Technical Interview Question
Why is thyristor SCR considered as a charge controlled device?
A thyristor is triggered into forward conduction state when the charge is injected by applying the gate signal (voltage between gate & cathode terminal). As the SCR is switched on by the current or charge, it is known as a charge controlled device.
What is a transformer?
A transformer is a passive electrical device that transfers electrical energy from one electrical circuit to another, or multiple circuits.
Energy equations for capacitor and inductor
Energy for capacitor: E = 1/2 CV^2 Energy for inductor: E = 1/2Li^2
What is the role of a transistor in a circuit?
Mainly used for two reasons: 1.Switching: to switch ON/OFF the flow of current in a circuit. The switching depends on the input voltage or current. 2.Amplification: to increase or amplify the input signal that has very low power into a high power signal.
Which motor has high starting torque and starting 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.
What is a Marx circuit?
Marx circuit or Marx generator is a circuit that charges a number of capacitors in parallel & then discharges them in series to generate high voltage. It is used for creating high voltage using a low voltage source.
What is the difference between resistance grounding system and resistance earthing system?
Resistance Grounding: In this system, the load neutral is connected to the ground using resistor/s to limit & carry the fault current in case of unbalanced conditions. Resistance Earthing: this is used to protect the equipment from fault condition by carrying the current through earth cable.
What is the difference between resistors in series and resistors in parallel?
Resistors in parallel: 1/Requiv = 1/R1 + 1/R2 + .... + 1/Rn Resistors in series: Requiv = R1 + R2 + ... + Rn
Kirchhoff's Current Law (KCL)
The sum of all currents entering a node is zero. Conservation of current
Kirchhoff's Voltage Law (KVL)
The sum of all voltage drops around a loop is zero.
Why use the VCB in a high transmission system? Why can't a ACB be used?
VCB stands for Vacuum circuit breaker & ACB stands for Air Circuit Breaker. The VCB uses the vacuum as its arc quenching medium While ACB uses air as its arc quenching medium. The vacuum has very fast arc quenching & recovering properties as compared to ACB thus they are used is HT while ACB is used in LT.
What is Electrical Engineering?
Electrical Engineering is the field of engineering that generally deals with the study and application of electricity, electronics, and electromagnetism.
What is a vacuum circuit breaker?
A circuit 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 arc quenching ability as compared to air & they are used in high voltage circuits.
Buffer amplifier (voltage follower)
vin = vo
Electrical Units
1. Parameter: Voltage, Unit: Volt, Symbol: V or E, Description: Unit of Electrical Potential 2. Parameter: Current, Unit: Ampere, Symbol: I or i, Description: Unit of Electrical Current 3. Parameter: Resistance, Unit Ohm, Symbol: R or Ω, Description: Unit of DC Resistance 4. Parameter: Conductance, Unit: Siemen, Symbol: G or ℧, Description: Reciprocal of Resistance. 5. Parameter: Capacitance, Unit: Farad, Symbol: C, Description: Unit of Capacitance 6. Parameter: Charge, Unit: Coulomb, Symbol: Q, Description: Unit of Electrical Charge 7. Parameter: Inductance, Unit: Henry, Symbol: L or H, Description: Unit of Inductance 8. Parameter: Power, Unit: Watts, Symbol: W, Description: Unit of Power 9. Parameter: Impedance, Unit: Ohms, Symbol: Z, Description: Unit of AC Resistance 10: Parameter: Frequency, Unit: Hertz, Symbol: Hz, Description: Unit of Frequency
What are the types of rectifiers?
1. Uncontrolled rectifier; these rectifiers are made from diodes & their output only depends on the input supply. They are not controlled by any external means 2. Half wave rectifier; that converts half i.e. positive half or negative half-wave of the AC cycle into DC. 3. Full-wave rectifier: it converts both positive & negative half of AC into DC. a) Bridge rectifier; It is made of 4 diodes to converts full AC wave into DC. b)Center tap rectifier; it utilizes a center tap transformer with only 2 diodes to convert full AC into DC. 4. Controlled Rectifier: these rectifiers are made from SCR (thyristors) & their output voltage can be controlled by varying the firing angle. a)Half controlled: These rectifiers are made from SCR as well as diodes. b)Full controlled: it is purely made from SCR & provides full control over the voltage through the external triggering pulse.
