UPPCL-JE’21 EE: Daily Practices Quiz 26-Nov-2021

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UPPCL-JE’21 EE: Daily Practices Quiz

UPPCL-JE’21 EE: Daily Practices Quiz 26-Nov-2021

Each question carries 1 mark.
Negative marking: 1/4 mark
Total Questions: 06
Time: 06 min.

Q1. A GTO can be turned on by applying
(a) Positive gate signal
(b) Positive drain signal
(c) Positive source signal
(d) None of the above

Q2. Schering bridge is used to measure
(a) Dielectric loss
(b) The inductance
(c) Low resistance
(d) Mutual Inductance

Q3. 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 starting current reduces but starting torque

Q4. The starting torque of a wound rotor induction motor can be increased by adding:
(a) external capacitance to the rotor circuit
(b) external inductance to the rotor circuit
(c) a combination of inductance and capacitance to the rotor circuit
(d) external resistance to the rotor circuit

Q5. Which device is used in sub-stations to improve the power factor?
(a) Synchronous reactor
(b) Series inductor
(c) Synchronous condenser
(d) Series capacitor

Q6. Ferranti effect in overhead line is experienced when?
(a) Sending end voltage is more than receiving end voltage
(b) Sending end voltage is less than receiving end voltage
(c) Sending end voltage is equal to receiving end voltage
(d) Voltage at all points in line is equal.

SOLUTIONS

S1. Ans.(a)
Sol. Gate turn off (GTO), like an SCR is a device four-layer, three junction semiconductor devices with three external terminals (anode, cathode and gate). GTO can be turned on and off by positive pulse and negative pulse respectively to the gate terminal.

S2. Ans.(a)
Sol. Schering bridge is used to measure dielectric loss. It is an AC bridge circuit that is used for measuring the insulating properties of electrical cables and equipment. It has the advantage that the balance equation is independent of frequency.
Maxwell bridge is used to measure the value of unknown inductance.
Heaviside bridge is used to measured mutual inductance in terms of a known self-inductance.
Kelvin bridge is used to measure the value of low resistance.

S3. Ans.(c)
Sol. Starting torque is directly proportional to the rotor resistance of an induction motor.

In slip ring induction motor the ends of the rotor windings are externally connected by a variable rheostat. So more the resistance, more the torque. When we add resistance to the rotor, the torque is high, the slip is high and the current is reduced. Therefore, slip ring induction motor is used for providing high starting torque.

S4. Ans.(d)
Sol. In an induction motor:
Generally, the initiation of these motors occurs when it draws full line voltage which is 6 to 7 times higher than the full load current. This high current can be controlled by external resistance connected in series with the rotor circuit.
the external resistance reduces high resistance as soon as the motor obtains normal speed and increases the starting torque of a motor. The tweaking of external resistance also aids in decreasing of rotor and stator current but improves the power factor of a motor.

S5. Ans.(c)
Sol. for power factor improvement, synchronous condenser is used.
Synchronous condenser is basically a synchronous motor operating at no-load and leading power factor.

S6. Ans.(b)
Sol. Ferranti effect in overhead transmission lines is experienced at light load condition in this phenomenon, V_R>V_S ie, receiving end voltage is greater than sending end voltage. To overcome this problem, shunt reactors are used at receiving end of lines.