Electrical Machines and Transformers
For transformers, know the equivalent circuit, the ideal transformer relationship (V1/V2 = N1/N2 = I2/I1), and the meaning of the short-circuit (impedance) and open-circuit (core-loss) tests. Per-unit impedance governs voltage regulation and fault current. Understand that transformer impedance limits fault current and that vector groups (e.g., Dyn11) determine phase shift and zero-sequence behaviour - a delta winding blocks zero-sequence current, and the clock number gives the LV-to-HV phase displacement.
For induction machines, master slip: s = (Ns − Nr)/Ns, where Ns = 120f/p. Know that torque is proportional to slip at low slip, peaks at breakdown torque, and that starting current is high (several times rated) with low starting torque in a simple squirrel-cage motor. Understand why star-delta and soft starters reduce starting current.
For synchronous machines, know the difference between cylindrical-rotor and salient-pole types, and that the machine can supply or absorb reactive power depending on excitation (over-excited supplies Q, under-excited absorbs Q).
Worked example: A 4-pole, 50 Hz induction motor runs at 1440 rpm. Ns = 120×50/4 = 1500 rpm. Slip s = (1500−1440)/1500 = 0.04, or 4%.
Traps: forgetting to convert rpm to synchronous speed correctly; assuming slip is zero at full load; and misreading transformer vector-group clock notation.