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Electrical Interview Questions 1) What is Electrical Engineering? > Electrical Engineering is the field of Engineering that generally deals with the study and _ application of electricity, electronics, and electromagnetism. 2) Whatis electricity ? »> Electricity is a general term used for all phenomena caused by electric charge whether static or in motion. 3) What are the types of electricity ? » There are two types,(1) Static Electricity and (2) Current Electricity. 4) Whatis static electricity ? » Static electricity means electricity at rest in contradistinction to dynamic or current electricity the effects of which are purely due to the electrostatic field produced by the charge. As it is obtained by rubbing two substance such as glass and silk it is also called frictional electricity. 5) Whatis current electricity ? > Current electricity means the electricity in motion the effects of which are due to the flow of electrons in a conductor. 6) What are the types of current electricity ? » There are two types :- (1) Direct current and (2) Alternating current. 7) What are the different methods of producing electricity ? Give examples. »> The methods are -— i) By means of frictions- Static electricity is produced. fi) By means of chemical action in cells and batteries. fii) By means of mechanical driving- Generator produces electricity in two dissimilar methods. iv) By means of heat — Thermal electricity is produced. v) By means of lighting effect — Electricity is produced in photo electric cell. 8) Explain the difference between direct and alternating current? » Direct current (DC) is the flow of electric charge in only one direction. It is the steady state of a constant-voltage circuit. » Alternating current (AC) is the flow of electric charge that periodically reverses direction. If the source varies periodically, particularly sinusoidal, the circuit is known as an alternating current circuit. 9) Where is D.C. used ? » (1)Battery charging, (2)Electroplating, (3)Electrolysis, (4) Relays, (5) Traction motors, (6) Cinema projector. 10) Where is A.C. used ? > (1)House hold appliances, (2) Fan, (3) Refrigerators, (4)Power driving motors. (5)Radio and T.V. Set etc. 11) What is active, reactive, apparent & complex power? > 1) _Active power: It is the actual power that is delivered to the load such as transformer, induction motors, generators, etc and dissipated in the circuit. It is denoted by P & its unit is watts W. > Il) Reactive Power: The powers that continuously bounce back and forth between source and load is known as reactive power. It is denoted by Q & its unit is VAR voltage-ampere reactive. > II!) Apparent Power: It is the product of voltage & current without referencing the phase difference between them. It is a combination of active power & reactive power. It is denoted by S & its unit Volt-Ampere, VA. a II!) Complex power: it is the product of voltage & current with reference to the phase difference between them. it is the complex sum or vector sum of the active power & reactive power. It is denoted by S & measure in VAR. 12) What is a leading & lagging power factor & how can you improve it? State the methods of power factor correction? >» Aunilateral is a type of circuit whose properties change with the direction of current flow or the voltage. The properties of a bilateral circuit do not change with changing the current direction or supply voltage. 15) What is a linear & non-linear circuit? > Ina linear circuit, the relation between the current & voltage is linear i.e. directly proportional. The circuit parameter such as frequency, resistance, inductance, capacitance, etc. remains constant with varying current & voltage. While in the non-linear circuit, the current & voltage does not have a linear relationship. The electrical parameters of such circuits changes with varying voltage & current. 16) What could be the reason for the current to double in a linear circuit?*** » There are two reasons for increasing the current: 1) Either the total resistance of the circuit is reduced by half 2) Or the supply voltage to the circuit is doubled. 17) Why is Battery Rating in Ah (Ampere hour) and not in VA or Watts? >» A battery converts chemical energy into electrical energy which is the charge stored inside the chemicals. The amount of current it can supply is for a said time thus Ampere-hour Ah is the unit for its rating. While the batteries supply direct current which has no phase or frequency thus there is no concept of P.F or reactive power, thus no need for expressing it in VA and its rated in Ah. 18) What is a primary & secondary cell? >» The primary cell is a non-rechargeable battery that cannot be recharged by any means. They are disposable & cannot be used once they are fully discharged. They are mostly used in toys, handheld devices & remote controllers, etc. »> The secondary cell is a rechargeable battery that can be recharged several hundreds of times (depends on its life cycle). Their initial cost is expensive compared to the primary cell. They are mostly used in cell phones, vehicles, generators, etc. 19) What are the limitations of ohm’s law? > Ohm law is not applicable to a unilateral circuit or a non-linear circuit. The criterion for ohm’s law is that the resistance must be constant which also depends on the temperature. Whereas, the resistance of a non-linear or unilateral circuit varies with voltage & current. Thus it is not applicable in such a circuit. Also, the temperature must remain constant. 20) Does current lead or lag the voltage in an inductive or capacitive circuit? > The current lags the voltage in an inductive circuit while the current leads the voltage in a capacitive circuit. 