SSC JE 2010 Electrical question paper with Solution

The basic sources of error in instruments are friction, temperature and ageing of various parts. The swamping resistance is a resistor with negative temperature coefficient connected in series with the instrument(moving coil) in order to reduce the error due to the variation of resistance of the moving coil with the variation of temperature.

The resistance of swamping resistor may be taken nearly three times that of the coil so that the possible error of the instrument will be reduced by about 75%.

Conducting materials having the negligible temperature coefficient of resistance like Manganin can be used as shunts for these instruments.

Induction heating is the process of heating an electrically conducting object (usually a metal) by electromagnetic induction, through heat generated in the object by eddy currents.

Since Eddy current heating require high frequency therefore high-frequency current ( 10 KHz to 400 KHz), is utilized for conversion of electromechanical energy into heat.

If the capacitors are connected in parallel then their effective capacitance is

C = C1 + C2 + C3 —— Cn

Since 4 capacitors of 10μF are connected in parallel therefore

C = (10 + 10 + 10 + 10)μF = 40μF

DC current supplied to the rotor field winding is known as excitation in a synchronous motor. As the speed of the synchronous motor is constant, the magnitude of back emf remains constant provided the flux per pole is constant. The magnitude of back emf can be changed by field excitation. If the excitation is changed at a constant load, the magnitude of armature current and power factor will change.

If the excitation is more than 100%, then the motor is said to be overexcited and if the excitation is less than 100%, then the motor is said to be under-excited. The magnitude of armature current varies with excitation. The current has large values at both low and high values of excitation. In between, it has a minimum value corresponding to a certain excitation for which power factor is unity.

In power engineering & electrical engineering, the V curve is the graph showing the relation of armature current as a function of field excitation in synchronous machines. The purpose of the curve is to show the variation in the magnitude of the armature current as the excitation voltage of the machine is varied.

The inverted V-curves of a synchronous motor show how power factor varies with excitation (or if) when the motor input is constant. The curve for p.f. looks like inverted V cure

Why the electric potential of Earth is zero?

For the measurement of level, we have a standard or a zero level, that is sea level and for the measurement of temperature, a zero of temperature, that is the temperature of melting ice, so also for the measurement of potential, we must have a standard or a zero of potential. The potential of the earth is taken to be zero of potential. For this purpose, the earth has been chosen because of its huge size, so that the small charges with which we have to deal with, if given to it or taken from it, produce no change in its potential.

Thus, a body whose potential is higher than that of the earth is said to have positive potential and one whose potential is lower than that of the earth is said to have a negative potential.

Diversity factor = Sum of individual maximum demand/ Station maximum demand.

Diversity helps to improve the load factor and economic operation of the power plant.

Load factor = Average load/ Maximum Demand

Both load factor and diversity factors are inversely proportional to maximum demand. Higher load factor and diversity factor means steady consumption or supply which means less maximum demand thus the initial and running cost will be low. Therefore the cost per unit power generation will be less.

The diversity factor can be equal to or greater than 1. If the value of the diversity factor is greater than 1, then it is a good diversity factor, and 1.0 represents a poor diversity factor.

Resistance Welding usually requires high current (over 80 amperes) and it can need above 12,000 amperes in spot welding. The current is either DC current or low-Frequency Alternating current.

Resistance welding has two undesirable effects on a plant’s power distribution system; high momentary peak loads and poor power factor. The most frequently considered method for eliminating these problems is the use of direct current rather than alternating current to form the weld. This reduces both the power demand imposed on the distribution system and the total energy
required to make a resistance weld.

These improvements are due to the higher power factor inherent direct power supplies, and to the fact that the power demand can be distributed over all three phases of the power supply system.

The tariff is the rate at which electrical energy is supplied to the consumers.

In a two-part tariff, the rate of electricity is based on the maximum demand of the consumer and the units consumed. The total charge is split into two components the fixed charges and running charges. The fixed charges depend on the maximum demand of the consumer while the running charges depend upon the number of units consumed by the consumer.

Total charge = Rs(y x Kw + z x kwh)

y = Charge per kW of maximum demand

z = Charge per kwh of maximum demand

The industrial consumers with appreciable maximum demand are charged with this type of tariff.

Advantages of this tariff are

1. It is simple and can be easily understood by consumers.
2. The fixed charges which depend on the maximum demand but independent of units consumed are recovered.

1. With the increase in transmission voltage, for the same amount of power to be transmitted current in the line decreases which reduces I²R losses (or copper losses). This will lead to an increase in transmission efficiency.
2. Consider electrical power transmitted at voltage V and current I. The power transmitted P is given byP = IVFor a fixed amount of electrical power transmitted, a higher voltage would give a smaller current. To see this relationship more clearly, one can write

I = P/V

Thus for a fixed power transmitted P, the current I is inversely proportional to the voltage V. Hence Small current can be achieved from large voltage.

1. Better Efficiency: With lesser Line losses due to reduced I²R loss, the efficiency of transmission increases.
2. Improved Voltage Regulation: With the decreased Line current the voltage drop across the line will decrease and this will improve the voltage regulation as the receiving end voltage will be more. Voltage regulation is defined as (Vs-Vr/Vs)
3. The percentage of line drop is very small for high voltage transmission.

Characterstics of FET

• A FET is essentially a unipolar device as the current is conducted by only one type of carriers (electrons for n-channel and holes for p-channel FET).
• A FET is also known as a three-terminal amplifying device. Its terminals are known as the source, gate, and drain.
• A FET is characterized by high input impedance, usually of the order of several  hundred megaohms,
• FETs are more temperature stable simple to fabricate and occupies less area on the silicon chip.
• FET is a voltage-controlled device whereas BJT is a current controlled device
• FET is a voltage-controlled device. The input gate to source voltage controls the output current.