RRB JE Electrical question paper with solution 2014 | RRB JE Electrical

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RRB JE Electrical question paper with solution 2014 | RRB JE Electrical

 

Ques.1. Cores of an electric motor are generally made from lamination of:

  1. Carbon
  2. Silicon Steel
  3. Cast iron
  4. None of the above

Answer.2. Silicon Steel

Explanation:-

An electric motor consist of mainly two parts

  1. Stator
  2. Rotor

Stator

The stator core is the part containing the main magnetic flux circuit of machines and the place where to lay in the stator winding. In order to reduce the eddy current loss and magnetic hysteresis loss caused by a rotating magnetic field in the iron core, the stator core is formed by a lamination of 0.5 mm thick silicon steel sheet, with varnish coated on both sides of the silicon steel sheet to ensure insulating. 

These laminations are insulated from one another by a coating of varnish and stacked together. Radical ventilating ducts are provided at intervals of about 8 cm along the axial length of the stator so as to improve the circulation of cooling air.

Rotor

The rotor is composed of a core, the winding, and a turning shaft, and the rotor core is also a part of the magnetic circuit of the motors. The rotor is also subjected to a rotating magnetic field though of lesser frequency. Hence, the rotor is built up of laminations of higher thickness than those of the stator. Ventilating ducts are provided in the same positions on its axial length as the stator. 

 

Ques.2. For which of the following application, a D.C motor is preferred over and A.C Motor?

  1. Variable speed operation
  2. High-speed operation
  3. Low-speed operation
  4. Fixed speed operation

Answer.1. Variable speed operation

Explanation:-

ADVANTAGES OF DC MOTORS:

Variable speed operation:- The attractive feature of the dc motor is that it offers a wide range of speed control both above and below the rated speeds. This can be achieved in dc shunt motors by methods such as armature control method and field control method. This is one of the main applications in which dc motors are widely used in fine speed applications such as in rolling mills and in paper mills.

High starting torque: D.C series motors are termed as best suited drives for electrical traction applications used for driving heavy loads in starting conditions. DC series motors will have a starting torque as high as 500% compared to normal operating torque. Therefore dc series motors are used in the applications such as in electric trains and cranes.

Accurate steep less speed with constant torque: Constant torque drives is one such the drives will have motor shaft torque constant over a given speed range. In such drives, shag power varies with speed. Quick starting, stopping, reversing and acceleration.

Free from harmonics, reactive power consumption and many factors which makes dc motors more advantageous compared to ac induction motors.

 

Ques.3. The dynamic resistance of a diode is defined as

  1. The ratio of small change in voltage to a small change in current
  2. The ratio of small change in voltage to the square of the small change in current
  3. The ratio of applied voltage to current
  4. None of the above

Answer.1. The ratio of small change in voltage to a small change in current

Explanation:-

When a diode uses A.C signal, and when the input variation is very small, the signal is referred to as small signal. Within the small limit of use, in the small portion of the V-l characteristic, even though the total characteristic is nonlinear, it is assumed that the device is linear and analysis is performed. To determine the resistance offered within this small region of operation, static resistance is not going to give complete behavior of the diode. Thus, a resistance known as dynamic resistance is useful.

Dynamic resistance is the ratio of a small change in voltage to the corresponding change in the current. Thus,

r = dv/di

 

Ques.4. One meter is equal to

  1. 10−6 micron
  2. 106 micron
  3. 10−3 micron
  4. 103 micron

Answer.2. 106

Explanation:-

1 meter is equal to 1000000 micrometer

1 meter = 106 micron

 

Ques.5. With the addition of impurities the resistance of the semiconductor

  1. Increases
  2. Decreases
  3. First increase than decrease
  4. First decrease than increase

Answer.2. Decreases

Explanation:-

Impurity Level in n-type Semiconduction: In an n-type semiconductor the impurity level appears close to the conduction band and therefore, at ordinary temperature electron can easily move from the impurity level to the conduction band and this thing contributes highly towards electrical conductivity.

Impurity Level in p-type Semiconductor: In a p-type semiconductor, the impurity level appears close to the valence band. Therefore at ordinary temperature electrons acquire enough energy to move easily from the top of the valence band to the impurity level and leaves positive holes in the valence band which is free to move within the semiconductor and this thing contributes highly toward conductivity.

It is clear, therefore, that the addition of donor or acceptor impurity increases the conductivity above the intrinsic level of the semiconductor. In n-type material the majority carriers are electrons and minority carriers are holes while in the case of p-type material the situation is reversed.

 

Ques.6. Creeping occurs in:-

  1. Ammeter
  2. Energy Meter
  3. Voltmeter
  4. None of these

Answer.2. Energy Meter

Explanation:-

Creeping in energy meter is the phenomenon in which the aluminum disc rotates continuously when only the voltage is supplied to the pressure coil, and no current flows through the current coil. In other words, the creeping is the kind of error in which the energy meter consumes a very small amount of energy even when no load is attached to the meter.

Creeping occurs due to

  1. Overcompensation for friction.
  2. Supply voltage is more than normal voltage
  3. Stray magnetic field
  4. Vibration

To eliminate this, two holes are drilled in the disc 180° opposite to each other. When this hole comes under the shunt magnet pole, it gets acted upon by a torque opposite to its rotation. This restricts its rotation, on no load condition.

 

Ques.7. The term RADAR stands for

  1. Radio direction and reflection
  2. Radio detection and ranging
  3. Radio waves dispatching and receiving
  4. Random Dispatching and receiving

Answer.2. Radio detection and ranging

Explanation:-

Radar stands for Radio Detection and Ranging. Radars use the electromagnetic spectrum for transmitting and receiving signal.

 

Ques.8. Which of the following is an active component of the circuit?

  1. Transistor
  2. Resistor
  3. Capacitor
  4. Inductor

Answer.1. Transistor

Explanation:-

Components that require an external source of power, and/or produce again in a circuit, are called active component

or

An active network transfers energy from an external source to an output signal, which may be generated within the network (as in an oscillator) or is controlled by an input signal. such as transistor, amplifier, oscillator

 

Ques.9. A transistor has an emitter current of 8mA and α of 0.99. Which of the following could be the collector current?

  1. 7.92 mA
  2. 5.00 mA
  3. 81 mA
  4. 7.84 mA

Answer.1. 7.92 mA

Explanation:-

D.C current gain α can be expressed as

α = IC/IE

Where

IC = collector current

IE = Emitter current

Given

α = 0.99

IE = 8mA

IC = α × IE

IC = 7.92 mA

 

Ques.10. An accurate ammeter must have a resistance of

  1. High value
  2. Low value
  3. Very low value
  4. Very high value

Answer.3. Very low value

Explanation:-

An ammeter is connected in series with the load andTo measure current, you want most of the current to pass through it. Hence ammeter is connected in series with the low resistance

An ammeter measures the value of current flowing in the circuit, so current should flow inside ammeter to give proper result. it acts like a closed switch so it indicates the current. And it has very low resistance to ensure the correct measurement of current in the circuit. If it is connected in parallel across any load then all current in the circuit will choose the lower resistive path (i.e ammeter) to cause its circuit to be damaged. Hence it is used in series.

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