Exam Question

(a) List the factors that determine the starting torgue of the three-phase induction motor. How does this torque generally compare with the value of the rated torque.

(b) The low-voltage release of an a.c. motor-starter consists of a solenoid into which an iron plunger is drawn against a spring. The resistance of the solenoid is 3S ohm. When connected to a 220 V, 50 Hz a.c. supply the current taken is at first 2 A, and when the plunger is drawn mto the "full-in' position the current falls to 0.7 A. Calculate the inductance of the solenoid for both postions of the plunger, and the maximum value of flux-linkages in Weber-turns for the "full-in" position of the plunger.