UPPCL JE Electrical question paper with Explanation Set-2 -2018

Sulfation:- “A lead-acid battery can be charged and recharged only a finite number of times because the use of the battery causes crystalline lead sulfate to be deposited and accumulate on the surface of the positive and negative electrodes of the battery. 

It occurs when batteries are subjected to prolonged undercharge conditions. During normal use, soft sulfate crystals form and dissipate as part of the normal charge and discharge cycle. During periods of prolonged undercharge, the sulfate converts to hard crystals and deposit on the negative plates i.e Lead. With time the crystals grow in size and become hard, covering the lead plates completely. This coverage deteriorates the overall efficiency and power storage capability of the battery. Sulfation also increases the internal resistance of a battery. This means a higher charging voltage is needed to regenerate active plate material when charging.

Trickel Charging:- Trickle charging means charging a fully charged battery by passing a small current at a rate equal to its self-discharge rate, thus enabling the battery to remain at its fully charged level.  Due to leakage action and other open-circuit losses, the battery deteriorates even when idle or on open-circuit. Hence, to keep it fresh, the battery is kept on a trickle charge. The rate of trickle charge is small and is just sufficient to balance the open-circuit losses. Trickel charging is the best way to prevent sulfation because it keeps a lead-acid battery in a fully charged state hence lead sulfate does not form.

If area of hysteresis loop of a material is large, the hysteresis loss in the material will be Large. Hysteresis loss is directly proportional to the area under the curve on B-H axis.

Hysteresis Loss

When a magnetic material is subjected to one cycle of magnetization, B always lags behind H so that the resultant B-H curve forms a closed loop, called hysteresis loop. For the second cycle of magnetization, a similar loop is formed if a magnetic material is located within a coil through which alternating current (50 Hz frequency) flows, 50 loops will be formed every second. This hysteresis effect is present in all those electrical machines where the iron parts are subjected to cycles of magnetization e.g. armature of a d.c. machine rotating in a stationary magnetic field, transformer core subjected to alternating flux etc.

When a magnetic material is subjected to a cycle of magnetization (i.e. it is magnetized first in one direction and then in the other), an energy loss takes place due to the domain friction in the material. That is, the domains of the material resist being turned first in one direction and then in the other. Energy is thus expended in the material in overcoming this opposition. This loss is in the form of heat and is called hysteresis loss. It is so called because it results due to the hysteresis effect in a magnetic material.

Hysteresis loss is present in all those electrical machines whose iron parts are subjected to cycles of magnetization. The obvious effect of hysteresis loss is the rise of temperature of the machine. It can be shown that hysteresis energy loss per cycle is directly proportional to the area of the hysteresis loop.

 Steinmetz Hysteresis Law

To eliminate the need for finding the area of the hysteresis loop for computing the hysteresis loss, Steinmetz devised an empirical law for finding the hysteresis loss. He found that the area of the hysteresis loop of a magnetic material is directly proportional to 1.6 the power of the maximum flux density established.

Hysteresis Loss = K_h × B_M^1.67 × f × v watts

where Kh is the Hysteresis coefficient depends upon the material. The smaller the value of Kh of a magnetic material, the lesser is the hysteresis loss. The armatures of electrical machines and transformer cores are made of magnetic materials having low hysteresis coefficient in order to reduce the hysteresis loss. The best transformer steels have Kh values around 130, for cast steel they are around 2500 and for cast iron about 3750.

Bm = Maximum flux density
f = frequency
v = Volume of the core

Factor affecting Hysteresis Loss

• The hysteresis loss is directly proportional to the area under the hysteresis curve i.e. area of the hysteresis loop.
• It is directly proportional to frequency i.e. number of cycles of magnetization per second.
• It is directly proportional to the volume of the material. 

Importance of Hysteresis Loop

• The shape and size of the hysteresis loop *largely depends upon the nature of the material. The choice of a magnetic material for a particular application often depends upon the shape al size of the hysteresis loop. A few cases are discussed below by way of illustration:
• The smaller the hysteresis loop area of a magnetic material, the less is the hysteresis loss. The hysteresis loop for silicon steel has a very small area.  For this reason, silicon steel is widely used for making transformer cores and rotating machine which is subjected to rapid reversals of magnetization.
• The hysteresis loop for hard steel indicates that this material has high retentivity and coercivity. Therefore, hard steel is quite suitable for making permanent magnets. But due to the large area of the loop, there is greater hysteresis loss. For this reason, hard steel is not suitable for the construction of electrical machines.

