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DamodarValleyCorp Technical Interview Questions and Answers
Interview Questions
1.What is inrush current?
Inrush current is the current drawn by a piece of electrically operated equipment when power is first applied. It can occur with AC or DC powered equipment, and can happen even with low supply voltages.
2.In a Tap changing transformer where is the tap connected, is it connected in the primary side or secondary side?
Tapings are connected to high voltage winding side, because of low current. If we connect tapings to low voltage side, sparks will produce while tap changing operation due to high current.
3.Why transformer ratings are in kva?
Since the power factor of transformer is dependent on load we only define VA rating and does not include power factor .In case of motors, power factor depend on construction and hence rating of motors is in KWatts and include power factor.
4.What is difference between fuse and breaker?
Fuses are burned at the time of over current flows in the circuit but breakers are just open(not burn) at the time of over current flow. Fuses are used in only one time but breakers are used by multiple number of times.
5.What is the difference between delta-delta, delta-star transformer?
Delta-delta transformer is used at generating station or a receiving station for Change of Voltage (i,e) generally it is used where the Voltage is high & Current is low.Delta-star is a distribution kind of transformer where from secondary star neutral is taken as a return path and this configuration is used for Step down voltage phenomena.
6.Capacitor is load free component but why ampere meter shows current when capacitor bank breaker close?
As we know that Electrical is having two type of load, Active and Reactive .Capacitor is a reactive load which is not considering as a load,& its factor is Isin@ .Meter is design based on Current RMS value because of it meter is showing the current RMS value.
7.What is meant by regenerative braking?
When the supply is cut off for a running motor, it still continue runningdue to inertia. In order to stop it quickly we place a load (resistor) across thearmature 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 loadthe motor stops quickly. This type of breaking is called as "RegenerativeBreaking".
8.Why is the starting current high in a DC motor?
In DC motors, Voltage equation is V=Eb-IaRa (V = Terminal voltage,Eb= Back emf in Motor,Ia = Armature current,Ra = Armature 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.
9.What are the advantages of star-delta starter withinduction motor?
(1)The main advantage of using the star delta starter is reduction ofcurrent during the starting of the motor. Starting current is reduced to 3-4 timesOf current of Direct online starting.(2). Hence the starting current is reduced; thevoltage drops during the starting of motor in systems are reduced
10.Why Delta Star Transformers are used forLighting Loads?
For lighting loads, neutral conductor is must and hence the secondarymust be star winding. And this lighting load is always unbalanced in all threephases. To minimize the current unbalance in the primary we use delta windingin the primary. So delta / star transformer is used for lighting loads
11.What is the difference between MCB & MCCB,Where it can be used?
MCB is miniature circuit breaker which is thermal operated and use forshort circuit protection in small current rating circuit. MCCB moulded case circuitbreaker and is thermal operated for over load current and magnetic operation forinstant trip in short circuit condition. under voltage and under frequency may beinbuilt. Normally it is used where normal current is more than 100A
12.Define IDMT relay?
It is an inverse definite minimum time relay. In IDMT relay its operatingis inversely proportional and also a characteristic of minimum time after whichthis relay operates. It is inverse in the sense ,the tripping time will decrease asthe magnitude of fault current increase.
13.What are the transformer losses?
TRANSFORMER LOSSES - Transformer losses have two sources-copper loss and magnetic loss. Copper losses are caused by the resistance ofthe wire (I2R). Magnetic losses are caused by eddy currents and hysteresis inthe core. Copper loss is a constant after the coil has been wound and therefore ameasureable loss. Hysteresis loss is constant for a particular voltage and current.Eddy-current loss, however, is different for each frequency passed through thetransformer
14.What is SF6 Circuit Breaker?
SF6 is Sulpher hexa Fluoride gas.. if this gas is used as arc quenchingmedium in a Circuit breaker means SF6 CB.
15.What is Automatic Voltage regulator (AVR)?
AVR is an abbreviation for Automatic Voltage Regulator. It is importantpart in Synchronous Generators; it controls the output voltage of the generator bycontrolling its excitation current. Thus it can control the output Reactive Power ofthe Generator.
16.what is the one main difference between UPS &inverter ? And electrical engineering & electronics engineering ?
uninterrupted power supply is mainly use for short time meansaccording to ups VA it gives backup. ups is also two types : on line and offline .online ups having high volt and amp for long time backup with with high dcvoltage. but ups start with 12v dc with 7 amp. but inverter is start with 12v,24,dcto 36v dc and 120amp to 180amp battery with long time backup
17.What is encoder, how it function?
An encoder is a device used to change a signal (such as a bitstream) ordata into a code. The code may serve any of a number of purposes such ascompressing information for transmission or storage, encrypting or addingredundancies to the input code, or translating from one code to another. This isusually done by means of a programmed algorithm, especially if any part isdigital, while most analog encoding is done with analog circuitry
18.what is the diff. btwn. electronic regulator andordinary rheostat regulator for fans?
The difference between the electronic and ordinary regulator is that inelectronic reg. power losses are lessi.e.for as we decrease the speed theelectronic reg. give the power needed for that particular speed but in case ofordinary rh type reg. the power wastage is same for every speed and no power issaved. In electronic regulator triac is employed for speed cntrl.by varying thefiring angle speed is controlled but in rheostat ctrl resistance is decreased bysteps to achieve speed control
19.what is the difference between synchronous generator & asynchronous generator?
In simple, synchronous generator supply's both active and reactive power but asynchronous generator(induction generator) supply's only active power and observe reactive power for magnetizing.This type of generators are used in windmills
20.Why use the VCB at High Transmission System ? Why can't use ACB?
Actually the thing is vacuum has high arc queching property compare to air because in VCB ,the die electric strengths equal to 8 times of air . That y always vaccum used as inHT breaker and air used as in LT .
