21. In the two-wattmeter method of three-phase power measurement of a balanced load, if the reading of one meter is −200W, then the power factor of the load is ?
The reading of two wattmeters can be expressed as
W1 = VLILcos(30 + φ)
W2 = VLILcos(30 − φ)
(1)the power factor in a 3 phase power measurement of two wattmeter shows equal reading
In this, we have to prove that the power measured by the two wattmeter i.e. the sum of the two wattmeter readings is equal to root 3 times of the phase voltage and line voltage (√3VLIL Cosϕ) which is the actual power consumed in a 3 phase balanced load.
( φ = 0) Unity Power factor
W1 = VLILcos(30)
W2 = VLILcos(30)
W1=W2
(2)In the Two wattmeter method of measuring power in a balanced three-phase circuit, one wattmeter shows zero and the other positive maximum. The load power factor is
( φ =60) The load power factor is = 0.5. Since at this power factor one wattmeter shows zero and the other shows a positive maximum value of power. W1 = VLILcos(90)
W2 = VLILcos(-30)
Cos (-φ) = Cos φ
W1=0, W2= (Positive value) total power is measured by wattmeter W2
v NoW,
When PF is less than 0.5 but greater than 0 (90° > φ > 60°)
lets assume φ = 80
W1 = VLILcos(30+80) = VLILcos(90+20)
W2 = VLILcos(30-80) = VLILcos(-50)
cos (90° + θ) = - sin θ (so W1 Value we got negetive )
Cos (-φ) = Cos φ ( and W2 Value in Positive )
22. An ideal transformer will not transform ?
1. Power
2. Current
3. Frequency
4. Voltage
1. Power
2. Current
3. Frequency
4. Voltage
An ideal transformer is an imaginary transformer which has
- no copper losses (no winding resistance)
- no iron loss in core
- no leakage flux
In other words, an ideal transformer gives output power exactly equal to the input power.
The efficiency of an idea transformer is 100%.
# Transformer basically transforms voltage, but there are current transformer too.
# Transformer does not effect frequency and power, both remains the same.
# However frequency is one major factor in designing of transformer.
23. The candle power of a lamp placed normal to a working plane is 60 CP. Find
the distance if the illumination is 15 lux ?
Illumination = 15 lux
candle power = 60 CP
$h^2$ = candle power/illumination
$h^2$ = 60/15 = 4
h = 2 meter
24. Four statements are given below. Identify the correct statement ?
There are two major classifications of logic gates.
1)Basic Gates
2)Derived Gates.
Basic Gates
Basic logic gates are the fundamental logical operations from which all the functions no matter how complex can be derived.
AND, OR and NOT are the three main basic gates which are massively used in digital systems.
Derived Gates
The logic gates which are derived from the basic gates such as AND, OR, NOT gates are called derived gates.
These derived gates have their own unique Symbols, Truth Tables and Boolean Expressions.
Here we will explore the most common derived gates such as NAND Gate, NOR Gate, EX-OR Gate, and EX-NOR Gate.
EX-OR Gate The Exclusive-OR, or XOR function is an interesting and useful variation on the basic OR function.
Verbally, it can be stated as, "Either A or B, but not both.
" The XOR gate produces a logic 1 output only if its two inputs are different.
If the inputs are the same, the output is a logic 0
25. The type of damping used for moving iron instruments is ?
There are two general types of moving iron instruments
1-Repulsion type
2-Attraction type
Damping torque is normally pneumatic(air friction damping), the damping device consisting of an air chamber and a moving vane attached to the instrument spindle.
Advantages #-The instruments are suitable for use in AC and DC circuits.
#-The instruments are robust, owing to the simple construction of the moving parts.
#-The stationary parts of the instruments are also simple.
#-Instrument is low cost compared to moving coil instrument.
#-Torque/weight ratio is high, thus less frictional error.
26. The combined inductance of two coils connected in a series is 0.6H and
0.1H, depending on the relative directions of the currents to the coils. If one of
the coils when isolated has a self-inductance of 0.2 H, then calculate the natural
inductance ?
The combined inductance of two coils connected in series
L = L1 + L2 + 2M
In series adding case
L1 + L2 + 2M = 0.6 H ——-–(1)
In series opposing case
L1 + L2 − 2M = 0.1 H ——–-(2)
Subtracting eq. (2) from eq. (1) we get
4M = 0.5 H
M = 0.125 H
Let L1 = 0.2 H (since the coil when isolated, its self-inductance is 0.2 H)
Putting the value of M & L1 in equation (2)
0.2 + L2 + 2 ×0.125 = 0.06
0.2 + L2 = 0.35
L2 = 0.15 H
27. Lower efficiency, lower power factor, are the features of which of the
following motors ?
#-Single-phase induction motors are not self-starting without an auxiliary stator winding driven by an out of phase current of near 90°. Once started the auxiliary winding is optional.
#-the single-phase induction motors develop about 50% of the output as that of three phase induction motors.
#-The efficiency of single phase induction motors is less compared to that of three phase induction motors.
#-The power factor of Single Phase Induction motor is low as compared to that of three-phase induction motor. Single Phase motors are mostly used in domestic appliances such as mixer grinder, fans, compressors, etc
28. In a 4 pole. 20 kW, 200 V wave wound DC shunt generator, the current in
each parallel path will be ?
P=VI
20*1000 = 200 * I
I = 100A
For wave winding parallel path = 2
So current in each path = 100/2 = 50 A
29. In excess-three code the 4-bit group that is used is ?
We can add 0011 to each four-bit group in binary coded decimal number (BCD) to get desired excess-3 equivalent.
30. Observe the given figure. The maximum power to the load resistor is when RL
= ?
The maximum power transfer theorem states that the DC voltage source will deliver maximum power to the variable-load resistor only when the load resistance is equal to the source resistance.
Hence when Ri = RL the transfer of power will be maximum.
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