• The hysteresis loop for wrought iron shows that this material has fairly good residual magnetism and coercivity. Hence it is suitable for making cores of electromagnets.

Nucleus: The dense structure at the center of an atom. Protons and neutrons are found inside the nucleus of an atom.

The number of protons in the nucleus of an atom determines the type of element the atom is. The number of neutrons is usually similar to the number of protons but is not always the same. Protons and electrons have an electric charge. The protons are positively charged and the electrons are negatively charged. Neutrons are electrically neutral; they have no charge.

Semiconductors can be classified into two types—intrinsic and extrinsic. Intrinsic semiconductors are pure elements like Si and Ge. At 0 K, the valence band is completely filled and the conduction band is empty.

At room temperature, some covalent bonds of an intrinsic semiconductor are broken and liberate some electrons from the orbit and these electrons move freely within the material like gas molecules and are called free electrons. When electrons are liberated from the covalent bonds, then an empty space Is left behind. This empty space (absence of electrons in the covalent bond) is called the hole. Hence an electron-hole pair is generated. These electrons and holes are responsible for conduction of current. In intrinsic semiconductors, the number of electron and holes are equal and very small so its conductivity is very low to be used in different electronic applications.

Under the influence of the external electric field, an equilibrium condition is disturbed and electrons move in a direction opposite to that of holes. Holes can be thought of as positively charged particles. The movement of electrons in the conduction band and that of holes in the valance band constitutes current in a semiconductor. Electrons and holes together are known as Charge carders. Thus in a semiconductor, current is constituted by two types of carriers: (a) electrons and (b) holes. In metals, conduction is due to electrons only.

It is also possible that some of the free electrons in conduction band may combine with holes in the valence band. This process is known as Recombination. In the process of recombination, an electron loses energy.

Intrinsic semiconductor conductivity can be increased either by heating or by adding impurities to them. With the rise in temperature, more and more electron hole pairs are fanned and more charge carriers are available for conduction. Thus the conductivity of intrinsic semiconductors increases with the increase in temperature (and hence the resistivity decreases with increase in temperature). Increasing conductivity by heating is not a suitable method, therefore we prefer increasing their conductivity by adding impurities to them. These impurities have special characteristics properties and can be taken of trivalent (In, B. Al) or pentavalent (As, Sb, P) elements.

The magnetic susceptibility is given as

χ_m = C/(T − θ)

Where

C = curie constant = 0.2

T = Critical Temperature

θ = Curie Temperature

The difference in critical temperature and paramagnetic Curie temperature is (T − θ) = 0.01

χ_m = 0.2/0.01

χ_m = 20

Capacitance is the ratio of the change in an electric charge in a system to the corresponding change in its electric potential.

Mathematically, the self-capacitance of a conductor is defined by

where

q is the charge held by the conductor,

V is the Voltage

From the above equation, it is clear that the capacitance is directly proportional to the charge stored on the plate. The larger the charged stored larger will be the capacitance.

The gain of the negative feedback amplifier is given as

A = V_out/V_in

Where

A is the voltage gain of the amplifier without feedback = 100
V_out is the output voltage =?
V_in is the input voltage =30 mV

100 = V_out/30mV

V_out = 3000mV = 3V

Feedback factor β is the ratio of feedback voltage to the output voltage

β = V_F/V_out

0.03 = V_F/3

V_F = 0.09V

The amount of energy that a battery can store is caged its capacitor. A water tank, for example, with a capacity of 8000 liters can hold at most 8000 liters. Similarly, a battery can only store a fixed amount of electrical energy, typically marked on the side of the battery by the manufacturer.

Actually battery capacity is measured in the amount of charge it can hold. Now we know the current is charge per unit time, therefore the product of current and time gives us the charge. That is why battery capacities are measured in milliampere-hour in case of cellphone batteries or ampere-hour in case of automotive batteries.

Battery capacity = charge stored = current x time or amp-hr.

The capacity of a battery is measured in amp hours (Ah). This indicates the amount of energy that can be drawn from the battery before it is completely discharged A battery of 100 Ah should ideally give a current of 2 amps for 50 hours (i.e. 2 amps times 50 hours equals 100 amperes).