Quiz: Electrical Engineering
Each question carries 1 mark.
Negative marking: 1/4 mark
Time: 10 Minute
Q1. A 3~phase synchronous motor, connected to infinite bus, is operating at no load at normal excitation. The field excitation of the motor is first decreased to zero and then increased in the reverse direction. The armature current of the synchronous motor will:
(a) Remain constant
(b) First decrease and then increase
(c) increase continuously
(d) First increase and then decrease
Q2. If a resistance is inserted in the rotor circuit of a slip ring induction motor, then compared to direct line starting:
(a) Both the starting current and torque reduce
(b) Both the starting current and torque increase
(c) The starting current reduces but starting torque increases
(d) The staring current reduces but starting torque remains same
Q3. Starting torque can be obtained in the case of a single-phase induction motor with identical main and auxiliary windings by connecting:
(a) A capacitor across the mains
(b) A capacitor in series with the machine
(c) A capacitor in series with the auxiliary winding
(d) Equal value capacitors in series with the main and the auxiliary windings
Q4. Armature shunting method of speed control of dc shunt motor is preferred over armature resistance method due to:
(a) Better speed regulation
(b) Less loss in external resistance
(c) Simplicity in control circuit
(d) Reduced cost of controller
Q5. Distribution transformers have core losses:
(a) More than full load copper losses
(b) Equal to full load copper losses
(c) Less than full load copper losses
(d) Negligible compared to full load copper losses.
Q6. Hollow conductors are used in the transmission lines to:
(a) Improve heat dissipation
(b) Reduce corona loss
(c) Reduce skin effect
(d) Reduce the line inductance
Q7. In a junction transistor, the doping level of collector region is
(a) Higher than emitter region
(b) Lower than base region
(c) is higher than base region but lower than emitter region
(d) independent of the doping of base and emitter regions.
Q8. The equivalent resistance of the circuit is:
(a) 200 Ω
(b) 400 Ω
(c) 600 Ω
(d) 1600 Ω
Q9. The purpose of a coupling capacitor in an amplifier is to:
(a) Control the output
(b) Provide impedance matching
(c) Provide DC isolation between amplifier and load
(d) Increase the bandwidth
Q10. A circuit requires a capacitor of 100μf, 25V. The capacitor can be:
(a) Paper capacitor
(b) Electrolytic capacitor
(c) Ceramic capacitor
(d) Any type of capacitor
Sol. In a 3-phase synchronous motor, when the field excitation is decreased to zero then consequently air gap flux is decreased also, in order fulfill this reduced air-gap flux motor will draw more armature current form the supply and when field excitation further increased in the reverse direction then for maintaining the motor operation, motor will draw more armature current from the supply end.
Sol. In slip ring induction motor, starting current is inversely proportional to the rotor-resistance and starting torque is directly proportional to the rotor-resistance.so, insertion of resistance in the rotor circuit will result reduction in starting current and increment in starting torque.
Single-phase capacitor-start induction motor.
Sol. In dc shunt motor, armature shunting methods gives better speed regulation in comparison with armature resistance method and at reduced speed considerable waste of energy is obtained in armature resistance method.
Sol. Core loss is a fixed loss and copper loss is a variable loss. Hence core losses are less the full load copper loss because at full load Cu loss is always more than the core loss in distribution transformers.
Sol. By the use of hollow conductors in the transmission lines, diameter of conductor is increased in order to reduce corona loss.
Sol. In junction transistor, emitter region is highly doped and base region is lightly doped and collector region is moderately doped.
In section I.
= 400 + 200 + 200 | | 800Ω
= 800 | | 800 Ω
= 400 Ω
Then circuit would be as,
In section II.
= 400 + 200 + 200 | | 800 Ω
= 800 | | 800 Ω
= 400 Ω
Now circuit would be as,
∴ Total resistance
= 400 + 100 + 100 = 600 Ω
Sol. Coupling capacitor passes the ac but blocks the dc.so, it provides DC isolation between amplifier and load.
Sol. Capacitor rated 100 µf is Electrolytic type of capacitor, say, Electrolytic capacitor.
They are made with capacitance values from 0.1 µF up to 2,700,000 µF (2.7 F), and rated voltages values from 4 V up to 630 V.