What is the difference between circuit breaker & isolator?
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. 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.
What is an inductor?
A coil of wire which resists changes in electric current passing through it. When the current flowing through an inductor changes, the time-varying magnetic field induces a voltage in the conductor. V = L di/dt. Inductors are DC shorts and act like resistors do when they are in parallel or in series
What is a conductor?
A conductor is an object or type of material that allows the flow of charge in one or more directions
What is the principle of a motor?
A current-carrying conductor (loop-shaped) placed inside a magnetic field experiences a mechanical force whose direction is given by Fleming's left-hand rule.
What is the difference between fuse and breaker?
A fuse is made up 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. A 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.
What is the difference between a generator and an alternator?
A generator & an alternator both convert the mechanical energy into electrical energy. The key difference between them is that the generator has a stationary magnetic field i.e. the armature rotates inside a fixed magnetic field. while in an alternator, the magnetic field rotates inside a conductor windings (stator). So the stationary part of the alternator is phase conductors.
What is a linear & non-linear circuit?
A linear circuit is an electronic circuit that follows the superposition rule. This means that the output of the circuit F(x) when a linear combination of signals ax1(t) + bx2(t) is applied to it is equal to the linear combination of the outputs due to the signals x1(t) and x2(t) applied separately: F(ax1(t)+bx2(t))=aF(x1(t))+bF(x2(t)). It is called a linear circuit because the output of such a circuit is a linear function of its inputs. The electronic components' values such as frequency, resistance, inductance, capacitance, etc. remain constant with varying current & voltage. Linear circuits are important because they can process analog signals without introducing intermodulation distortion. This means that separate frequencies in the signal stay separate and do not mix, creating new frequencies While in the non-linear circuit, the current & voltage does not have a linear relationship. The electronic components' values of such circuits change with varying voltage & current.
What is a magnetic starter?
A magnetic starter is a device designed to provide a safe starting method to electric motors with a heavy load. It includes a contactor as an essential component, while also providing power-cutoff, under-voltage, and overload protection.
What are transformer losses & their types?
A real transformer has the following losses: Copper Losses(Winding Resistance): the losses occur due to the current flowing through the windings that causes resistive heating of the conductors. Core or Iron Losses: there are two types of core or iron losses. 1. Hysteresis Losses: these losses occur due to the magnetic field reversal. The magnetization & demagnetization of the core consumes some energy, know as hysteresis loss. 2. Eddy Current Losses: the losses due to the current generated inside the core. The eddy current causes the resistive heating of the core.
What are rectifiers?
A rectifier is an electrical device that converts the alternating current into a direct current.
What is reverse power relay?
A reverse power protection relay is a directional protection device that protects the generator from taking current back from the grid & causes what is known as a generator motoring effect. it can damage the generator.
What will happen if a DC supply is given to the primary of a transformer?
A transformer is essentially an inductor (coil) that has high inductance & low resistance. For a DC supply, there will be no inductance & there will be no mutual induction between the windings. Also, the low resistance will draw a huge amount of current that will damage the coil & the insulation. Thus, a transformer won't operate on DC and may explode with fire.
What is a transistor composed of?
A transistor is made of different combinations of P-type & N-type semiconductors. The doping & combination & different shapes of semiconductors form different types of transistors with different electrical properties.
What is a capacitor?
A two terminal electrical device used to store electrical energy in the form of electric field between the two plates. Also known as a condenser and the SI unit of its capacitance measure is Farad (F). Capacitors are DC open circuits
What is a unilateral & bilateral circuit?
A unilateral circuit is a circuit whose properties change with changing the direction of current flow or the supply voltage. In other words, unilateral circuits only allow for current to flow in one direction, example is a diode rectifier. Whereas a bilateral circuit is a circuit whose properties do not change with changing the current flow direction or supply voltage. In other words, bilateral circuits all current to flow in both directions, example is a transmission line(give supply from other direction, circuit properties are the same).
