SSC JE electrical question paper 2019 with solution SET-2

Answer.1. Isolation transformer

Explanation:-

The transformers which are particularly designed to provide electrical isolation between primary and secondary circuits, without a change in voltage and current level are called isolation transformers.

The turns ratio of such isolation transformers is 1:1 i.e N₁ and N₂. Hence, isolation transformers are also called 1:1 transformers.

The isolation transformer greatly reduces any voltage spikes that originate on the supply side before they are transferred to the load side. Some isolation transformers are built with a turns ratio of 1:1. A transformer of this type has the same input and output voltages and is used for the purpose of isolation only. The main function of the isolation transformer is to reduce the voltage spike before it reaches the load.

Answer.1. I₁ + I₂ + I₆ = I₃ + I₄+ I₅

Explanation:-

According to Kirchhoff’s law states that the current entering a node is always equal to the current leaving a node.

From the above Diagram

Current Flowing towards the Point: I₁, I₂, I₆ 

Current Flowing Away from the Point: I₃, I₄, I₅

Hence I₁ + I₂ + I₆ = I₃ + I₄+ I₅

Answer.3. Coasting

Explanation:-

The coasting period is from T3 to T4 i.e, from C to D. At the instant, the power supply to the motor will be cut off and the speed falls on account of friction, windage resistance, etc. During this period, the train runs due to the momentum attained at that particular instant. The rate of the decrease of the speed during the coasting period is known as coasting retardation.

Answer.1. Neon lamp

Explanation:-

Neon-lamp holders A lot of industrial electronic equipment uses neon lamps to indicate when they have been energized from the mains. If the lamp fails to glow, it means something is wrong with the supply circuit, the equipment, or the indicator itself. The neon lamps used are quite small and housed in holders 5 mm to 10 mm in diameter.

Answer.2. Amperes

Explanation:-

FUSE RATINGS

You can determine the physical size and type of a fuse by looking at it. Fuses are rated by current, voltage, and time-delay characteristics to aid in the proper use of the fuse. To select the proper fuse, you must understand the meaning of each of the fuse ratings.

CURRENT RATING

The current rating of a fuse is a value expressed in amperes that represents the current the fuse will allow without opening. The current rating of a fuse is always indicated on the fuse. To select the proper fuse, you must know the normal operating current of the circuit. If you wish to protect the circuit from overloads (excessive current), select a fuse rated at 125 percent of the normal circuit current.

In other words, if a circuit has a normal current of 10 amperes, a 12.5-ampere fuse will provide overload protection. If you wish to protect against direct shorts only, select a fuse rated at 150 percent of the normal circuit current. In the case of a circuit with 10 amperes of current, a 15-ampere fuse will protect against direct shorts, but will not be adequate protection against excessive current.

VOLTAGE RATING

The voltage rating of a fuse is NOT an indication of the voltage the fuse is designed to withstand while carrying current. The voltage rating indicates the ability of the fuse to quickly extinguish the arc after the fuse element melts and the maximum voltage the open fuse will block.

Answer.4. Reduce the effects of armature reaction

Explanation:-

A Universal motor can also refer as the AC series motor which can work on both AC and DC supply. Its construction is very similar to DC series motor.

In a normal dc. motor if the direction of both field and armature current is reversed, the direction of torque remains unchanged. So when the normal d.c.series motor is connected to an a.c supply, both field and armature current get reversed and unidirectional torque gets produced in the motor hence motor can work on a.c. supply.

But the performance of such motor is not satisfactory due to the following reasons:

1. There are tremendous eddy current losses in the yoke and field cores, which causes overheating.
2. Armature and field winding offer high reactance to a.c. due to which the operating power factor is very low.
3. The sparking at brushes is a major problem because of high voltage and current.

Now to compensate for increased in the armature reaction, it is necessary to use compensating winding. The flux produced by this winding it opposite to that produced by the armature and effectively neutralizes the armature reaction. An increase in the number of armature coils will increase the armature reaction and may cause more commutation problems. More number of armature coils will increase the armature reactance also.

Conductive Compensated Winding

If such a compensating winding is connected in series with the armature the motor is said to be ‘conductively compensated‘. For motors to be operated on a.c. and d.c. both, the compensation should be conductive.

In the conductively compensated type of motor, an additional compensating winding is placed in slots cut directly into the pole faces. The strength of this field increases with an increase in load current and thus minimizes the distortion of the main field flux by the armature flux called armature reaction,. The compensating winding is connected in series with the series field winding and the armature. Although conductively compensated motors have a high starting torque, the speed regulation is poor. A wide range of speed control is possible with the use of resistor-type starter-controllers.

Inductive Compensated Winding

If compensating winding is short-circuited on itself the motor is said to be ‘inductively compensated‘. In this compensating winding acts as a secondary of transformer and armature as its primary.

This winding is placed so that it links the cross-magnetizing flux of the armature, which acts as the primary winding of a transformer. Because the magnetomotive force of the secondary is nearly opposite in phase and equal in magnitude to the primary magnetomotive force, the compensating winding flux nearly neutralizes the armature cross flux. This type of motor cannot be used in DC. Because of its dependency on induction, the operating characteristics of an inductively compensated motor art very similar to those of the conductively compensated motor.

Answer.3. 2

Explanation:-

Indian Electricity Rules specify that the maximum load on a power sub-circuit should not exceed 3000 watts and the number of outlets should be limited to two.

Answer.2. E₁ − I₁R₁ & I₃R₃

Explanation:-

For the given network the potential at point a can be found by applying KVL.

For loop 1

V_a = E₁ − I₁R₁

V_a = I₃R₃

For loop 2

V_a = E₂ − I₂R₂

Answer.4. 2³

Explanation:-

Voltage Regulation: When the transmission line is energized, the current is flowing in the line. Due to this current, a voltage drop occurs because of the resistance and inductance of the line. So, the receiving-end voltage (V_R) is always less than the sending-end voltage (V_s).

The difference produced in the voltage at the receiving end of the transmission line from no load to full load conditions is known as voltage regulation of the line and it is expressed as a percentage of the receiving-end voltage (V_R). Mathematically, the voltage regulation is given as

${V_R}\% = \dfrac{{{V_S} – {V_R}}}{{{V_R}}} \times 100$

It is always desirable that the voltage regulation of the line should be low, or in other words, the increase in the load current should lead to a small difference in the receiving-end voltage.