Electrical Interview questions Part 16

Query:What is the function of interposing current transformer? 
Resolution:The main function of an interposing current transformer is to balance the currents supplied to the relay where there would otherwise be an imbalance due to the ratios of the main current transformers.  Interposing current transformer are equipped with a wide range of taps that can be selected by the user to achieve the balance required. 

Query:What are Motor Generator Sets and explain the different ways the motor generator set can be used ? 

Resolution:Motor Generator Sets are a combination of an electrical generator and an engine mounted together to form a single piece of equipment. Motor generator set is also referred to as a genset, or more commonly, a generatorThe motor generator set can used in the following different ways: 

              1.Alternating current (AC) to direct current (DC)
              2.DC to AC
              3.DC at one voltage to DC at another voltage
              4.AC at one frequency to AC at another harmonically-related frequency

Query:What is power quality meter ?
Resolution:Power Quality meters are common in many industrial environment. Small units are now available for home use as well. They give operators the ability to monitor the both perturbations on the power supply, as well as power used within a building, or by a single machine or appliance. In some situations, equipment function and operation is monitored and controlled from a remote location where communication is via modem, or high-speed communication lines.So we can understand the importance of power measurement through power quality meters.

Query:What is the different between digital phase converter and ordinary phase converter?
Resolution:Digital phase converter are a recent development in phase converter technology that utilizes proprietary software in a powerful microprocessor to control solid state power switching components. This microprocessor, called a digital signal processor (DSP), monitors the phase conversion process, continually adjusting the input and output modules of the converter to maintain perfectly balanced three-phase power under all load conditions.

Query:Explain the operation of variable frequency transformer?
Resolution:A variable frequency transformer is used to transmit electricity between two asynchronous alternating current domains. A variable frequency transformer is a doubly-fed electric machine resembling a vertical shaft hydroelectric generator with a three-phase wound rotor, connected by slip rings to one external ac power circuit. A direct-current torque motor is mounted on the same shaft. Changing the direction of torque applied to the shaft changes the direction of power flow; with no applied torque, the shaft rotates due to the difference in frequency between the networks connected to the rotor and stator.The variable frequency transformer behaves as a continuously adjustable phase-shifting transformer. It allows control of the power flow between two networks .

Query:What is the main use of rotary phase converter ?
Resolution:Rotary phase converter will be converting single phase power into true balanced 3 phase power,so it is often called as single phase to three phase converter .Often the advantages of 3 phase motors, and other 3 phase equipment, make it worthwhile to convert single phase to 3 phase so that small and large consumers need not want to pay for the extra cost of a 3 phase service but may still wish to use 3 phase equipment.

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Electrical Interview questions Part 15

Query:How to determine capacitor tolerance codes?
Resolution:In electronic circuits, the capacitor tolerance can be determined by a code that appears on the casing. The code is a letter that often follows a three-digit number (such as 130Z).The first two are the 1st and 2nd significant digits and the third is a multiplier code. Most of the time the last digit tells you how many zeros to write after the first two digits and these are read as Pico-Farads.

Query:Why most of analog o/p devices having o/p range 4 to 20 mA and not 0 to 20 mA?

Resolution:4-20 mA is a standard range used to indicate measured values for any process. The reason that 4ma is chosen instead of 0 mA is for fail safe operation .For example- a pressure instrument gives output 4mA to indicate 0 psi, up to 20 mA to indicate 100 psi, or full scale. Due to any problem in instrument (i.e) broken wire, its output reduces to 0 mA. So if range is 0-20 mA then we can differentiate whether it is due to broken wire or due to 0 psi.

Query:Two bulbs of 100w and 40w respectively connected in series across a 230v supply which bulb will glow bright and why?

Resolution:Since two bulbs are in series they will get equal amount of  electrical current but as the supply voltage is constant across the bulb(P=V^2/R).So the resistance of 40W bulb is greater and voltage across 40W is more (V=IR) so 40W bulb will glow brighter.
 

Query:What is meant by knee point voltage?

Resolution:Knee point voltage is calculated for electrical Current transformers and is very important factor to choose a CT. It is the voltage at which a CT gets saturated.(CT-current transformer).
 