What is ACSR cable and when do we use it?
ACSR stands for "Aluminum Conductor Steel Reinforced". Its outer strands are made of highly pure aluminum to have better conductivity while the center strands from steel to increase the tensile strength of the cable. It is used for overhead transmission lines. It has better conductivity & low weight as well as cost.
What is the difference between active and passive components?
Active components are devices or components that require an external source to carry out their operation and produce energy in the form of voltage or current. Ex) Diodes, transistors Passive components are devices or components that do no require an external source to carry out their operation and store or maintain energy in the form of voltage or current. Ex) Resistor/Capacitor
What is the difference between analog and digital circuits?
Analog circuit can process an analog input signal that has continuously varying voltage. An analog circuit can convert an analog signal into a digital signal. Digital circuit can only process a digital signal i.e. a signal that has only two levels (1 or 0 / High or low). They are used in logic-based circuit designing to process complicated functions like in computer & cellphones etc.
What is active, reactive, apparent & complex power?
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. 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. 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. 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.
What is meant by reverse polarity and how can it 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 switch will cutoff only the neutral wire from the appliance connected to such outlets. It can be easily fixed by swapping the wire.
What is an ideal transformer?
An ideal transformer is an imaginary transformer in which no losses occur at all. In other words, the transformer Input power is equal to the output power of the transformer i.e. they have 100% efficiency. It is just a theoretical transformer because there must be some losses in a real transformer. Transformer input Power = Transformer Output Power. PIN = POUT.
Why are motors rated in kW instead of kVA
As we know the transformer is rated in KVA because its PF (Power Factor) depends on the nature of the loads. However, Motor has a fixed Power factor, i.e. motor has defined power factor (P.F) and the rating has been mentioned in kW or HP on Motor. In more clear words, Motor only consumes active power and provides mechanical power in HP or kW at the motor shaft and that is the reason for motor rating in Watts.
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.
Define the terms 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. Capacitance equation: C = Q/V where C is capacitance, Q is electrical charge in coulombs, and V is voltage in volts. Inductance: inductance is the ability of a conductor to resist or oppose any change in the current flowing through it. The current generates a magnetic field whose strength varies with the current. It is denoted by L & is measured in Henry H.
What is the role of a capacitor in AC and DC circuits?
In an AC circuit, capacitor reverses its charges as the current alternates and produces a lagging voltage (in other words, capacitor provides leading current in AC circuits and networks). In a DC Circuit, the capacitor once charged with the applied voltage acts as an open switch.
What is the difference between capacitors in parallel and capacitors in series?
In series: 1/CT = 1/C1 + 1/C2 + ... 1/Cn In parallel: CT = C1 + C2 +.... Cn
What is a Zener diode?
It is a special type of diode that not only conducts current in forward but also in reverse direction once the Zener voltage is reached. It is used in reverse bias to maintain a constant voltage over a range of currents. It is a safety device used as a voltage regulator or to protect other components from overvoltage.
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.
What is the maximum power transfer theorem?
It mentions the condition for maximum power transfer from source to load. It states that in a linear, bilateral network, the maximum external power transferred from source with a finite internal resistance to the load is obtained when the external load resistance equals the internal resistance of the source or Thevenin's resistance of the circuit.
What are the methods used for p.f. correction to nullify the lagging current?
Most loads that are inductive cause a lagging power factor. Methods used to fix this are: 1.Static Capacitor: the capacitor helps in providing a leading current that eliminates the lagging component of current and improves the power factor. 2. Synchronous Condenser: it is an over-excited synchronous motor with no load that also provides a leading power factor. 3. Phase Advancer: a simple AC exciter which is connected on the main shaft of the motor and operates with the motor's rotor circuit for power factor improvement. Phase advancer is used to improve the power factor of induction motor in industries
What is a motor starter?
