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Q.1 Which loss in a transformer varies significantly with load?
a) Hysteresis loss
b) eddy current loss
c) copper loss
d) core loss
Ans is C) copper loss
Transformer has two type of major Losses
1. Iron Losses
2. Copper Losses
Iron losses are constant while copper losses are variable with load.
Iron Losses(Pi) = Hysteresis losses(Ph) + Eddy current losses(Pe),
Both Ph and Pe are independent of Load Current i.e. Iron losses
are constant with changing load.
Copper losses(Pc) = Conduction losses in winding conductor which are proportional to square of Current
Q.2 For a perfectly matched transmission line, the voltage standing wave ratio is
a) Zero
b) Unity
c) Maximum
d) Minimum
Ans is b) unity
When a transmission line load impedance is same as that of the characteristic impedance, the line is said to be matched
. In a matched line, maximum transmission occurs. The reflection will be zero. The standing wave ratio
S = (1 – R)/(1 + R). For R = 0, the SWR is unity for matched line
short circuits, open circuits, and purely reactive loads having infinite VSWR
Q.3 A 15 V source is connected across a 15 ohm resistor. How much energy is used in three minutes?
a) 75 Wh
b) 0.75 Wh
c) 90 Wh
d) 0.90 Wh
Answer: Option b) 0.75 Wh
As P=V2/R = (15)2 /15 =15 w
Or
I= V/R=15/15=1 and P=I2R =(1)2(15)=15
Time = 3 minutes = 3/60 hours
Energy= Pt = (15 *3)/60 = 3/4 =0.75 Wh
Q.4 Reduction in supply voltage by 10% will change the torque of the motor by _________
a) 40%
b) 19%
c) 9.5%
d) No change
Answer: Option b) 19%
T is directly proportional to square of the voltage…So
Torque which can be delivered by the motor is proportional
to square of the operating voltage. When the supply voltage
increases by 10%, it becomes 1.1 times and therefore torque
becomes 1.1^2 = 1.21 times i.e. 21% increase.
When supply voltage reduces by 10%, it becomes 0.9 times, therefore
torque becomes 0.9^2 times i.e. 0.81 times i.e. 19% less.
Hence we can say that 10% change in supply voltage results
in 20% change in torque.
Q.5 What operation is performed in 2nd quadrant of the 4 quadrant Operation (NESCOM TEST 2018)
a) Forward Braking
b) Forward motoring
c) Reverse motoring
d) Reverse braking
Answer: Option a)Forward Braking
Four Quadrant Operation of any drives or DC Motor means that the machine operates in four quadrants.
They are Forward Braking, Forward motoring, Reverse motoring and Reverse braking.
A motor operates in two modes that are Motoring and Braking.
A motor drive capable of operating in both directions of rotation and of
producing both motoring and regeneration is called a Four Quadrant variable speed drive
In motoring mode, the machine works as a motor and converts the electrical energy into mechanical energy,
supporting its motion. In braking mode, the machine works as a generator and converts mechanical energy into electrical energy
and as a result, it opposes the motion In the I quadrant power developed is positive and the machine is working as a motor
supplying mechanical energy. The I (first) quadrant operation is called Forward Motoring.
II (second) quadrant operation is known as Forward Braking.
In this quadrant, the direction of rotation is positive, and the torque is negative,
and thus, the machine operates as a generator developing a negative torque, which opposes the motion.
III (third) quadrant operation is known as the reverse motoring.
The motor works, in the reverse direction. Both the speed and the torque have negative values while the power is positive.
IV (fourth) quadrant, the torque is positive, and the speed is negative.
This quadrant corresponds to the Reverse braking.
Compressor, pump and fan type load requires operation in the I quadrant only.
As their operation is unidirectional, they are called one quadrant drive systems.
In hoist drives, a four-quadrant operation is needed
Q.6 A four quadrant operation requires__________
a. two full converters in series
b. two full converters connected back to back
c. two full converters connected in parallel
d. two semi converter connected back to back
Answer: Option b) two full converters connected back to back
Full converter is operated in two quadrants.
Hence in order to obtain four quadrants the drive requires another two quadrants,
this will obtained by reversal of current. Reversal of current can be done by connecting two full converters in back to back
Q.7 Buchholz relay is used for___________
a. protection of a transformer against all internal faults
b. protection of a transformer against all external faults
c. both a and b
d. neither a nor b
Answer: Option a) protection of a transformer against all internal faults
Buchholz relay is the gas detector relay used to protect transformers from internal or incipient faults,
which operates when the oil level in the conservator pipe of a transformer is lowered
by the accumulation of gas caused by a poor connection or by incipient breakdown of insulation.
It is most famous gas-actuated relay which is installed in an oil-immersed transformer.
Q.8 A certain appliance uses 350 W. If it is allowed to run continuously for 24 days, how many kilowatt-hours of energy does it consume?
a. 20.16 kWh
b. 201.6
c. Percent Overshoot
d. Rise time
Answer: Option b) 201.6 kWh
Total kWH = power * time
=350 (watt) *24 (days)
=350(watt)*24 *(24) hours
=201600 watt hour
=201.6 kWH
Q. 9 Horizontal lines on the S-plane are the lines of constant ______________
a. Peak time
b. Settling time
c. Percent Overshoot
d. Rise time
Ans is a) Peak Time
As T_p=Pi/W_d where W_d is imaginary part of a complex pole.
Now as shown in the above figure, on S-plane horizontal lines represents constant imaginary part of the imaginary poles and Peak time in inversely proportional to it. So as W_d is constant therefore Peak time will also be constant. So ans is constant peak time.
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III (third) quadrant operation is known as the reverse motoring.
The motor works, in the reverse direction. Both the speed and the torque have negative values while the power is positive.
IV (fourth) quadrant, the torque is positive, and the speed is negative.
This quadrant corresponds to the Reverse braking.