The extremely high input impedance of a MOSFET is Primarily because of

(A) Absence of its channel
(B) Depletion of current carriers
(C) Extremely small leakage current of its gate capacitor
(D) Negative VGS

Which stage of the power supply uses a zener as the main component?

a)      Rectifier

b)      Voltage divider

c)      Regulator

d)      Filter

Answer : c)      Regulator

The armature current of a dc motor fed from a thyristor power convertor contains ripple. The ripple in the armature current affects

a)      Commutating capability of the motor

b)      Overload capacity of the motor

c)      Torque capability of the motor

d)      Control ability of the speed of the motor

Which of the following configurations is used for both motoring and generative braking?

a)      First quadrant chopper

b)      Second quadrant chopper

c)      Two quadrant chopper

d)      Four quadrant chopper 

A three-phase fully-controlled thyristor bridge converter is used as line commutated inverter to feed 50 kW power 420 V dc to a three phase, 415 V(line), 50 Hz ac mains. Consider dc link current to be constant. The rms current of the thyristor

a)      119.05 A

b)      79.37 A

c)      68.73 A

d)      39.68 A

In a self-controlled synchronous motor fed from a variable frequency inverter

a)      The rotor poles invariably have damper windings

b)      There are stability problems

c)      The speed of the rotor decides stator frequency

d)      The frequency of the stator decides the rotor speed

The output voltage waveform of a three-phase square-wave inverter contains

a)      Only even harmonics

b)      Both odd and even harmonics

c)      Only odd harmonics

d)      Only triple harmonics

In a single-pulse modulation of PWM inverters if pulse width is 72° then

a)      Third harmonic will be eliminated

b)      fifth harmonic will be eliminated

c)      seventh harmonic will be eliminated

d)      none of these

In case of voltage source inverter, freewheeling diode can be avoided for the load of

a)      inductive nature

b)      capacitive natures

c)      resistive nature

d)      back emf nature

In a three phase voltage source inverter operating in square wave mode, the output line voltage is free from

a)      3^rd harmonic

b)      7^th harmonic

c)      11^th harmonic

d)      13^th harmonic

The quality of output ac voltage of a cycloconverter is improved with

a)      Increase in output voltage at reduced frequency

b)      Increase in output voltage at increased frequency

c)      decrease in output voltage at reduced frequency

d)      decrease in output voltage at increased frequency

how many switches are used to construct a three-phase to three-phase cycloconverter?

a)      3

b)      6

c)      12

d)      18

Compared to a single phase half-bridge inverter, the output power of a single phase full-bridge inverter is higher by a factor of

a)      12

b)      8

c)      4

d)      2

A four quadrant chopper cannot be operated as

a)      One quadrant chopper

b)      Cycloconverter

c)      Inverter

d)      Bidirectional rectifier

If an ac chopper is feeding an inductive load, the firing pulse to the SCR

a)      May have a width equal to turn ON time of the SCR

b)      Should be a series of pulses of short duration

c)      Should be a single pulse of long duration

d)      Should be train of pulses of duration equal to the conduction period of the SCR

Ac regulators are widely used in

a)      Traction drives

b)      Fan drives

c)      Synchronous motor drives

d)      Slip power recovery scheme of slip-ring induction motor

A single phase ac voltage controller is controlling current in a purely inductive load. If the firing angle of the SCR is α, what will be the conduction angle of the SCR?

a)      Π

b)      Π-α

c)      (2Π-α)

d)      2Π

In a 3-phase bridge rectifier circuit for HVDC transmission, the ratio of outout dc voltage to input ac voltage is

a)      0.5

b)      1.0

c)      1.71

d)      2

A three phase full coverter is supplying a purely resistive load at 300 V dc for 0° firing angle. The output voltage for 90° firing angle would be

a)      0 V

b)      40 V

c)      50 V

d)      100 V

A thyristorised, three phase, fully controlled converter feeds a dc load that draws constant current. Then the input ac line current to the converter has

a)      An rms value equal to the dc load current

b)      An average value equal to the dc load current

c)      A peak value equal to the dc load current

d)      A fundamental frequency component, whose rms value is equal to the dc load current