Electrical Machines
What the domain tests
Electrical machines questions cover transformer action, turns ratio, efficiency and basic motor operation. You should explain how a changing flux links windings, distinguish DC and AC machines, and calculate voltage, current and efficiency in a transformer.
Core rules
- Transformer: alternating primary current produces changing flux, inducing emf in the secondary. Ideal relation Vs/Vp = Ns/Np; current ratio Ip/Is = Ns/Np.
- Transformers work on AC, not steady DC. Ratings are in VA because heating depends on voltage and current, not load power factor alone.
- DC motor: back emf opposes supply; speed rises until back emf limits current. Starting current is high without a starter or control.
- AC induction motor: stator rotating field induces rotor currents; rotor runs slightly below synchronous speed. Slip s = (ns − nr)/ns.
Worked example
A transformer has 1000 primary turns and 50 secondary turns, connected to 240 V AC. Secondary voltage: Vs = Vp × Ns/Np = 240 × 50/1000 = 12 V. A 24 W secondary load draws Is = 24/12 = 2 A. Ideal primary current: Ip = P/Vp = 24/240 = 0.1 A. If efficiency is 90%, input power is 24/0.9 = 26.7 W, so Ip = 26.7/240 = 0.111 A.
Traps that cost marks
The turns ratio is easily inverted. Use VA for transformer rating, not W. Do not apply the ideal current ratio when efficiency is given. In induction motors, slip is small at normal load but never zero at full load.