controls
The 24-volt control circuit is powered by a transformer rated in VA; total connected VA must not exceed that rating. Typical thermostat terminals: R (power), C (common), W (heat), Y (cooling compressor), G (fan), O/B (reversing valve). Residential control components include contactors, relays, sequencers, defrost boards, and time delays. Electrical safety: lockout/tagout, discharge capacitors before touching them, verify with a meter, and never bypass safety switches.
Traps
Mixing up MCA and MOP; assuming a run capacitor reads good without checking microfarads against the rating (tolerance is usually ±6–10% and it must be tested out of circuit); forgetting that a weak capacitor causes high amp draw and hot motors; reading 24 V controls with a load-free meter and missing a high-resistance connection; and sizing conductors without the 125% factor for continuous motor loads.
Worked example
A run capacitor rated 45 µF measures 42 µF out of circuit. Ten percent tolerance gives a floor of 40.5 µF, so 42 µF is within tolerance; keep looking at the motor windings, bearings, and supply voltage.