21) Define the term Capacitance and Inductance » Capacitance: capacitance is the ability of a component to store charge between two plates when there is a potential difference applied. It is denoted by C & it is measured in Farads F. » Inductance: inductance is the ability of a conductor to resist or oppose any change in the current. The current generates a magnetic field whose strength varies with the current. It is denoted by L & is measured in Henry H. 22) Why the Capacitors works on AC only? » Generally capacitor gives infinite resistance to dc components (i.e., block the dc components). It allows the ac components to pass through. 23) What is the maximum power transfer theorem? > It mentions the condition for maximum power transfer from source to load. It states that n a linear, bilateral network, the maximum power will be transferred from source to the load when the external load resistance equals the internal resistance of the source or Thevenin’s resistance of the circuit. Red wire: This color wire is a secondary live wire in a 220 volt circuit and used in some types of interconnection. You can join the red wire to another red wire or to a black wire Blue and Yellow wire: These wires are also used to carry power but are not wiring the outlets for common plug-in electrical devices. They are used for the live wire pulled through the conduct. You will see yellow wire in the fan, structure lights, and switched outlets. White and Gray: This color wire is used as a neutral wire. It carries the current (unbalanced load) to the ground. You can join white and gray only to other white and gray wires Green: It is connected to the grounding terminal in an outlet box and run from the outlet box to the ground bus bar within an electric panel. 28) Explain The Working Principal Of The Circuit Breaker? » Circuit Breaker is one which makes or breaks the circuit. It has two contacts namely fixed contact & moving contact. Under normal condition the moving contact comes in contact with fixed contact thereby forming the closed contact for the flow of current. During abnormal & faulty conditions (when current exceeds the rated value) an arc is produced between the fixed & moving contacts & thereby it forms the open circuit. Arc is extinguished by the Arc Quenching media like air, oil, vacuum etc. 29) What Is a Vacuum Circuit Breaker? » Acircuit breaker breaks the circuit by opening the contact terminals. During the opening, an arc is generated between the terminals that can be quenched using various mediums. In VCB, the medium for arc quenching is a vacuum. The vacuum has a high voltage are quenching ability as compared to air & they are used for in high voltage circuits. 30) What is the difference between MCB & MCCB? >» The MCB stands for “miniature circuit breaker” & it is used for current rating lower than 100 amps with interrupting ratings of below 18k Amps. Its tripping characteristics cannot be adjusted & they are used for domestic purposes. The MCCB stands off “Molded case circuit breaker”. It has a high current rating of around 2500 Amps, where its interrupting ratings are between 10K to 200k Amps. Also, its tripping characteristics can be adjusted. They are used in industries. Vv 31) What is the difference between a single pole and a double pole circuit breaker? » Single-pole breakers are wired with one hot wire and one neutral wire. When there is an overload in a single-pole breaker’s circuit, only that particular breaker trips. » Double-pole breakers have two hot wires that are connected by a single neutral wire. That means if there’s a short circuit on either of the poles’ hot wires, both trip. 32) What Is the Difference Between Fuse And Breaker? » The fuse is made of a metal wire called fuse link or element that melts when the current exceeds its limit. it works automatically & it is a one- time use device that needs to be replaced. » The circuit breaker is an electromechanical switch that opens the circuit during overcurrent or short-circuits. It works automatically as well as manually & it can be used again by resetting the lever. 33) What is the difference between circuit breaker & Isolator? > Circuit Breaker: A Circuit Breaker is a protective electromechanical device used to control the flow of current same like a fuse. It automatically breaks the circuit in case of fault conditions like short circuit and overload. It can also manually break the circuit. It is ON-load & OFF load device, it means it operates in both ON/OFF supply condition. Isolator: An isolator is a mechanical switch used for isolating or disconnecting power supply in substations. It is an off-load device i.e. it is operated when the power supply is off. v » The alternator and generator both work on the principle of Faraday’s law of electromagnetic induction. An alternator is a device that converts mechanical energy into AC electrical energy. It always induces an alternating current. Alternators are very efficient. A generator is a mechanical device which converts mechanical energy to either AC or DC electrical energy. It can generate either alternating or direct current. Generators are considered less efficient. The major difference between the alternator and the generator is that in alternator, the armature is stationary and the field rotates whereas in the generator, armature rotates and field is stationary. 39) What Are The Advantages Of Star- Delta Starter With Induction Motor? » The main advantages of star delta starter are: To decrease the starting current required for the induction motor because it is 6 to 7 times higher than full load current which can damage the windings of the motor. To eliminate the voltage drop problem because of the huge amount of starting current results in a voltage drop along the consumer line which may damage other electrical appliances. Its operation is very simple The cost of this starter is comparatively very cheap. It has a good torque to current performance. 40) Why is the starting current high in dc motor? »> The de motor has no back emf. At the starting of the motor, the armature current is controlled by the resistance of the circuit. The resistance of the armature is low, and when the full voltage is applied at the standstill condition of the motor, the armature current becomes very high which damage the parts of the motor. 