Electrical Machines and Transformers
Core concepts
Understand transformer EMF equation E = 4.44fN*Phi, turns ratio, equivalent circuit referral, losses (core vs copper), efficiency and regulation, and why open-circuit and short-circuit tests separate the two loss components. For DC machines, grasp the EMF equation, armature reaction, commutation, and the different torque-speed characteristics of shunt, series, and compound motors. For induction machines, know slip, rotor frequency, torque-slip curve, starting methods, and the equivalent circuit. For synchronous machines, understand excitation, V-curves, power-angle relation, and parallel operation.
Traps examiners set
- Transformer efficiency peaks when copper loss equals core loss, not at full load.
- A series DC motor must never start unloaded (it races).
- Slip is positive in motoring, negative in generating, greater than one in braking.
- Synchronous motors run only at synchronous speed regardless of load, until pull-out.
- Open-circuit test is done on the low-voltage side; short-circuit test on the high-voltage side.
Worked rule of thumb
Efficiency is maximum when variable (copper) loss equals constant (core) loss. If full-load copper loss is Pcu and core loss is Pc, the load fraction for peak efficiency is sqrt(Pc/Pcu). Example: Pc = 400 W, Pcu = 900 W gives a fraction of sqrt(0.444) is about 0.667, so peak efficiency near two-thirds load.
Practice strategy
Draw the power-flow diagram for every machine; it prevents confusion between input, output, and losses.