Practice Questions
- What do you understand by skin effect?
Ans. Skin Effect: The tendency of a high-frequency alternating current to flow through only the outer surface of a conductor. At high frequency, the waves inside the conductor are highly attenuated inside the conductor and tend to confine in outer surface only.
- Explain maximum power transfer theorem in detail?
Ans. The maximum power transfer theorem states that in order to obtain maximum power from a source having a finite internal resistance, the resistance of the load must equal the resistance of the source as viewed from its output terminals across the load resistor. The theorem results in maximum power transfer and not maximum efficiency. If the resistance of the load is greater than the resistance of the source, then efficiency is higher since a higher percentage of the source power is transferred to the load, but the magnitude of the load power is lower in this case as the total circuit resistance goes up.
- What do you understand by capacitor banks?
Ans. Shunt capacitor banks are primarily used to improve the power factor in the network. Since most practical devices have inductive nature so to compensate the large current drawn at their operation capacitor banks are used for power factor improvement. They improve voltage stability and reduce network losses.
- What are the uses of Reactive power?
Ans. Reactive power (VARs) is required to maintain the voltage to deliver active power (watts) through transmission lines from the generator. Reactive powers are a must for the flow of active power.
Physical Significance :
Reactive power is the power consumed in an ac circuit because of the expansion and collapse of magnetic (inductive) and electrostatic (capacitive) fields. Unlike true power reactive power is not a useful power because it is stored in the circuit itself. This power is stored by inductors, because they expand and collapse, their magnetic fields in an attempt to keep the current constant, and by capacitors because they charge and discharge in an attempt to keep the voltage constant. Circuit inductance and capacitance consume and giveback reactive power. Reactive power is a function of a system's amperage. The power delivered to the inductance is stored in the magnetic field when the field is expanding and returned to the source when the field collapses. The power delivered to the capacitance is stored in the electrostatic field wen the capacitor is charging and returned to the source when the capacitor discharges. None of the power delivered to the circuit by the source is consumed; all is returned to the source. The true power, which is the power consumed, is thus zero. We know that alternating current constantly changes; thus, the cycle of expansion and collapse of the magnetic and electrostatic fields constantly occurs.
Therefore, we conclude that reactive power is the rate of energy flow between the source and the reactive components of the load (i.e., inductances and capacitance.) It represents a lossless interchange between the load and the source.
- Why do we use, mostly 3 ϕ systems in industries over 1 ϕ systems.
Ans. For Three-phase machines the controls are smaller, lighter in weight, and more efficient than those of single-phase equipment. More power is supplied to them in the same period that can be supplied by a single-phase power circuit. However, three-phase machines and controls are more complex. Only about 75% as much copper wire is required for distributing three-phase power as required for distributing the same amount of single-phase power.
- What is inductor?
Ans. An inductor is a passive electronic component that stores energy in the form of a magnetic field (kinetic energy). An inductor is a wire loop or coil.
- What is impedance matching and why it is done?
Ans. Impedance matching is the technique the input impedance of an electrical load is made equal to the output impedance of its corresponding signal source to maximize the power transfer from source to load and minimize signal reflection from the load.
Previous Interview Questions
- Draw the diagram of a high pass filter and explain its transfer function. [ESE]
Ans. Diagram of high pass filter :
where,
At frequency f=0,(s=0) the transfer function output is zero. At f=infinite, the transfer function has a finite value.
- What is the difference between grounding and earthing? If yes, then what is it? [ESE-2015]
Ans. In an electrical wiring system, the ground or earth is a conductor that provides a low impedance path to the current to prevent hazardous voltages from appearing on the equipment and ensures the safety of the electrical device.
Grounding is achieved by connecting the live part to the earth. Whereas earthing can be achieved by connecting the part which does not carry current under normal conditions to the earth.
- Explain the working of tube light? [ESE]
Ans. Tube light is a long glass tube which contains inert gases. In a tube light, electrical energy is converted to radiant energy, and it has a starter and choke coil which helps to make inert gases into a plasma state, hence giving out radiant energy.
- What is inductance? Explain in detail. [ESE]
Ans. Inductance is a property of conductance in which a change in the current flowing through it induces a voltage in both the conductor itself i.e. self-inductance and in nearby conductors, i.e.mutual inductance.
V=L(di/dt)
The voltage induced in a conductor depends on the rate of change of electric current flowing through the inductor.
- What is the filter? [ESE]
Ans. A filter is a device or network that is frequency selective and is used to select the frequency components from input to be available at the output.
- What is KCL, KVL? [ESE-2013]
Ans. KCL: Algebraic sum of all the currents entering any node is zero. It is based on the law of conservation of charge.
KVL: Algebraic sum of all the voltages drop in a closed loop or path is zero. It is based on the law of conservation of energy.
- What is the difference between active and passive devices? [ESE]
Ans. The devices which are capable to supply energy for the infinite duration of time are called active devices i.e. they deliver energy while the devices that dissipate energy are called passive devices.
8. What is Ground in Electrical network?
Ans. Voltage is a differential quantity. The ground is a reference point from which all the voltages are measured.
9. What is the slew rate? [ESE]
Ans. Slew rate is defined as the rate of change of the output of any device. Ideally, it should be infinite to avoid distortion of the output.
10. What is Norton theorem? [ESE-2005]
Ans. Norton theorem is a technique to represent a network containing many linear passive and independent or dependent elements by a current source in shunt with a resistor. The value of the current source is calculated by short-circuiting the points or nodes across which the load is connected and the value of the resistor will be calculated by replacing the independent sources with their internal impedance across open circuited load terminals.
11. What is the basic requirement of DC power supply? [ESE]
Ans. It should be free from ripples so that the Output voltage should be constant and the load is not damaged because of fluctuations of supply.
12. What is the frequency of pulsating DC of FWR without a filter? [ESE]
Ans. FWR is full wave rectifier which rectifies the AC signal conducting for the full cycle the frequency of pulsating DC will be twice the frequency of the input.
13. The difference between ‘FWR’ and bridge rectification is? [ESE]
Ans. Peak inverse voltage of diode in FWR is 2Vm while peak inverse voltage of diode in bridge rectifier is Vm.
14. What is the last stage of the power supply? [ESE]
Ans. A voltage regulator is the last stage of the power supply as it regulates the output and is free from ripples.
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