41) What is the slip of an induction motor? »> The percentage difference between the synchronous speed Ns & the rotor speed N of an induction motor is called slip. it is denoted by S. The rotor speed of the induction motor is always less than its synchronous speed. 42) Why can't a series motor be started on no-load? » Aseries motor should never be started at no load. With no mechanical load on the series motor, the current is low, the counter-EMF produced by the field winding is weak, and so the armature must turn faster to produce sufficient counter-EMF to balance the supply voltage. The motor can be damaged by overspeed. 43) Explain the principle of Induction Motor » We need to give double excitation to make a DC motor to rotate. In the DC motor, we give one supply to the stator and another to the rotor through brush arrangement. But in induction motor, we give only one supply, so it is interesting to know how an induction motor works. It is simple, from the name itself we can understand that here, the induction process is involved. When we give the supply to the stator winding, a magnetic flux gets produced in the stator due to the flow of current in the coil. The rotor winding is so arranged that each coil becomes short-circuited. The flux from the stator cuts the short-circuited coil in the rotor. As the rotor coils are short-circuited, according to Faraday’s law of electromagnetic induction, the current will start flowing through the coil of the rotor. When the current through the rotor coils flows, another flux gets generated in the rotor. Now there are two fluxes, one is stator flux, and another is rotor flux. The rotor flux will be lagging with respect to the stator flux. Because of that, the rotor will feel a torque which will make the rotor to rotate in the motor mechanically and electrically, but regenerative braking can be done at sub synchronous speed if the variable frequency source is available. 46) What is Plugging Breaking? >» In this method the terminals of supply are reversed, as a result the generator torque also reverses which resists the normal rotation of the motor and as a result the speed decreases. During plugging external resistance is also introduced into the circuit to limit the flowing current. The main disadvantage of this method is that here power is wasted. 47) What is dynamic breaking? >» In this method of braking the motor which is at a running condition is disconnected from the source and connected across a resistance. When the motor is disconnected from the source, the rotor keeps rotating due to inertia and it works as a self-excited generator. When the motor works as a generator the flow of the current and torque reverses. During braking to maintain the steady torque sectional resistances are cut out one by one. 48) What Is Meant By Armature Reaction? »> The effect of armature flux on main flux is called armature reaction. The armature flux causes two effects on the main field flux: e@ The armature reaction distorts the main field flux. e [It reduces the magnitude of the main field flux. 49) Which Motor Has High Starting Torque And Staring Current Dc Motor, Induction Motor Or Synchronous Motor? » The DC series motor has the highest starting torque out of all motors & that is why they are used in electrical machinery requiring high starting torque like cranes, hoist, etc. 50) What is a Universal Motor? A universal motor works on either DC or single phase AC supply. When the universal motor is fed with a DC supply, it works as a DC series motor. When current flows in the field winding, it produces an electromagnetic field. The same current also flows from the armature conductors. When a current carrying conductor is placed in an electromagnetic field, it experiences a mechanical force. Due to this mechanical force, or torque, the rotor starts to rotate. The direction of this force is given by Fleming's left hand rule. When fed with AC supply, it still produces unidirectional torque. Because, armature winding and field winding are connected in series, they are in same phase. Hence, as polarity of AC changes periodically, the direction of current in armature and field winding reverses at the same time. Thus, direction of magnetic field and the direction of armature current reverse in such a way that the direction of force experienced by armature conductors remains same. Thus, regardless of AC or DC supply, universal motor works on the same principle that DC series motor works. 51) What are some of the most common causes of transformer humming? » Electric hum around transformers is caused by stray magnetic fields causing the enclosure and accessories to vibrate. Magnetostriction is a second source of vibration, in which the core iron changes shape minutely when exposed to magnetic fields. Transformer noise is produced by the core. The amount of noise is generally fixed by the design of the transformer. 52) What is the voltage regulation of the transformer & why is it important? > The voltage regulation of a transformer is the percentage change in the secondary voltage from no load to full load condition. Ideally, the secondary voltage remains the same throughout the load, in which case the voltage regulation is zero. But practically it varies with the power factor of the load. The voltage regulation value provides the efficiency of the transformer & it is best to prefer a transformer with low voltage regulation. 