Electrical Systems, Motors, and Controls
Circuits, motors, and control logic
Start with Ohm's law and power relationships for single and three phase, then apply them to actual parts: contactors and relays, heat sequencers, defrost boards, 24 V transformers, thermostats and the common wire, run and start capacitors, and motor overloads. Motor types you must recognize: PSC, ECM constant torque, ECM constant airflow, and three-phase induction. Synchronous speed equals 120 times frequency divided by the number of poles, so a four-pole motor at 60 Hz is nominally 1,800 rpm and about 1,725 rpm under load with slip.
Read the nameplate: RLA or FLA, LRA, minimum circuit ampacity, maximum overcurrent protection. Conductors, disconnects, and breakers are selected from those values, using NEC Article 440 for hermetic motor-compressors, not from habit.
Traps: substituting a run capacitor without matching microfarads; confusing locked rotor amps with running amps; condemning a compressor that is only humming because the capacitor or contactor failed; overlooking a rubbed control wire shorting to ground; forgetting to check the common side of the 24 V circuit when a thermostat display is blank.
Worked example: a 240 V single-phase unit drawing 12 A uses 2,880 VA. If the nameplate lists 24 A minimum circuit ampacity and 40 A maximum overcurrent protection, size the wire and breaker from those nameplate numbers; the observed amp draw is a performance check, not a sizing value.