Question

44. The direction of rotation of an induction motor depends on a. the frequency of the input voltage b. the strength of the magnetic field set up c. the phase sequence of the input supply lines and order in which they are connected to the stator windings d. whether the windings are connected in delta or star determines the speed of the rotating magnetic field a. the number of poles in the motor b. the frequency of the supply d the voltage and phase sequence of the supply the number of poles and the frequency of the supply 46. The speed of rotation of an induction motor is always le b. aay than that of the rotating magnetic field c. always the same d. as no relatio areater than that of the rotating magnetic field as that of the rotating magnetic field n to the speed of the rotating magnetic field 47. The maximum torque that an AC induction motor can produce without stalling is called a. stalling torque b. critical torque c. starting torque d. breakdown torque 48. If the ratio of the output real power to the apparent power of an induction motor is 0.77, what does this mean? 49. A change of 5% of supply voltage to an induction motor will produce a change of approximately a. b. c, d. 4% in the rotor torque 5% in the rotor torque 2% in the rotor torque 16% in the rotor torque Real power (kW) produces work and heat. Reactive power (KVAR) establishes the magnetic field. An Induction motor does not require reactive power to operate a. True b. False

Answer 1

(c) option If we interchange any of the two phase winding connection to the stator then direction of Rotating magnetic filed changes and hence direction of rotation of induction motor changes. [Q 45] Ans (d) N_s = 120 f/p [Q 46] (a) Ans Rotor speed = Slip times the stator rotating magnetic field, S = N_s - N_r/N_s N_s rightarrow 120 f/p = RMF speed N_r rightarrow rotor speed [Q.47] (a) Stalling torque- max^m torque during stalled condition. [Q 48] ratio of output real power to the apparent power of an induction motor is called the efficiency of the motor 77% efficiency \r\n T alpha V^2 If V increases by 5%, it becomes 1.05 times & therefore torque becomes (1.05)^2 = 1.1025 times i.e., 0.25% increases If V reduces by 5%, it becomes 0.95 time & therefore torque becomes 0.9025 times i.e., 9.75% less [Q 50] False [Q 51] pair of poles = 3 Therefore Total no. of poles = 6 frequency