Electrical Machines and Transformers
What this domain tests
The paper checks the working principle of DC machines, three-phase induction motors and transformers, plus simple calculations on turns ratio, slip and efficiency.
Core rules
- A transformer works on mutual induction and only with AC. EMF equation: E = 4.44 f N Φm.
- Turns ratio: V1/V2 = N1/N2 = I2/I1 (ignoring losses). A step-down transformer raises current in the same ratio.
- Transformer losses: copper loss (I²R, varies with load) and iron loss (hysteresis and eddy current, nearly constant).
- Induction motor synchronous speed: Ns = 120f/P. A 4-pole, 50 Hz motor has Ns = 1500 rpm; slip = (Ns − N)/Ns.
- A three-phase induction motor is self-starting; a DC motor needs a starter to limit starting current because there is no back EMF at standstill.
Worked example
A transformer has 240 V on the primary and 12 V on the secondary, and the secondary supplies 5 A. Find the turns ratio and the primary current. N1/N2 = 240/12 = 20, so it is a 20:1 step-down transformer. I1 = I2 × N2/N1 = 5 × (1/20) = 0.25 A - ignoring magnetising and loss currents, which make the real primary current slightly higher.
Traps that cost marks
Connecting a transformer to DC and expecting output; treating an induction motor as running at synchronous speed; using slip the wrong way round; confusing copper loss (load-dependent) with iron loss (constant).