Query:What is reverse power relay?

Resolution:Reverse Power flow relay are used in generating stations's protection. A generating stations is supposed to fed power to the grid and in case generating units are off,there is no generation in the plant then plant may take power from grid. To stop the flow of power from grid to generator we use reverse power relay.

Query:What will happen if DC supply is given on the primary of a transformer?
Resolution:Mainly transformer has high inductance and low resistance.In case of DC supply there is no inductance ,only resistance will act in the

-->electrical circuit. So high  electrical current will flow through primary side of the transformer.So for this reason coil and insulation will burn out.

Query:What is the difference between isolators and  -->electrical circuit breakers? What is bus-bar?
Resolution:Isolators are mainly for switching purpose under normal conditions but they cannot operate in fault conditions .Actually they used for isolating the CBs for maintenance. Whereas CB gets activated under fault conditions according to the fault detected.Bus bar is nothing but a junction where the power is getting distributed for independent loads.

Query:What are the advantage of free wheeling diode in a Full Wave rectifier?
Resolution:It reduces the harmonics and it also reduces sparking and arching across the mechanical switch so that it reduces the voltage spike seen in a inductive load

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Electrical Interview questions Part 14

Q:How do you select a cable size (Cu & Al) for a particular load?
A:At first calculate the electrical current of the load, after that derate the electrical current considering derating factor(depending on site condition and laying of cable) after choose the cable size from cable catalog considering derating electrical current.After that measure the length of cable required from supply point of load to load poin. Calculate the voltage drop which will max 3% (resistance and reactance of cable found from cable catalog of selecting cable) if voltage drop>3%
then choose next higher size of cable.

Q:What are HRC fuses and where it is used?
A:HRC stand for "high rupturing capacity" fuse and it is used in distribution system for electrical transformers.

Q:Which power plant has high load factor?
A:All base load power plants have a high load factor. If we use high efficiency power plants to supply the base load,we can reduce the cost of generation.Hydel power plants have a higher efficiency than thermal & nuclear power plants.

Q:Mention the methods for starting an induction motor?
A:The different methods of starting an induction motor
DOL:direct online starter

Star delta starter
Auto transformer starter
Resistance starter
Series reactor starter

Q:What is the difference between earth resistance and earth electrode resistance?
A:Only one of the terminals is evident in the earth resistance. In order to find the second terminal we should recourse to its definition: Earth Resistance is the resistance existing between the electrically accessible part of a buried electrode and another point of the earth, which is far away.
The resistance of the electrode has the following components:
(A) the resistance of the metal and that of the connection to it.
(B) the contact resistance of the surrounding earth to the electrode.

Q:What is use of lockout relay in ht voltage?
A:A lock-out relay is generally placed in line before or after the e-stop switch so the power can be shut off at one central location. This relay is powered by the same electrical source as the control power and is operated by a key lock switch. The relay itself may have up to 24 contact points within the unit itself. This allows the control power for multiple machines to be locked out by the turn of a single key switch.

Q:What is the power factor of an alternator at no load?
A:At no load Synchronous Impedance of the alternator is responsible for creating angle difference. So it should be zero lagging like inductor.

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Electrical Interview questions Part 13

Q:How electrical power is generated by an A.C Generator? 

A:For the generation of elect power we need a prime mover which supplies mechanical power input to the alternator, can be steam turbines,or hydro turbines .When poles of the rotor moves under the armature conductors which are placed on the stator ,field flux cut the armature conductor ,therefore voltage is generated and is of sinusoidal in nature...due to polarity change of rotor poles(i,e) N-S-N-S.

Q:Why an ac solenoid valve attract the plunger even though we interchanges the terminal? Will the poles changes?
A:Yes because the poles changes for every half-cycle of ac voltage so the polarity of AC voltage is continuously changing for every half cycle. so, interchanging of terminals in ac system does not show any difference. That's why the ac solenoid attract the plunger even though it's terminals are interchanged. 

Q:What is derating?, why it is necessary, it is same for all means for drives, motors,and cables. 