Motor Starter is a device that connects in series with the motor to decrease the starting current (that could damage the windings in normal conditions) and gradually increase current after starting the motor (in other words start or stop the motor) and provide overload protection.
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.
What is power?
Power is the amount of energy per unit time. P= IV
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.
What is a coulomb?
The charge transported by a constant current of one ampere in one second: 1 C = 1 A * 1 s. One coulomb is the magnitude of electrical charge 6.24 x 10^18
What is a leading & lagging power factor?
The cosine of the angle between voltage & current is called the power factor. If the current leads the voltage, it causes a leading power factor. If the current lags voltage the power factor is lagging.
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
What will the current be if the total resistance in a series circuit doubles?
The current has a linear inverse relation with the resistance. Thus, doubling the resistance will decrease the current by half.
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.
Why should the current transformer's secondary not be open when there is current flow 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.
What do the different colors on wires indicate? What is the difference between ground and neutral?
The different colors of the wires are used for phase indication purposes. They represent different phases, the neutral & earth cable. The color code may differ around the world but usually, the earth wire remains the same i.e green with yellow stripes. Ground: a conductor that provides a low-impedance path to the earth to prevent hazardous voltages from appearing on equipment (high voltage spikes). Neutral: a circuit conductor that normally completes the circuit back to the source
What is meant by insulation voltage in cables?
The insulation voltage or insulation level is the rated voltage of a cable at which the cable can withstand it for its lifespan without rupturing or breakdown.
What is meant by knee point voltage?
The knee point voltage of a current transformer is the minimum voltage at which a minimum increase (like 10%) of voltage will increase the magnetizing current by a large margin (like 50%).
What is a laser diode?
The laser diode converts electrical energy into light energy just like an LED but it creates coherent light. It is a PIN junction diode that creates a beam of light by a combination of holes & electrons in the intrinsic region. They are used in optical communication, pointer, printer, etc.
What are the advantages of Star Delta Starter with induction motors?
The main advantages of star delta starter are: 1.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. 2. 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. 3. Its operation is very simple 4. The cost of this starter is comparatively very cheap. 5. It has a good torque to current performance.
What are the different methods for starting an induction motor?
The methods used for starting an induction motor are as follows: 1.DOL: direct online starter 2.Star delta starter 3.Autotransformer starter 4.Resistance starter 5.Series reactor starter
What is latching current & holding current?
The minimum initial current required to maintain thyristor in forward conduction state immediately after turning ON is called latching current. The gate signal is removed after providing a latching current. The holding current is the minimum current required to maintain the thyristor in forward conduction otherwise the thyristor will return to forward blocking state. The latching current is the current required to switch on the thyristor while the holding current is the minimum limit for turning off the thyristor. The latching current is always greater than holding current.
What is current?
The net rate of flow of electric charge through a region. I = dQ/dt. If the electrical charge is positive, the direction of current is in the same direction as electrical charge. If electric charge is negative, the direction of current is in the opposite direction of the direction of the electric charge.
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.
What is a volt(V)
The potential difference between two points the will impart one joule of energy per coulomb of charge. V(x) = E(joules)/Q(Coulomb)
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 cellphones, vehicles, generators, etc. We have discussed both in details in different types of batteries.
How do you decide what size electrical wire to use?
The proper size of the wire mainly depends on the amount of load current & voltage supplied as well as the length of the cable. Usually, the voltage drop in cables should not be greater then 2.5% of the supply. 1.Measure the supply voltage. 2.Calculate its 2.5% allowable voltage drop. 3.Measure the required length of the cable in meters. 4.Divide the allowable voltage drop with the length. 5.The resultant is the max allowable voltage-drop per meter & your required cable size should have a lower voltage drop than this. 6.Calculate your load current. The cable charts provide this info such as voltage-drop per meter, current capacity, cable size, etc. match your calculation with the chart & find you required cable size.
Why do we improve the power factor?
