# SSC JE electrical 2019 question paper with solution SET-1

Ques.91.  ______starting method CANNOT be used for starting a 3-phase squirrel cage induction motor.

1. Direct-on-line
2. Auto-transformer
3. Rotor-resistance
4. Star-delta

Explanation:-

Squirrel-cage induction motors are the most common and are either three phases or a single phase. These motors contain a rotor in which the conductors, made of either aluminum or copper, are arranged in a cylindrical format resembling a ‘squired cage’.

Squirrel-cage induction motors have no external electrical connections to the rotor which is made of solid, uninsulated aluminum or copper bars short-circuited at both ends of the rotor with solid rings of the same metal.

Since in squirrel cage induction motor the metal conductor bars are embedded in the surface of the rotor, therefore, it’s resistance is fixed and can’t vary & even no external resistance can be added therefore rotor-resistance method can’t be used in squirrel cage induction motor.

Ques.92. A three-phase induction machine, when it is operating like a motor, the range of slip is given by

1. −1 > s > −2
2. 0 ≤ s ≤ 1
3. 1 < a < 2
4. 0 > s > −1

Answer.3. 0 ≤ s ≤  1

Explanation:-

### Torque speed characteristic of induction motor

Motoring mode: 0 ≤ s ≤  1

For this range of slip, the load resistance in the circuit is positive, i.e. torque developed is in the direction in which the rotor rotates.

• In this region the value of slip lies between 0 to 11 i.e., slip is positive.
• The motor rotates in the same direction as that of a rotating magnetic field.
• At s = 0 (synchronous speed), the torque produced by the motor is zero because the induced voltage in the rotor is zero when N= Ns.
• The torque increases as the slip increases while the air gap flux remains constant.
• The torque-slip characteristic from no-load to somewhat beyond full-load is almost linear.

Generating mode: s <0

• In this operating mode, the slip s is negative i.e., s < 0. The slip will be negative if and only if the rotor speed N is greater than the synchronous speed Ns (N> Ns). However, the rotor and R.M.F both rotate in the same direction.
• In this region, the motor acts as a generator and return the power back to a.c. source.

Braking or plugging Mode: s > 1

• In this region, the value of slip is greater than 1 and the rotor rotates in the opposite direction of the rotating magnetic field.
• This is achieved by interchanging any two phases of the stator supply.

Ques.93. The average value of a sinusoidal wave is:

1. 0.637 × maximum value
2. 0.5 × maximum value
3. 2 × maximum value
4. √2 × maximum value

Explanation:-

The average value of the sine wave over one complete cycle is actually zero. Hence, for a sine wave, the average value is defined over half the period. The average value expressed in terms of peak value is given by

Average value = 0.637 × peak value

Ques.94. Identify the INCORRECT statement with regards to synchronous motors.

1. The field winding is placed on the rotor
2. It is a self-starting machine
3. The armature winding is placed on the stator
4. It is a constant speed motor

Answer.2. It is a self-starting machine

Explanation:-

A synchronous motor is not a self-starting motor. You can check out this article why synchronous motor is not self starting

Ques.95. _____ power plant is the plant which uses a non-conventional source of energy.

1. Wind
2. Thermal
3. Nuclear
4. Hydroelectric

Explanation:-

Conventional energy:- Conventional energy resources are energy resources that have been traditionally used for many years. These resources are also widely used at present and likely to be depleted.

Non-conventional energy: These are alternate energy resources to the conventional energy resources which are being considered to be used on large scale. A non-conventional source of energy is also called a renewable source of energy. The types of Non-conventional source of energy are

1. Wind Energy
2. Solar energy
3. Biomass energy
4. Geothermal energy
5. Ocean tidal energy
6. Ocean wave energy
7. Ocean thermal energy conversion

Ques.96. Which of the following is the correct relation regarding coupled coils?

1. M = (L1 + L2)/2
2. M = (L1 − L2)/2
3. M ≤ (L1L2)0.5
4. M > (L1L2)0.5

Explanation:-

Mutual inductance between two coupled coils is given by the relation

M = K√L1L2

Where

L1L2 = Inductance of the coil

K = Coefficient of coupling

When K = 1 coils are said to be tightly coupled and if K is a fraction the coils are said to be loosely coupled.

Here

Kmax =1

Kmin =0

∴ m ≤ (L1 L2 )0.5

Ques.97. Which of the following fuel has the least calorific value?

1. Wood
2. LPG
3. Kerosene oil
4. Hydrogen

Explanation

• In the above option, the wood has the lowest calorific value. Its value is around 17000 – 20000 kilojoule/kg.
• The calorific value of LPG is 55000 kilojoule/kg.
• The calorific value of Kerosene oil is 45000 kilojoule/kg.
• The calorific value of hydrogen oil is 150000 kilojoule/kg.

Ques.98. Two decimal coils A and B of 1000 turns each lies in the parallel plane such that 80% of the flux produced by one coil links with the other. If a current of 5A flowing in A produces a flux of 0.05 mWb, then the flux linking with coil B is:

1. 0.04 mWb
2. 4 mWb
3. 0.004 mWb
4. 0.4 mWb

Explanation

Flux in coil A = 0.05 mWb = 5 × 10−5 wb = φA

No. of turns NA = NB = 1000

Flux linkage in a coil with B = Flux linkage in coil A × 80/100

= 0.8 × 5 × 10−5

= 4 × 10−5 wb =0.04 mwb

Ques.99. Identify the machine shown in the circuit.

1. DC short shunt compound generator
2. DC short shunt compound motor
3. DC shunt motor
4. DC long shunt compound generator

Answer.4. DC long shunt compound generator

Explanation

Long-shunt compound generator in which the shunt field is connected in parallel will both the series winding and the armature, as shown in Fig.  In long shunt connection, the series field winding carries the current same as that of shunt field winding at no-load.

The current relations of the long-shunt connection of a d.c. compound generator is

la = If + IL

Two fields of the compound generators may be cumulative or differential wound. If it is cumulative the two fields are additive and the two fields are subtractive if it is differential wound.

The essential difference between the two is that in the short-shunt connection, the armature current excites the series field, whereas, in the long-shunt connection, the load current excites the series field.

Ques.100. If N = linkage flux, then the linkage flux per unit current is defined as:

1. Inductance
2. Magnetomotive force
3. Leakage coefficient
4. Inductive reactance

Explanation

The inductance L of this coil is defined as the flux linkage per unit current and its unit is Henry.

L =Λ/I

Where

I = current

For SSC JE 2018 SET-1  Electrical paper with complete solution Click Here

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