2 A Triac control of a three-phase, 30 kW, 400 V, 50 Hz, six-pole, star- connected squirrel-cage induction motor such that it operates at a full load efficiency of 0.85 p.u. and power factor 0.8 lagging. Calculate the average and rms current rating and maximum voltage rating required of the triacs. [Ans: 32 A, 45 A, 326 V] 3 The motor in Problem 15.2 drives a load characterised by the relation T₁ = koo where T₁ is the shaft torque and wo is the motor speed. Operation is required to give rated torque at rated speed of 960 rpm and also to supply the appropriate load at speed of 640 rpm at which the power factor is dropped to 0.6. If the motor operates at full voltage at its upper speed, estimate roughly, the change of thyristor firing-angle necessary to achieve satisfactory operation of the motor at the lower speed. Assuming that the motor impedance is unchanged between the two speeds. [Ans: 37° ≤a ≤90°]

A) At what percentage of slip is the pull-out or breakdown torque of a typical 3-phase squirrel- cage induction motor reached? 21. a) 3% slip b) 10% slip c) 20% slip d) none of the above B) If load is increased beyond the breakdown torque, what happens to the speed of the motor? a) it stops b) it may speed up beyond its rated rpm c) its speed will not change, but its current will go down d) none of the above

1. The rotor resistance and reactance per phase of a three-phase wound rotor induction motor are 0.035 and 0.15 ohm respectively. If an additional resistance is added, the starting torque is one half of maximum torque and the full load torque is three-fourth of the maximum. Determine the following: a. Full load speed b. speed at maximum torque c. value of additional resistance per required to achieve these conditions.