The reasons for improving the power factors are stated below: Large Line Losses (Copper Losses): Line losses (I2R) depend on current. The low power factor draws a large amount of current as compared to the high power factor. Large kVA rating and Size of Electrical Equipment: PF is inversely proportional to KVA. Low PF equipment with a high KVA rating is larger in size. Large Conductor Size and Cost: we need large conductors to transmit the heavy current required due to low power factor. Poor Voltage Regulation and Large Voltage Drop: The large current due to low PF causes a high voltage drop that needs to be regulated more often than usual. Low Efficiency: The losses due to the high current flow & voltage drop deteriorate the efficiency of the system. The efficiency is maximum at PF=1.
How do you know if a circuit is inductive or capacitive or purely resistive?
The total impedance of the circuit can be used to identify the circuit. If the imaginary part of the total impedance is positive, then it is an inductive circuit. It the imaginary part is negative, and then it is a capacitive circuit. If it is zero then the circuit is purely resistive.
What are the various kinds of cables used for transmission?
The type of cables are: 1. Low tension or LT cable t is used for transmission of voltage below 1000v 2. High tension or HT cable used for transmission of up to 11kv. 3. Super tension cable is used for handling voltage up to 33kv 4. Extra high tension cable can handle voltage up to 66 kV. 5. Extra super tension cable is used for transmission of voltage up to 132kV.
What is the voltage regulation of the transformer & why is it important?
The voltage regulation of a transformer is the percent change in the secondary voltage from no load to full load condition, equation: Regulation Percentage = (Eno-load -Efull-load)/ Efull-load * 100. "Full-load" means the point at which the transformer is operating at maximum permissible secondary current. 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 is important because it is the measure of how well a transformer can maintain constant secondary voltage given a constant primary voltage and wide variance in load current. You want the transformer to have a low voltage regulation as the secondary voltage is more stable and the better voltage regulation the transformer will provide.
How many types of faults occur in a three phase power system?
There are two main types of faults in the three-phase system: 1. Open circuit fault: a) Single-phase open fault b) Double phase open fault c) Three-phase open fault 2. Short circuit fault: a)Single line to ground fault(LG) b) Double line to ground fault(LLG) c) Line to line fault(LL) d) Three phase short circuit fault (LLL)
What are the types of semiconductors?
There are two main types of semiconductors: 1. Intrinsic semiconductor: the semiconductor that has same number of electrons & holes. They exist in their natural form. 2.Extrinsic semiconductor: these semiconductors have foreign impurity (electrons or holes) inserted artificially through the process called doping. The impurities are intentionally added to change its electrical properties. Extrinsic semiconductors can other be: N-Type: The dopant in this type of semiconductor increases the number of free electrons. So the majority carriers are electrons in N-type. OR P-Type: the dopant of such semiconductors adds in the excess number of holes. The majority carriers in P-type semiconductors in holes.
What could be the reason for the current to double in a linear circuit?
There are two reasons for increasing the current: 1. The total resistance of the circuit is reduced by half. 2. The supply voltage to the circuit is doubled
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. Equation: Efficiency, η = Output Power / Input Power All-Day efficiency: the ratio of output power in kilo Watt-Hour (kWh) to the input power in kWh of the transformer for 24 hours. Equation: ηall_day = Output in kWh / Input in kWh (for 24 hours) Condition for Maximum Efficiency: The copper loss must be equal to iron loss; which is the combination of hysteresis loss & eddy current loss. Cu Loss = Iron Loss->Wcu = Wi, cu and i are subscripts
Why are transformers rated in kVa, not in kW?
When manufacturers design a transformer, they have no idea which kind of load will be connected to the transformer. Further, the load may be resistive(R), inductive(L), capacitive(C) or mixed load (R, L, C). This means there would be a different power factor at the secondary (load) sider. 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 at y amount of voltage". This way they go for VA as in (voltage X amperes) instead of W in the case of rating a transformer.
How do capacitors work?
Whenever voltage is applied across the capacitor's terminals (charging the capacitor), an electric field develops across the separation and current starts to flow and continue to travel until the voltage across both the negative and positive (Anode and Cathode) plates become equal to the voltage of the source (applied voltage). These two plates are separated by a dielectric material which is used to increase the capacitance of the capacitor.
