Q4. a) Explain the significance of the root mean square value of an alternating current or
voltage wave form; Define the form factor of such a wave form. (6) b) A 72 KVA transformer supplies a heating and lighting load of 12 kW at unity power factor and a motor load of 70 KVA at 0.766 (lagging) power factor. Calculate the
minimum rating of the power-factor improvement capacitors which must be connected
in the circuit to ensure that the transformer does not become overloaded (10)
Q5. a) Show how the power that is transferred across the air gap of the three-phase induction motor is represented. Explain the terms. What portion of this is useful power? (6) b) A 440V shunt motor takes an armature current of 30A at 700 rev/min. The armature resistance is 0.7 Ohm. If the flux is suddenly reduced 20 percent, to what value will the armature current rise momentarily? Assuming unchanged resisting torque to motion, what will be the new steady values of speed and armature current? Sketch graphs showing armature current and speed as functions of time during the transition from initial to final, steady-state conditions. (10)
Q6. (a) What are the factors which determine the synchronous speed of a motor? (6) (b) A three-phase induction motor is wound for four poles and is supplied from a 50 Hz System, Calculate: (i) The synchronous speed (ii) The speed of the rotor when the slip is 4 percent (iii) The rotor frequency when the speed of the rotor is 600 r/min. (10)
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