Electrical Engineering ⇒ Topic : Double squirrel-cage moter

DOUBLE SQUIRREL-CAGE MOTOR

   Fig.(a) shows a double squirrel-cage rotor. There are two sets of squirrel-cage windings. There are two sets of squirrel-cage windings. The upper cage (starting cage)arranged nearer to the air gap is made of high resistivity material brass, aluminium, bronze etc. while the lower cage (running cage) is made of copper, the two cages are separated by narrow slits. As a result the lower cage has high permanence for leakage flux due to Which the leakage reactance of lower cage is considerably higher than that of upper cage. The upper and lower cages may have either common short-circulating end rings or each of the cages has its own short-circuiting ring

                                                          Fig.(a) Double squirrel-cage rotor

The two cages, sometimes, may be made of cast aluminium. In this case a high active resistance for the upper cage is obtained decreasing its area of cross-section.

   At starting the frequency of rotor currents is high and is equal to supply frequency. The current is distributed between the upper 'and lower cages in the inverse proportion of their impedance and since the lower cage has a very high leakage reactance, its impedance is very large as compared with that of upper cage. Thus the current in the lower cage is small and good starting torque.

               As the motor speed increases, the frequency of rotor currents decreases and this results in reduction of leakage reactance of lower cage. When the motor runs near aboutits full speed, the reactance of the cages becomes negligible and therefore the current divides itself in the inverse ratio of resistance. Since the resistance of upper cage is about 5 to 6 times that of lower cage, most of the current is carried by the lower cage and the motor has the running characteristic of the low-slip general-purpose motor. The torque-slip (speed) characteristics of individual cages as well as resultant (or total) area shown in Fig.(b)This motor, however, is better adapted to adjust to suddenly applied loads, since at increased values of slip, the high-resistance winding again develops significant torque. With a starting current no more than 5 1/2 times full-load current, the double-squirrel-cage motor is 2 known as a low-starting current, high-starting-torque, low-slip motor.The double-cage induction motor is ideal for compressors

                                                 Fig.(b) Torque-slip curves of double-squirrel-cage motor.