Electrical Systems, Motors, and Controls
Fundamentals
Ohm's law and power relationships drive every electrical question. Single-phase power in watts equals volts times amps times power factor. Three-phase power equals volts times amps times 1.732 times power factor. Control circuits are typically 24 volts from a step-down transformer rated in VA; size the transformer for the connected load including the contactor coil and any accessories. Know the standard thermostat terminals: R power, C common, G fan, W heat, Y cool, O/B reversing valve.
Motors and starting components
PSC motors run with a run capacitor and have three speed taps; ECM and variable-speed motors are electronically commutated and sensitive to proper line voltage and control signals. A bulging or leaking capacitor is a routine failure; test microfarads against the rating. Know RLA versus LRA, overloads, start relays, and the difference between an open winding and a shorted winding. Motors must be matched to rotation and airflow direction.
Protection and disconnects
Minimum circuit ampacity (MCA) is calculated with 125 percent of the largest motor load plus the other loads; maximum overcurrent protection (MOP) sizes the breaker or fuse. Conductors are sized to ampacity, not simply to the breaker. A disconnecting means must be within sight of and readily accessible from the equipment. Bonding and grounding are not optional, and GFCI protection applies where the code requires it.
Traps
Questions often confuse voltage drop with overload, or assume a larger breaker fixes a tripping unit rather than checking capacitors, bearings, and refrigerant pressures. Do not assume a control board is bad before verifying 24 volts at the board.
Worked example
A three-phase 208-volt compressor drawing 12 amps at 0.9 power factor: watts = 208 x 12 x 1.732 x 0.9 = about 3,890 watts, or roughly 3.9 kW of input power.