Electrical Fundamentals and Motor Controls
Basics
Ohm's law (E = I × R) and power (P = E × I) underpin everything. Series circuits share current and divide voltage; parallel circuits share voltage and divide current. Know the meters: a voltmeter across the load, an ammeter in series or a clamp around one conductor, and a megohmmeter for insulation resistance - never on an energized circuit. A capacitor can hold a lethal charge after shutdown.
Motors and controls
Read the nameplate: horsepower, voltage, full-load amps, service factor, phase, hertz, insulation class, enclosure, frame. A contactor switches the motor; an overload relay protects the motor from sustained overcurrent by sensing heat; fuses and breakers clear short circuits and grounds. Overloads do not replace short-circuit protection. A three-phase motor reverses when any two of the three leads are swapped. Wye-delta and soft starters reduce inrush, which can be six times running current. VFDs control speed and require shielded, properly grounded cable runs.
Traps
Assuming overloads protect wiring (they protect the motor); testing insulation with a megohmmeter on a live circuit; confusing control voltage with line voltage; ignoring the difference between grounded and bonded; forgetting that a stop button in a ladder is normally closed.
Worked example
A 24 V control relay coil measures 120 Ω. Current = E ÷ R = 24 ÷ 120 = 0.2 A. A coil drawing far less suggests an open winding; far more suggests a shorted coil.