Electrical Machines and Equipment
This domain examines transformers, induction motors, synchronous machines and DC machines: their construction, their operating principles, and the calculations examiners favour - EMF, slip, efficiency and loading.
Transformers
A transformer transfers power between two circuits at the same frequency through mutual induction. EMF per phase E = 4.44 f N Φm, so the volts-per-turn is fixed by the core flux. Core (iron) loss is constant at rated voltage and frequency; copper loss varies with the square of load. Maximum efficiency occurs when copper loss equals iron loss. Distribution transformers are judged on all-day efficiency, which weights energy over 24 hours because they sit idle for much of the day.
Induction motors
Synchronous speed Ns = 120f ÷ P. The rotor runs slightly slower, and slip s = (Ns − N) ÷ Ns. Motors are self-starting, robust and cheap, which is why they dominate industry. Starting methods: direct-on-line for small ratings, star-delta (starting torque reduced to one-third), autotransformer and soft starter/VFD for controlled acceleration. Power factor is poor at light load and improves towards full load.
Synchronous and DC machines
Alternators run at exactly synchronous speed and set the grid frequency; excitation controls reactive power and terminal voltage. DC machines use a commutator; shunt, series and compound characteristics are standard questions.
Worked example: slip
A 4-pole induction motor runs at 1440 rpm on a 50 Hz supply. Ns = 120 × 50 ÷ 4 = 1500 rpm. Slip = (1500 − 1440) ÷ 1500 = 0.04, i.e. 4%. At no load slip is near zero; at rated load typically 2–5%.
Traps
Using 4.44 in a DC machine question; saying the rotor of an induction motor runs at synchronous speed (it cannot, or there would be no torque); treating iron loss as variable with load; and forgetting that a star-delta starter reduces phase voltage to 1/√3, so torque falls to one-third.