Ques.91. ______starting method CANNOT be used for starting a 3-phase squirrel cage induction motor.
Direct-on-line
Auto-transformer
Rotor-resistance
Star-delta
Answer.3. Rotor-resistance
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 > s > −2
0 ≤ s ≤ 1
1 < a < 2
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:
0.637 × maximum value
0.5 × maximum value
2 × maximum value
√2 × maximum value
Answer.1. 0.637 × 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.
Ques.95. _____ power plant is the plant which uses a non-conventional source of energy.
Wind
Thermal
Nuclear
Hydroelectric
Answer.1. Wind
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
Wind Energy
Solar energy
Biomass energy
Geothermal energy
Ocean tidal energy
Ocean wave energy
Ocean thermal energy conversion
Ques.96. Which of the following is the correct relation regarding coupled coils?
M = (L1 + L2)/2
M = (L1 − L2)/2
M ≤ (L1L2)0.5
M > (L1L2)0.5
Answer.3. 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?
Wood
LPG
Kerosene oil
Hydrogen
Answer.1. Wood
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:
0.04 mWb
4 mWb
0.004 mWb
0.4 mWb
Answer.1. 0.04 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.
DC short shunt compound generator
DC short shunt compound motor
DC shunt motor
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:
Inductance
Magnetomotive force
Leakage coefficient
Inductive reactance
Answer.1. Inductance
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
Λ = Flux linkage
I = current
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