Exam Question

(a) Wound rotor induction motor: Limitation of high inrush current and low starting torque is overcome – in this design of induction motor – by connecting the three phase windings of the rotor to external three variable resistors. On start, rotor windings are connected to full strength of externally star connected resistors. This ensures that the major portion of rotor currents is active in nature (rather than reactive) and hence results in high starting torque. As the rotor begins to accelerate, external resistances are decreased until full rated speed is achieved when external resistors are completely removed (electrically) from the rotor circuit.
(b) Double cage: Rotor of this type of induction motor has two concentric sets of rotor bars; outer rotor bars which are closer to stator windings and the inner rotor bars which are closer to rotor shaft. Outer conductors are made of high resistance design (smaller cross sectional area of conductor bars). Inner conductor bars are made of low resistance design (bigger cross sectional area). When motor is started, the rotating magnetic field induces rotor currents in the outer conductors (flux takes finite time to penetrate the inner bars). This current is mostly active in nature, which causes the starting torque to be greater. As motor begins to accelerate and flux has managed to penetrate through to inner bars of lower resistance, currents are mainly induced in the inner bars.