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.

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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]
The motor in Problem 15.2 drives a load characterised by the relation
TL = k o where T, is the shaft torque and o 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.
3
[ Ans: 37° <a<90°]
Transcribed Image Text: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] The motor in Problem 15.2 drives a load characterised by the relation TL = k o where T, is the shaft torque and o 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. 3 [ Ans: 37° <a<90°]
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