Electrical Systems, Motors, and Controls
Ohm's law and circuits
Ohm's law (V = I × R) and the power equation (W = V × A, or I²R) drive almost every electrical question. In series circuits, current is common and voltages add; in parallel circuits, voltage is common and currents add. Residential control circuits run at 24 V from a step-down transformer, and transformer VA must exceed the total VA of all connected loads.
Motors and starting components
Know PSC motors (run capacitor only, no start winding switch), ECM/constant-torque and constant-airflow motors (programmed, often powered continuously at low voltage), and capacitor-start arrangements using a start capacitor plus potential relay. RLA is running load amps; LRA is locked-rotor amps. A weak run capacitor causes high amp draw, slow starts, and overheating; test microfarads against the nameplate.
Controls and sequence
Understand contactors, relays, fan centers, defrost boards, pressure and float switches, high-limit safeties, and lockout boards that require a manual reset. Trap: a safety that keeps tripping is a symptom - never bypass it. Another: thermostat terminals are not interchangeable; R, W, Y, G, and O/B each serve a distinct function.
Worked example
A 240 V resistance heater measures 12 Ω. I = V ÷ R = 240 ÷ 12 = 20 A, and P = 20 × 240 = 4,800 W, about 16,400 Btu/h. If amp draw measures 16 A instead, the element resistance has risen - check the element before condemning the contactor.