Ques 81. Three equal impedances are first connected in delta across a 3-phase balanced supply. If the same impedances are connected in star across the same supply
Phase currents will be 1/3 rd of the previous value
Line currents will be 1/3 rd of the previous value
Power consumed will be 1/3 rd of the previous value
Power consumed will be 3 times the previous value
Answer.3. Power consumed will be 1/3 rd of the previous value
Explanation:
Let us suppose Vs be the supply voltage per phase.
So the line voltage of the supply will be 3Vs.
Now assume any type of load; for simplicity, let’s assume it a resistive load which is R per phase
For Delta connected load:
Calculation of per phase power; PD= I2R
Where I is load current (per phase) And,
I = √3Vs/R {as line voltage of the supply is directly applied to the phase of the delta load}
So,
Pd = (√3Vs/R )2R = 3Vs2/R watts per phase.
For 3 phases:
P3D = 3Pd = 3*3Vs2/R = 9Vs2/R watts.
Now for Star connected load:
PS = I2R = (Vs/R)2R = Vs2/R watts
For 3 phases: P3S = 3PS = 3 Vs2/R watts
Conclusion:
P3S / P3D = 3Vs2/R / 9Vs2/R = 1/3
i.e The power ratio between Star to Delta is 1:3
Ques 82. The average value of the voltage wave V= 110 + 175 sin (314 t – 180°) volts is
110 V
175 V
165.57 V
206.7 V
Answer.1. 110 V
Explanation:
The average value of the second term is zero therefore answer is 110 V
Ques 83. A current from an ac source bifurcates into two branches A and B in parallel. Branch A is an inductor with 30 μH inductance and 1 Ω resistance. Branch B is another inductor with inductance L and 1.5 Ω resistance. For the ratio of currents in the branches to be independent of supply frequency, the value of L should be
30.5 μH
20 μH
45 μH
29.5 μH
Answer.3. 45 μH
Explanation:
The diagram of the above question can be constructed as
Ques 84. A universal motor is one which
Can run on any value of supply voltage
Has infinitely varying speed
Can operate ac as well as dc voltage
It can work as a single-phase or three-phase motor.
Answer.3. Can operate ac as well as dc voltage
Explanation:
The universal motor is so named because it is a type of electric motor that can operate on AC or DC power.
It is a commutated series-wound motor where the stator’s field coils are connected in series with the rotor windings through a commutator.
It is often referred to as an AC series motor. The universal motor is very similar to a DC series motor in construction but is modified slightly to allow the motor to operate properly on AC power.
This type of electric motor can operate well on AC because the current in both the field coils and the armature (and the resultant magnetic fields) will alternate (reverse polarity) synchronously with the supply.
Ques 85. If the centrifugal fuse of a single-phase resistance split induction motor does not operate after starting of the motor, the motor
Will run above normal speed
Will run below normal speed
Will draw very small current
Will draw very high current and over-heated
Answer.4. Will draw very high current and over-heated
Explanation:
If the centrifugal switch is failed to open then the starting winding will draw very high current and it may burn the winding
Ques 86. Alternators are usually designed to generate which type of ac voltage?
With fixed frequency
With variable frequency
Fixed current
Fixed power factor
Answer.1. With fixed frequency
Explanation:
In an alternating current electric power system, synchronization is the process of matching the speed and frequency of a generator or other source to a running network. An AC generator cannot deliver power to an electrical grid unless it is running at the same frequency as the network. If two segments of a grid are disconnected, they cannot exchange AC power again until they are brought back into exact synchronization.
Hence alternators are usually designed with the fixed frequency of AC voltage.
Ques 87. Three inductors each of 60 mH are connected in the delta. The value of inductance of each arm of the equivalent star connection is
10 mH
15 mH
20 mH
30 mH
Answer.2. 20 mH
Explanation:
For equivalent star connection
ZY = 1/3 ZΔ
= 60/3
= 20 mH
Ques 87. The magnetic field energy in an inductor changes from maximum value to minimum value in 5 msec when connected to an ac source. The frequency of the source
500
200
50
20
Answer.3. 50
Explanation: For a quarter part of wave pulse energy going down to the maximum value to minimum value. i,e T/4 = 5ms T =20ms f = 1/T =1/20ms = 50Hz
Ques 88. A voltage source having an open-circuit voltage of 150 V and internal resistance of 75 Ω, is equivalent to a current source of
2 A in series with 75 Ω
2 A in parallel with 37.5 Ω
2 A in parallel with 75 Ω
1 A in parallel with 150 Ω
Answer.3. 2 A in parallel with 75 Ω
Explanation:
Voltage = 150V
R = 75 Ω
I = V/R = 2A
If we remove the load resistance RL then it becomes an open circuit. In this case, the terminal voltage between points A and B is V.
In the next figure, if we remove the load resistance RL then all the current goes through the internal resistance Ri. The terminal voltage between A and B is the same as the internal resistance Ri voltage for the open circuit.
Hence the equivalent current source is 2 A in parallel with 75 Ω
Ques 89. A 300 kW alternator is driven by a prime mover of speed regulation 4% while the prime mover of another 200 kW alternator has a speed regulation of 3%. When operating in parallel, the total load they can take without any of them being overloaded is
