Electrical Machines and Transformers
What is tested
Construction and working of DC machines, three-phase induction motors and single-phase transformers, with simple numericals on slip, EMF and efficiency.
DC machines
The commutator is what makes a DC machine a motor or a generator. Its EMF equation is E = (P Φ N Z)/(60 A). Shunt, series and compound fields give different torque-speed behaviour; a shunt motor must never be started direct-on-line because back EMF is zero at standstill and starting current would be enormous.
Three-phase induction motor
Synchronous speed Ns = 120 f / P. Slip s = (Ns − N)/Ns and rotor frequency = s × f. A 4-pole, 50 Hz motor has Ns = 1500 rpm; at 4% slip it runs at 1500 × 0.96 = 1440 rpm. DOL starting suits small motors up to roughly 5 hp; star-delta starting cuts starting current and starting torque each to one-third.
Transformer
EMF per turn is E = 4.44 f N Φm. A transformer cannot change frequency; it transfers power at nearly constant VA. Losses are copper (I²R, rising with load squared) and iron (essentially constant). Efficiency peaks when copper loss equals iron loss. For a 5 kVA transformer at full load, 0.8 pf, 95% efficiency: output 4 kW, input 4.21 kW, total losses 210 W.
Traps
Using Ns instead of N in the slip formula; omitting √3 in three-phase current I = P/(√3 V_L cos φ); assuming a transformer changes frequency; and starting a shunt motor without a starter.