53) There is a Transformer and an Induction Machine. Those two have the same supply. For Which Device the load current will be maximum and why? 57) What is transformer efficiency & all-day efficiency? What is the condition for maximum efficiency? > Transformer Efficiency: The efficiency of the transformer is given by the output power divided by the input power. Some of the input power is wasted in internal losses of the transformer. Efficiency, n = Output Power / Input Power > All Day Efficiency: The ratio of energy delivered in Kilo Watt-Hour (kWh) to the energy input in kWh of the transformer for 24 hours is called all-day efficiency. Nal_day = Output in kWh / Input in kWh > Condition for Maximum Efficiency: The copper loss must be equal to the iron loss; which is the combination of hysteresis loss & eddy current loss. Cu Loss = Iron Loss Wa = Wi 58) Why the current transformer’s secondary should not be open when there is current flowing in its primary? » The current transformer is essentially a step-up transformer that increases the voltage & decreases the current on the secondary side. Under the open secondary condition, the primary current becomes the magnetizing current that generates a very high secondary voltage that can damage the insulation as well as can pose danger to personnel. 59) Why are transformers rated in KVA? » Note:- KVA= KV(killo Volt) * A(ampere) KW2= KV(killo Volt) * A(ampere) * Power factor We already know that Power factor only depends on the type of Loads like, iii) e Inductive Load - Lagging Power factor © Capacitive Load - Leading Power Factor ¢ Resistive Load - Unity Power Factor The Transformer is not a Load, it is a device which can transfer power not consume power. If you think that a Transformer is a Load that's wrong. So as the transformer does not consume power it can only transfer power with increasing and decreasing voltage and current that is why the Transformer always rated in KVA. When the transformer is designed the manufacturer does not know which type of load will be connected in future. And the power factor depends upon the load. If an Inductive Load is connected then the current will be lag which flows through the secondary winding of the transformer as well as the primary winding of the transformer. We also know that pure inductive and pure capacitive load does not practically exist. Every load has some resistance even it inductive or capacitive. For example, a motor is connected to the transformer which is inductive + resistive. So the motor draws both reactive(KVAR) and active power(KW). So the power supplied by the transformer is the vector sum of reactive(KVAR) power and active power(KW) that is KVA. The Copper loss(I2R) occurs due to the flow of the current in the transformer winding and the Iron or core Joss occurs due to the voltage. These losses do not depend on the power factor so that is why the transformer rating in KVA not KW. OR » When manufacturers design a transformer, they have no idea which kind of load will be connected to the transformer. The load may be resistive (R), inductive (L), capacitive (C) or mixed load (R, L, and C). Its mean, there would be different power factor (p.f) at the secondary (load) side. The output of real power may vary depending on the power factor. Thus the manufacturer denotes it as “this transformer can provide x number of amperes » Circuits are simplified. Voltages have same range in per unit in all parts of the system from EHV system to distribution and utilization. When expressed in the per unit system, apparatus parameters usually fall in narrow range regardless of apparatus size. 64) What are the various kinds of cables used for transmission? >» The types of cables based on the transmission voltage ratings are given below : e Low tension or LT cable t is used for transmission of voltage below 1000v e High tension or HT cable used for transmission of up to 11kv. e Super tension cable is used for handling voltage up to 33kv e Extra high tension cable can handle voltage up to 66 kV. e &xtra super tension cable is used for transmission of voltage up to 132kV. 65) What is meant by reverse polarity and how it can be fixed?*** » An electrical outlet has two wires i.e. neutral & hot or live wire. The Reverse Polarity means that the neutral wire is connected to the terminal where the hot wire is supposed to be. It can create shock hazards because the ON/OFF switches will cutoff only the neutral wire from the appliance connected to such outlets. 66) What is the difference between surge arrestor and lightning arrestor? » Both of them are used as protection devices for grounding the high voltage transients or surges. The surge arrestor is used inside the circuit to protect the components from high voltage spikes. The lightning arrestors are used outside the circuit such as on transmission tower to protect them from high voltage strikes of lightning. 67) What Is Critical Disruptive Voltage? > It is the minimum voltage required for the breakdown of the insulation (air) between a phase & neutral to discharge the current. It is the voltage at which the corona discharge starts. 68) How Many Types of Faults Occur in a 3 phase Power System? > The faults in the three-phase system are named below:- Open circuit fault: Single-phase open fault Double phase open fault Three-phase open fault Short circuit fault: Single line to ground fault (LG) Double line to ground fault (LLG) Line to line fault (LL) Three phase short circuit fault (LLL) 69) What Is Skin Effect? » The current density of the AC passing through a conductor tends to be very high near the surface & very low near the middle of its cross-section area. This phenomenon is called skin effect & directly proportional to the frequency. That is why; stranded conductors are used for power transmission. 70) How to reduce Skin Effect? >» The methods of reducing skin effects are: ACSR bundled conductor is used to reduce the skin effect. ... Using cable material with less magnetic permeability. ... Reducing the size of the conductor. Increasing the voltage by reducing the current which decreases the skin effect in the same conductor. 71) What is Ferranti Effect? » The effect in which the voltage at the receiving end of the transmission line is more than the sending voltage is known as the Ferranti effect. Such type of effect mainly occurs because of light load or open circuit at the receiving end. Capacitance and 
