Electrical Theory, Circuits, and Motor Controls
Domain 2 tests electrical fundamentals, AC/DC machine theory and the control logic that makes an elevator run safely. Expect ohms-law arithmetic, three-phase power, contactor and relay circuits, and the difference between overcurrent and overload protection.
Fundamentals that get used
Ohm's law and series/parallel rules let you check a control circuit: a 120 V coil with 240 Ω draws 0.5 A, and a coil in series with a resistor drops voltage in proportion to resistance. Three-phase induction motors run near synchronous speed with slip rising under load. Traction machines are increasingly driven by VVVF inverters that vary voltage and frequency to control speed and torque, while older gearless DC machines used motor-generator (Ward-Leonard) sets. Encoders supply speed and position feedback to the drive and controller.
Protection and control
NEC Article 620 covers elevator feeders, disconnects and wiring methods; Article 430 covers motor branch circuits. Overload protection is sized to the motor nameplate full-load amps and protects the motor from sustained excess current. Overcurrent protection handles short circuits and ground faults and protects conductors. Control circuits are ladder logic: seal-in contacts hold a relay picked up, interlocks in series prevent opposing motions, and normally closed stop circuits drop the main contactor.
Worked example
A 208 V, three-phase, 15 hp machine motor has a nameplate FLA of 21 A. An inverse-time breaker for a motor branch circuit may be sized up to 250% of FLA: 21 × 2.5 = 52.5 A. Because that is not a standard rating, NEC 240.6 permits the next higher standard size - 60 A. The overload heaters stay at nameplate FLA; the breaker is not the overload.
Traps
Oversizing overloads to stop nuisance trips; confusing braking resistors with running resistors; forgetting a stop circuit must fail safe (open on loss of power); wiring inverter drives as if they were across-the-line starters.