Electrical Machines, Control Circuits and Motor Protection
Candidates must understand DC machines (series, shunt, compound), AC induction motors (squirrel cage and wound rotor), synchronous machines, and transformers. Key concepts: slip in induction motors, synchronous speed Ns = 120f/P, torque-speed characteristics, and starting current (often 6–8 times full load).
Control circuits use contactors, relays, overloads, pushbuttons and limit switches. Know the difference between a contactor and a relay (contactors switch higher currents and have arc suppression), and between a normally open and normally closed contact. Understand latching (seal-in) circuits, start-stop control, and emergency stop wiring (fail-safe, normally closed).
Motor protection includes thermal overloads, magnetic circuit breakers for short circuit, and RCDs for earth leakage. Overloads protect against sustained overcurrent, not short circuit. A common trap is setting an overload to the motor's starting current rather than its full load current, or assuming an overload protects against short circuit.
Worked example: a 4-pole motor on 50 Hz has synchronous speed Ns = 120 × 50 / 4 = 1500 rpm. At 1440 rpm, slip = (1500 − 1440)/1500 = 4 percent.
Other traps: forgetting that a three-phase motor can run in reverse if two phases are swapped; confusing star and delta connection voltages; and assuming a contactor provides overload protection. Always check the control circuit's emergency stop is hard-wired and not reliant on software.