SESSION:9

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1)       A step up transformer which has 110/220 turns. Can we replace it with 10/20 turns? Turns ratio is same shall its rating be same? Give appropriate reason?

We cannot replace 110 / 220 turns Transformer with 10 / 20 turns. As the Flux produced in the core is directly proportional to number of turn therefore if number of turn is reduced then developed emf will also be reduced. Hence if we use 10/20 turn Transformer then Voltage ration will also change i.e. decrease.

2)       Why Alternator/Generator Rated in kVA. Not in kW?

The power √3 V_L I_L Cos φ delivered by the alternator for the same value of current, depends upon p.f. (Power Factor=Cos φ) of the load. But the alternator conductors are calculated for a definite current and the insulation at magnetic system are designed for a definite voltage independent of p.f. (Cos φ) of the load. For this reason apparent power measured in kVA is regarded as the rated power of the alternator.

3)       What is the main difference between contactor and Starter?

The magnetic starter is very similar to the magnetic contactor in design and operation. Both have the feature of operating contacts when the coil is energized. The important difference between contactors and starters is the use of overload heater element in the starter. (For protect the motor from overheating or over load protection).

4)       What are the causes of low power factor?

The main cause of low Power factor is Inductive Load. As in pure inductive circuit, Current lags 90° from Voltage, this large difference of phase angle between current and voltage causes zero power factor. Basically, all those circuit having Capacitance and inductance (except resonance circuit (or Tune Circuit) where inductive reactance = capacitive reactance (XL = Xc), so the circuit becomes a resistive circuit), power factor would be exist over there because Capacitance and inductance causes in difference of phase angle (θ) between current and voltage.

There are a lot of disadvantages of low Pf and we must improve Pf .

Following are the causes of low Power factor:

1. Single phase and three phase induction Motors (Usually, Induction motor   works at poor power factor i.e. at:

Full load, Pf = 0.8 -0.9

    Small load, Pf = 0.2 -0.3

    No Load, Pf may come to Zero (0).

2.  Varying Load in Power System (As we know that load on power system is varying. During low load period, supply voltage is increased which increase the magnetizing current which cause the decreased power factor)

3.  Industrial heating furnaces

4.  Electrical discharge lamps (High intensity discharge lighting) Arc lamps (operate a very low power factor)

5.  Transformers

6.  Harmonic Currents

5)       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.

6)       Why we can’t store AC in Batteries instead of DC.or Can we store AC in batteries instead of DC?

We cannot store AC in batteries because AC changes their polarity up to 50 (When frequency = 50 Hz) or 60 (When frequency = 60 Hz) times in a second. Therefore the battery terminals keep changing Positive (+ve) becomes Negative (-Ve) and vice versa, but the battery cannot change their terminals with the same speed so that’s why we can’t store AC in Batteries.

In addition, when we connect a battery with AC Supply, then It will charge during positive half cycle and discharge during negative half cycle, because the Positive (+ve) half cycle cancel the negative (-Ve) half cycle, so the average voltage or current in a complete cycle is Zero. So there is no chance to store AC in the Batteries.

7)       Why Motor rated in kW instead of kVA?

Motor is a device which converts Electrical power into Mechanical power. In this case, the load is not electrical, but mechanical (Motor’s Output) and we take into the account only active power which has to be converted into mechanical load. Moreover, the motor power factor does not depend on the load and it works on any P.F because of its design.

8)       Why Battery rating in Ah (Ampere hour) and not in VA?

Battery stores charge in the form of chemical energy and then converts it into electrical energy to utilize for a specific time. The amount of available charge is the capacity of a cell or battery which may be expressed in Ah (Ampere-hour). Moreover, in a charged battery, the numbers of molecules are limited to create a flow of electron in electric circuits, so, there must be a limited number of electrons in a cell/battery which they motivate through a circuit tofully discharge. Now we have the option to rate the battery capacity in Number of flowing electrons for a specific time, but, it would be a headache, because there are a vast number of electrons in it.  So we have another option (1C (Coulomb) = 6.25 x 1018electrons, or 6,250,000,000,000,000,000 electrons.

In addition,

1A (Ampere) = 1 coulomb of electrons per second and,

1h = 3600 Seconds

Therefore;

1Ah = (1A) x (3600s) = (C/s) x (3600s) = 3600 C.

∴ A (1 Ampere) = 1 Coulomb per second = C/s

9)        What is the Main Difference between Active and Passive Components?

Active Components:
Those devices or components which required external source to their operation is called Active Components.
For Example: Diode, Transistors, SCR etc…
Explanation and Example: As we know that Diode is an Active Components. So it is required an External Source to its operation.
Because,  If we connect a Diode in a Circuit and then connect this circuit to the Supply voltage., then Diode will not conduct the current Until the supply voltage reach to 0.3(In case of Germanium) or 0.7V(In case of Silicon). 

Passive Components:

Those devices or components which do not required external source to their operation is called Passive Components.
For Example: Resistor, Capacitor, Inductor etc…

 Explanation and Example: Passive Components do not require external source to their operation.
Like a Diode, Resistor does not require 0.3 0r 0.7 V. I.e., when we connect a resistor to the supply voltage, it starts work automatically without using a specific voltage. If you understood the above statement about active Components, then you will easily get this example.