Electrical Machines: Transformers, Motors and Generators
This domain tests electromagnetic machines: transformers, induction motors, DC motors and generators. The exam expects the working principle, turns ratio, losses, starting methods and basic protection of each machine.
Core rules
- A transformer works on mutual induction with alternating current. It does not change frequency. The turns ratio gives V1/V2 = N1/N2.
- Ideal transformer power is constant: V1I1 = V2I2. Real transformers have copper loss, core loss and reduced efficiency.
- A three-phase induction motor has a rotating magnetic field and runs near synchronous speed. Slip = (Ns - Nr)/Ns.
- A DC motor needs a starter to limit starting current. A generator converts mechanical energy to electrical energy by electromagnetic induction.
- Motors and generators need overload protection, proper earthing and cooling.
Worked example
A transformer has 460 primary turns and is connected to 230 V. Its secondary must supply 12 V. N2 = N1 × V2/V1 = 460 × 12/230 = 24 turns. If the secondary supplies 2 A, the ideal primary current is I1 = V2I2/V1 = 12 × 2/230 = 0.104 A. Real primary current is slightly higher because of losses.
Traps
Do not connect a transformer to DC and expect induction. Do not use the turns ratio upside down. Do not confuse motor and generator action. Do not ignore slip in an induction motor. Do not start a large DC motor without a starter.