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(b) flux control method(c) both the methods(d) none of these14.13For controlling the speed of dc motor of 150 hp rating, the following types ofconverters are normally used(a) single-phase full converters(b) single-phase dual converters(c) three-phase full converters(d) three-phase dual converters14.14A motor armature supplied through phase-controlled SCRs receives a smoothervoltage shape at(a) high motor speed(b) low motor speeds(c) rated normal motor speeds(d) none of these14.15A 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 tONand tOFFrespectively.
184Power Electronics14.16The advantage of tachometer speed control method for dc motors is that, itsenses(a) back emf(b) armature current(c) armature voltage(d) speedChapter 15: Control of A.C. Drives15.1In an a.c. motor control, the ratio of voltage to frequency is maintained atconstant 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.15.2A 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 areuseful.(c) In induction motor only fundamental and in heater harmonics as well asfundamental are useful.(d) In induction motor only harmonics and in heater only fundamental areuseful.15.3Speed control of induction motor can be effected by varying(a) Flux(b) Voltage input to stator(c) Keeping rotor coil open(d) None of these15.4Voltage induced in the rotor of the induction motor when it runs at synchronousspeed is(a) very near input voltage to stator(b) slip time the input voltage(c) zero(d) none of these15.5The speed and torque of induction motors can be varied by which of thefollowing means?(a) Stator voltage control(b) Rotor voltage control(c) Frequency control(d) All of these15.6An inverter capable of supplying a balanced three-phase variable voltage/variable frequency, its output is feeding a three-phase induction motor ratedfor 50 Hz and 440 V. The stator winding resistances of the motor are negligiblysmall. During the starting, the current inrush can be avoided without sacrificingthe 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 frequency15.7For the large a.c. motor control shown below, if the firing angle of SCRs in theinverters circuit is delayed thenFig. Q.15.7
185Multiple 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 increase15.8In the above system, if the firing angle of the SCRs of the controlled rectifier isdelayed, the motor speed will