SESSION:6

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1)       Why use the VCB at High Voltage Transmission System? Why can't use ACB?

Actually the thing is vacuum has high arc quenching property compared to air because in VCB, the dielectric strengths equal to 8 times of air. That why always vacuum CB is used in as HT breaker and air used as in LT Breaker.

2)       What is the difference between surge arrestor and lightning arrestor?

Lightning Arrestor is installed outside and the effect of lightning is grounded, whereas surge arrestor installed inside panels comprising of resistors which consumes the energy and nullify the effect of surge.

3)       What is the difference between synchronous generator & asynchronous generator?

In simple, synchronous generator supplies both active and reactive power but asynchronous generator (induction generator) supplies only active power and consume reactive power for magnetizing. This type of generators are used in windmills.

4)       Give two basic speed control scheme of DC shunt motor?

By using Flux Control Method: In this method a rheostat is connected across the field winding to control the field current. So by changing the current the flux produced by the field winding can be changed, and since speed is inversely proportional to flux speed can be controlled.

Armature Control Method: In this method a rheostat is connected across armature winding by varying the resistance the value of resistive drop (IaRa) can be varied, and since speed is directly proportional to Eb-IaRa the speed can be controlled.

5)       What is the basic principle of operation of motor?

Whenever a current carrying conductor is placed in a magnetic field it produce turning or twisting movement is called as torque.

6)       What is meant by armature reaction?

The effect of armature flu to main flux is called armature reaction. The armature flux may support main flux or opposes main flux.

7)       What is the Polarization index value? (PI value) and simple definition of Polarization Index?

Its ratio between Insulation Resistance (IR) values for 10 min to Insulation Resistance for 1 min. It ranges from 5-7 for new motors & normally for motor to be in good condition it should be Greater than 2.5.

8)       What will happen when power factor is leading in distribution of power?

If there is high power factor i.e. if the power factor is close to one:

1. Losses in form of heat will be reduced,

2. Cable becomes less bulky and easy to carry, and very cheap to afford, & it also reduces over heating of transformers.

9)       What is the advantages of VVVF drives over non VVVF drives for EOT cranes?

1. Smooth start and stop.

2. No jerking of load.

3. Exact positioning

4. Better protection for motor.

5. high/low speed selection.

6. Reliability of break shoe.

7. Programmable break control.

8. Easy circuitry

9. Reduction in controls

10. Increases motor life

10)    Why, when birds sit on transmission lines or current wires doesn't get shock?

It’s true that if birds touch the single one line (phase or neutral) they don't get electrical shock. If birds touch two lines than the circuit is closed and they get electrical shock. So if a human touch single one line (phase) then he doesn't get shock if he is in the air (not touching - standing on the ground if he is standing on the ground then touching the line (phase) he will get a shock because the ground on what we are standing is like line (ground bed - like neutral) and in the most of electric lines the neutral is grounded, so current will flow through our body to ground. So that means that human who touch the line closes the circuit between phase and neutral.

11)    Which motor has high Starting Torque and Staring current DC motor, Induction motor or Synchronous motor?

DC Series motor has high starting torque. We cannot start the Induction motor and Synchronous motors on load, but cannot start the DC series motor without load.

12)    What is ACSR cable and where we use it?

ACSR means Aluminium conductor steel reinforced, this conductor is used in transmission & distribution.

13)    How tube light circuit is connected and how it works?

A choke is connected in one end of the tube light and a starter is in series with the circuit. When supply is provided, the starter will interrupt the supply cycle of AC. Due to the sudden change of supply the chock will generate around 1000 volts. This volt will capable of to break the electrons inside the tube to make electron flow. Once the current passes through the tube the starter circuit will be out of part. Now there is no change of supply causes choke voltage normalized and act as minimize the current.

14)    What is Marx Circuit?

It is used with generators for charging a number of capacitor in parallel and discharging them in series. It is used when voltage required for testing is higher than the available.

15)    What is encoder, how it function?

An encoder is a device used to change a signal (such as a bit stream) or data into a code. The code may serve any of a number of purposes such as compressing information for transmission or storage, encrypting or adding redundancies to the input code, or translating from one code to another. This is usually done by means of a programmed algorithm, especially if any part is digital, while most analog encoding is done with analog circuitry.

16)    What are the advantages of speed control using thyristor?

1. Fast Switching Characteristics than MOSFET, BJT, IGBT

2. Low cost

3. Higher Accurate.

17)    Why Human body feel Electric shock? And in an Electric train during running, we did not feel any Shock?

Unfortunately our body is a good conductor of electricity. The golden rule is current takes the lowest resistant path if you have insulation to our feet as the circuit is not complete (wearing rubber footwear which doing some repairs is advisable as our footwear is a high resistance path not much current flows through our body).The electric train is well insulated from its electrical system.