b flux control method c both the methods d none of these 1413 For controlling

B flux control method c both the methods d none of

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(b) flux control method (c) both the methods (d) none of these 14.13 For controlling the speed of dc motor of 150 hp rating, the following types of converters are normally used (a) single-phase full converters (b) single-phase dual converters (c) three-phase full converters (d) three-phase dual converters 14.14 A motor armature supplied through phase-controlled SCRs receives a smoother voltage shape at (a) high motor speed (b) low motor speeds (c) rated normal motor speeds (d) none of these 14.15 A dc chopper circuit controls the average voltage across the dc motor by (a) controlling the input voltage (b) controlling the field current (c) controlling the line current (d) continuously switching-ON and OFF the motor for fixed durations of t ON and t OFF respectively.
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184 Power Electronics 14.16 The advantage of tachometer speed control method for dc motors is that, it senses (a) back emf (b) armature current (c) armature voltage (d) speed Chapter 15: Control of A.C. Drives 15.1 In an a.c. motor control, the ratio of voltage to frequency is maintained at constant value (a) to make maximum use of magnetic circuit. (b) to make minimum use of magnetic circuit. (c) to maximise the current from the supply to provide torque. (d) to minimize the current drawn from the supply to provide torque. 15.2 A single-phase voltage controller feeds an induction motor and a heater (a) In both the loads only fundamental and harmonics are useful. (b) In induction motor only fundamental and in heater only harmonics are useful. (c) In induction motor only fundamental and in heater harmonics as well as fundamental are useful. (d) In induction motor only harmonics and in heater only fundamental are useful. 15.3 Speed control of induction motor can be effected by varying (a) Flux (b) Voltage input to stator (c) Keeping rotor coil open (d) None of these 15.4 Voltage induced in the rotor of the induction motor when it runs at synchronous speed is (a) very near input voltage to stator (b) slip time the input voltage (c) zero (d) none of these 15.5 The speed and torque of induction motors can be varied by which of the following means? (a) Stator voltage control (b) Rotor voltage control (c) Frequency control (d) All of these 15.6 An inverter capable of supplying a balanced three-phase variable voltage/ variable frequency, its output is feeding a three-phase induction motor rated for 50 Hz and 440 V. The stator winding resistances of the motor are negligibly small. During the starting, the current inrush can be avoided without sacrificing the starting torque by suitably applying (a) low voltage at rated frequency (b) low voltage keeping V/F ratio constant (c) rated voltage at low frequency (d) rated voltage at rated frequency 15.7 For the large a.c. motor control shown below, if the firing angle of SCRs in the inverters circuit is delayed then Fig. Q.15.7
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185 Multiple Choice Questions (MCQs) (a) motor speed will increase (b) motor speed will decrease (c) the speed will not be effected (d) frequency of inverter output will increase 15.8 In the above system, if the firing angle of the SCRs of the controlled rectifier is delayed, the motor speed will
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