SESSION:4

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1)    What is the difference between isolators and circuit breakers? What is bus-bar?

Isolators are mainly for switching purpose under off load conditions but they cannot operate in fault conditions. There is no any arc quenching facility provided in Isolator. 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 lines for transmission.

2)    What are the advantage of Free-wheeling diode in a Full Wave rectifier?

It reduces the harmonics and it also reduces sparking and arcing across the mechanical switch so that it reduces the voltage spike seen in an inductive load.

3)    How do you select a cable size (Cu & Al) for a particular load?

At first calculate the load current, after that de-rate the electrical current considering derating factor (depending on site condition and laying of cable) after choose the cable size from cable catalogue considering de-rated current. After that measure the length of cable required from supply point of load to load point. Calculate the voltage drop which will max 3% (resistance and reactance of cable found from cable catalogue of selecting cable) if voltage drop>3%
then choose next higher size of cable.

4)    What are HRC fuses and where it is used?

HRC stand for "high rupturing capacity" fuse and it is used in distribution system for electrical transformers.

5)    Which power plant has high load factor?

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.

6)    Mention the methods for starting an induction motor?

            The different methods of starting an induction motor

a)     DOL: Direct Online Starter

b)     Star Delta Starter

c)     Auto Transformer Starter

d)     Resistance Starter

e)     Series Reactor Starter

7)    What is the difference between earth resistance and earth electrode resistance?

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.

8)    What is the power factor of an alternator at no load?

At no load Synchronous Impedance of the alternator is responsible for creating angle difference. So it should be zero lagging like inductor.

9)    How electrical power is generated by an A.C Generator?

For the generation of electric power we need a prime mover which supplies mechanical power input to the alternator, it can be steam turbine, or hydro turbine .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.

10) Why an AC solenoid attract the plunger even though we interchanges the terminal? Will the poles changes?

This is because Plunger is not magnetic material rather it is of iron. So it does not matter whether Poles are changing or not, plunger will be attracted.

11) What is derating? Why it is necessary, it is same for all means for drives, motors and cables. 

The current currying capacity 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, and distance between cables. Considering this condition actual current currying capacity of cable reduce than current currying capacity (which given in cable Catalogue) this is called derating.

12) Why temperature rise Test is conducted in bus bars and isolators? 

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.

13) 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?

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.

14) If one lamp connects between two phases it will glow or not?

If the voltage between the two phases are 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.

15) What is the count of HVDC transmission lines in India?

At present there are three HVDC transmission lines in India.

·         chandrapur to padghe(mumbai)--(1500 MW at ±500 kV DC)

·         rehand to delhi (1500 MW at ±500 kV DC)

·         talchal to kolar (2500 MW)

16) What is meant by regenerative braking?

When the supply is cut off for a running motor, it still continue running due to inertia. In order to stop it quickly we place a load (resistor) 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".

17) 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.

18) What are the advantages of star-delta starter with induction motor?

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 (DOL). Hence the starting current is reduced, the voltage drops during the starting of motor in systems are reduced.

19) Why Delta Star Transformers are used for Lighting Loads?

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.

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

It depends upon R= pl/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 to ensure first to make the connection and last to disconnect to Earth. This assures Safety for the person who uses the electrical instrument.

21) Why series motor cannot be started on no-load?

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