2 SKEE 2413 PART A: COMPULSORY Q.1 (a) If a three phase, 50 Hz induction motor is needed to drive a load at 950 rpm, how many poles should the motor have such that the slip is obtained as small as possible. Then, determine the synchronous speed and the corresponding slip. (3 marks) (b) Draw an equivalent circuit for one phase of an induction motor and the associated power-flow diagram. (8 marks) (c) Draw torque versus speed (T- ω ) characteristic for a typical three-phase induction motor and label in the plot the region where torque is proportional to slip. (3 marks) (d) An eight-pole 50-Hz ac induction motor produces an output power of 2 hp and has rotational losses of 100 W at a speed of 725 rpm. Determine: (i) the slip (ii) the frequency of the stator currents (iii) the frequency of the rotor currents and (iv) the rotor copper loss. [1 hp = 746 W] (5 marks) (e) A 440 V rms (line-to-line) 50-Hz three-phase star-connected induction motor draws 16.8 A at a power factor of 0.8 lagging. The stator copper losses are 350 W, the rotor copper loss is 120 W, and the rotational losses are 400 W. Find: (i) the power crossing the air gap P ag , (ii) the developed power P dev (iii) the output power P out and (iv) the efficiency. (6 marks)
3 SKEE 2413 Q.2 (a) Explain the classification of transmission line based on their length and the parameters considered to represent the model. (3 marks) (b) A steam power station spends RM30 million per annum (year) for coal used in generation. The coal has a calorific value of 5000 kcal/kg and costs RM3000 per ton. If the station has thermal efficiency of 33% and electrical efficiency of 90%, find the average load (in kW) for the station.
- Fall '20
- Electric motor, SKEE 2413