A:The current currying of cables will change depending upon the site temperature (location of site), type of run (it will run through duct, trench, buried etc.), number of tray, depth of trench, distance between cables. Considering this condition actual current currying capacity of cable reduce than current currying capacity (which given to cable Catalogue) this is called derating.

Q:Why temperature rise is conducted in bus bars and isolators? 

A:Bus bars and isolators are rated for continuous power flow, that means they carry heavy currents which rises their temperature. so it is necessary to test this devices for temperature rise.

Q:When voltage increases then current also increases then what is the need of over voltage relay and over current relay? Can we measure over voltage and over current by measuring current only?
A:No.We can't sense the over voltage by just measuring the current only because the current increases not only for over voltages but also for under voltage(As most of the loads are non-linear in nature).So,the over voltage protection & over current protection are completely different. Over voltage relay meant for sensing over voltages & protect the system from insulation break down and firing. Over current relay meant for sensing any internal short circuit, over load condition ,earth fault thereby reducing the system failure & risk of fire.So, for a better protection of the system.It should have both over voltage & over current relay.

Q:If one lamp connects between two phases it will glow or not?
A:If the voltage between the two phase is equal to the lamp voltage then the lamp will glow. When the voltage difference is big it will damage the lamp and when the difference is smaller the lamp will glow depending on the type of lamp.

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Electrical Inteview questions Part 12

Query:What is the count of hvdc transmission lines in India?
Resolution:At present there are three hvdc transmission lines in india
1)chandrapur to padghe(mumbai)--(1500 MW at ±500 kV DC)
2)rehand to delhi (1500 MW at ±500 kV DC)
3)talchal to kolar (2500 MW)

Query:What is meant by regenerative braking?

Resolution:When the supply is cutt off for a running motor, it still continue running due to inertia. In order to stop it quickly we place a load(resitor) across the armature winding and the motor should have maintained continuous field supply. so that back e.m.f voltage is made to apply across the resistor and due to load the motor stops quickly.This type of breaking is called as "Regenerative Breaking".

Query:Why is the starting current high in a DC motor?

Resolution:In DC motors, Voltage equation is V=Eb-IaRa (V = Terminal voltage,Eb = Back emf in Motor,Ia = Armature current,Ra = Aramture resistance).At starting, Eb is zero. Therefore, V=IaRa, Ia = V/Ra ,where Ra is very less like 0.01ohm.i.e, Ia will become enormously increased.

Query:What are the advantages of star-delta starter with induction motor?

Resolution:(1). The main advantage of using the star delta starter is reduction of current during the starting of the motor.Starting current is reduced to 3-4 times Of current of Direct online starting.(2). Hence the starting current is reduced , the voltage drops during the starting of motor in systems are reduced.

Query:Why Delta Star Transformers are used for Lighting Loads?

Resolution:For lighting loads, neutral conductor is must and hence the secondary must be star winding. and this lighting load is always unbalanced in all three phases. To minimize the current unbalance in the primary we use delta winding in the primary. So delta / star transformer is used for lighting loads.

Query:Why in a three pin plug the earth pin is thicker and longer than the other pins?

Resolution:It depends upon R=rho l/a where area(a) is inversely proportional to resistance (R), so if (a) increases, R decreases & if R is less the leakage current will take low resistance path so the earth pin should be thicker. It is longer because the The First to make the connection and Last to disconnnect should be earth Pin. This assures Safety for the person who uses the electrical instrument.

Query:Why series motor cannot be started on no-load?

Resolution:Series motor cannot be started without load because of high starting torque. Series motor are used in Trains, Crane etc.

Query:Why ELCB can't work if N input of ELCB do not connect to ground?

Resolution:ELCB is used to detect earth leakage fault. Once the phase and neutral are connected in an ELCB, the current will flow through phase and that much current will have to return neutral so resultant current is zero. Once there is a ground fault in the load side, current from phase will directly pass through earth and it will not return through neutral through ELCB. That means once side current is going and not returning and hence because of this difference in current ELCB wil trip and it will safe guard the other circuits from faulty loads. If the neutral is not grounded, fault current will definitely high and that full fault current will come back through ELCB, and there will be no difference in current.

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