SESSION:8

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1)       As per Ohm’s law, if voltage increases current also increases but according to P=VI, if Voltage increases Current decreases. Explain?

Actually, according to Ohm’s Law I= V/R, clearly Current is directly proportional to the Voltage, But according to P=VI or I=P/V, it shows that current is inversely proportional to the Voltage.

It depends on how you increase the voltage i.e whether we increase it by keeping the power of the source constant or not.
If the power of the source is constant then the current would decrease when voltage increasing. But if we don’t care about the power and just simply replace the battery with a new one’s with higher power rating this can increase the current.

In Transformer, when voltage increases then current decrease because power remains constant at both side power is P=VI

By Ohm’s Law, Current (I) is directly proportional to the Voltage (V) if Resistance (R) and Temperature remain same.
I = V/R or R=V/I or V=IR.

According to P=VI or I=P/V or V=P/I

It says that Current inversely proportional to the voltage if power remain same. As we know that in Transformer, if power remain same, and voltage increase, then current decreases in Step up Transformer. Also Voltage decreases when current increases as in Step down Transformer.

Also on the generation side, same story will be there if power is constant.
But if we improve the power, then current and Voltage both will be increase. 

2)       What is the difference between Battery and Capacitor?

There are many differences. But the major one is that Electrical Energy is stored in battery or cell in the form of chemical energy, and transformed again in the form of electrical energy, while in a capacitor, electrical charge or energy stored in the form of electrostatic field. 

3)       What is the difference between a VOLTAMETER and a VOLTMETER?

A VOLTAMETER is a device used to carry out electrolytes and a VOLTMETER is a high resistance device used for measuring potential difference or voltage between two points in an electrical Circuits.

4)       How much Watts Solar Panel We need for our Home Electrical appliances?

Suppose we want to power up 5 lights of 15 Watts and we need to use these 5 lights for 4 hours every day. So first we get a total Watts usage.

P_Total = 15 x 5 = 75Watts.

Than we multiply 75 Watts with 4 hours.

P_Daily = 75 x 4 = 300 Watts.

We are going to use 300 Watts daily. Let us say we are going to have complete sunshine 6 hours each day.

 Now we divide 300W with 6 hours, so we will get hourly power charge that we need. So here will be hour power charge that we need i.e watts of solar panel that we want for our electrical appliances. PHourly = 300 / 6 = 50W. 

So we need a 50 watt solar panel.

5)       Will a DC Shunt Motor operate on AC?

The Shunt winding has a large number of turns so that it has appreciable inductance. When A.C is applied to a shunt motor, the large inductive reactance of shunt winding will reduce the field current too much. Consequently, Shunt motor will not usually run on A.C Supply.

6)       Why the reactance of a system under fault condition is low and faults currents may raise dangerously high value. ? (With simple example)

Because the total Power is constant for a system, and under fault condition there is no load (Impedance Z = 0 in case of no load, there will be no reactance, so current will be high). So current will be too high, and when power is constant, and current increases, voltage will be decrease.

Example,

Suppose, (In normal condition)

 P= 10 watt, V = 5 Volts, and Current = 2 Amp.

But in Short circuit Condition, (When current is too high)

Then,

P = 10 Watts,    I= 10 A, so

 V = P/I = 10 Watts/10A=1 V.

In case of short circuit, there will be no load so when XL = Zero, then Current will be too high.

So we can see that, in case of short circuit, XL (inductive Reactance) =0, so Current increase, voltage decreases.

7)       Explain the statement that induction motor is fundamentally a Transformer?

The induction motor is fundamentally a transformer in which the stator is the primary and the rotor is short circuited secondary. This is evident; particularly when the rotor is stationary. The rotor current establishes a flux which opposes and, therefore, tends to weaken the stator flux. This causes more current to flow in the stator winding just as increase in secondary current in a transformer causes a corresponding increase in primary current. Very often the analysis of an induction motor is made on the same lines as the transformer with the modification that short circuited secondary is considering rotating.

Also note that the working principle of both (Transformer and Induction Motor) is same i.e. Faraday law’s of Electromagnetic induction or Mutual induction.

8)       What is the difference between Power Transformers and Distribution Transformers?

Those transformers installed at the ending or receiving end of long high voltage transmission lines are the power transformers. The distribution transformers (generally pole mounted) are those installed in the location of the city to provide utilization voltage at the consumer terminals.

·    Power transformers are used in transmission network of higher voltages for step-up and step down application (400 kV, 200 kV, 110 kV, 66 kV, 33kV) and are generally rated above 200MVA.

·    Distribution transformers are used for lower voltage distribution networks as a means to end user connectivity. (11kV, 6.6 kV, 3.3 kV, 440V, 230V) and are generally rated less than 200 MVA.

·    A power transformer usually has one primary and one secondary, and one input and output. A distribution transformer may have one primary and one divided or “Tapped” secondary, or two or more secondaries..

·    Power transformers generally operate at nearly full – load. However, a distribution transformer operates at light loads during major parts of the day.

·    The performance of the power transformers is generally judged from commercial efficiency whereas the performance of a distribution transformer is judged from all – day – efficiency.

·    The rating of a high transformer is many times greater than that of distribution transformer.

·    In Power Transformer the flux density is higher than the distribution transformer.

·    Power transformer’s primary winding always connected in star and secondary winding in delta while in distribution transformer primary winding connected in delta and secondary in star.

·    In The Sub station end of the transmission line, The Power Transformer Connection is Star-Delta.( For the purpose of Step down the Voltage Level)

·    In the star up of the  Transmission line (H-T), The Connection of the power Transformer is Delta – Star (For the purpose of Step Up the Voltage Level) But in case of Distribution Transformer, But Generally it is used in there-phase Step down distribution transformer( Delta – Star).

9)       What is the purpose of ground wires in over-Head Transmission lines?

Ground wires are bare conductors supported at the top of transmission towers. They serve to shield the line and intercept lighting stroke before it hits the current carrying conductors below. Ground wires normally do not carry current. Therefore, they are often made of steel. The ground wires are solidly connected to ground at each tower in transmission and distribution system.

10)    What is the difference between real ground and virtual ground?

Real ground is when a terminal is connected physically to the ground or earthed. Whereas virtual ground is a concept used in Op-Amps in which a node an assumed to have the potential that of the ground terminal.