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DFCCIL’21 EE: Daily Practices Quiz 03-Sep-2021
Each question carries 1 mark.
Negative marking: 1/4 mark
Total Questions: 06
Q1. A two-pole alternator running at 1500 r.p.m. its angular velocity will be____
(a) 192 rad/s
(b) 157 rad/s
(c) 212 rad/s
(d) 118 rad/s
Q2. In a split-phase induction motor, The two-stator winding’s_____
(a) Have equal R/X_L ratio
(b) Draw only the in-phase currents
(c) Are mutually displaced by 90⁰ electrically
(d) Draw equal current
Q3. Core lamination in transformer decreases____
(a) Eddy current loss
(b) Hysteresis loss
(c) Copper loss
(d) Leakage reactance
Q4. A three-phase induction motor running at 4 % slip, if the input to rotor is 1000 W, the mechanical power develop by the motor will be___
(a) 960 W
(b) 9600 W
(c) 96 W
(d) 0.96 W
Q5. Armature of DC machine is placed on the rotor to_____
(a) Reduce the losses
(b) Save iron
(c) Support commutation
(d) Decrease armature reaction
Q6. To obtain greater efficiency, the slip of an induction motor should be
(a) Low
(b) Very High
(c) High
(d) 1
SOLUTIONS
S1. Ans.(b)
Sol. The frequency of an AC machine is expressed by the formula
∴f =((P × N))/120
where f= Frequency, in hertz
P = the number of poles of the generator rotor
N = the speed of the generator rotor (in RPM)
∴f=2×1500/120=25
Angular velocity=2πf rad/s=2×3.14×25=157 rad/s
S2. Ans.(c)
Sol. The Split Phase Motor is also known as a Resistance Start Motor. It has a single cage rotor, and its stator has two windings known as main winding and starting winding. Both the windings are displaced 90⁰ degrees in space.
NOTE:
In Split phase motor or Resistance start motor:
In split phase motor, the main winding has low resistance and a high inductive reactance whereas the starting winding has high resistance and low inductive reactance.
The current in the two windings is not equal as a result, the rotating field is not uniform.
At the starting of the motor both the windings are connected in parallel.
S3. Ans.(a)
Sol. Eddy Current Loss is directly proportional to the square of lamination thickness. If more lamination is used for a given rotor axial length, the thickness of lamination decreases which result in decrease in Eddy current loss. Thus, the use of thin lamination reduces the Eddy Current Loss.
S4. Ans.(a)
Sol. The formula is given by:
Mechanical power = (1 – S) × Power input.
Power input = 1000 W
S = 4%
S = 4/100 = 0.04
∴ Mechanical power = (1 – 0.04) ×1000 = 0.96 × 1000= 960 W
S5. Ans.(c)
Sol. The armature winding of a DC machine is placed on the rotor to improve commutation i.e., to convert the alternating voltage produced in the winding into direct voltage at the brushes.
S6. Ans.(a)
Sol. Slip can be defined as the difference between the flux speed (Ns) and the rotor speed (N). Speed of the rotor of an induction motor is always less than its synchronous speed. To obtain greater efficiency, the slip of an induction motor should be low or less than one.
Rotor efficiency=(1-s).
