Q4. a) What is a commutator? Discuss its rectifying action in detail. (6) (b) Determine the line current taken by a 440 V, three-phase, star-connected motor having an output of 45 Kw at 0.88 (lagging) power factor and an efficiency of 93 per cent. (10)
Q5. a) Describe the effects of changes in speed, rotor current and torque as load is applied to an induction motor. How does the motor adjust its stator current with changes in mechanical load? (6) b) A 20 KVA, 2000/220V, single-phase transformer has a primary resistance of 2.1 Ω and a secondary resistance of 0.026 Ω. The corresponding leakage reactance’s are 2.5 Ω and 0.03 Ω. Estimate the regulation at full load under power-factor conditions of: (i) Unity
(ii) 0.5 (lagging)
(iii) 0.5 (Leading) (10)
Q6. a) Which of the following three motors has the poorest speed regulation: shunt motor, series Motor or cumulative compound motor? Explain. (6) b) An 18.65 KW,4-pole,50HZ, 3 phase induction motor has friction and windage losses of 2.5 percent of the output. The full load slip is 4% compute for full load (i) the rotor Cu loss (ii) the rotor input (iii) the shaft torque (iv) the gross electromagnetic torque. (10)
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