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

CRGO is cold rolled grain-oriented steel and as the name suggests, its grains(crystals) are aligned in the direction of rolling. when the magnetic flux passes through it, the magnetic domains (the region in grains with aligned magnetic moment) get aligned in one direction causing the minimum resistant path to flux causing lesser hysteresis loss.

Other advantages of high silicon steel are

1. Low copper loss due to lower grain resistance.
2. High magnetic permeability.
3. The lower thickness of lamination.

Role of Capacitors in Transistor Amplifiers

Regardless of the manner in which a capacitor is connected in a transistor amplifier, its behavior towards d.c. and a.c. is as follows.

A capacitor blocks d.c. i.e. a capacitor behaves as an “open*” to d.c. therefore, for d.c. analysis, we can remove the capacitors from the transistor amplifier circuit. A capacitor offers reactance (= 1/2πfC) to a.c. depending upon the values of f and C. In practical transistor amplifier circuits, the size of capacitors is so selected that they offer negligible (ideally zero) reactance to the range of frequencies handled by the circuits. Therefore, for a.c. analysis, we can replace the capacitors with a short i.e. by a wire. The capacitors serve the following two roles in transistor amplifiers:

(i) As coupling capacitors (ii) As bypass capacitors

(A As coupling capacitors. In most applications, you will not see a single transistor amplifier. Rather we use a multistage amplifier i.e. a number of transistor amplifiers are connected in series or cascaded. The capacitors arc is commonly used to connect one amplifier stage to another When a capacitor is used for this purpose, it is called a coupling capacitor: A coupling capacitor performs the following two functions:

(a) It blocks d.c. i.e. it provides d.c. isolation between the two stages of a multistage amplifier.

(b) It passes the a.c. signal from one stage to the next with little or no distortion.

(ii) As bypass capacitors. Like a coupling capacitor, a bypass capacitor also blocks dc and behaves as a short or wire (due to the proper selection of capacitor size) to an a.c. signal. But it is used for a different purpose. A bypass capacitor is connected in parallel with a circuit component (e.g. resistor) to bypass the a.c. signal and hence the name.

Ohm’s law of constant proportionality state that for a given conductor the ratio of voltage to current is constant i.e if the temperature, area, the length did not change the resulting current is directly proportional to the variation in the applied voltage. Hence the graph between voltage and current is linear.
R(constant) = V/I

A heterodyne frequency meter is an oscillator in which the tuning control is calibrated in terms of frequency. The oscillator used is stable and tuneable and depends upon the frequency range to be covered. Radio frequency can be measured by Heterodyne frequency Meter.

Good mechanical construction is also desirable. The oscillator of the heterodyne frequency meter must be isolated by a suitable buffer circuit arrangement so that its frequency is not affected by either the energy or the impedance of the circuits into which the frequency meter is connected when making measurements.

When a heterodyne frequency meter is to be used for interpolation purposes, it is desirable that the frequency be linearly proportional to the angle of rotation of the tuning dial to a high degree of precision.

The accuracy of the frequency indicated by a heterodyne frequency meter is ordinarily greater than the accuracy of the corresponding lumped-circuit wavemeter. This is because an oscillator generates a definite frequency for any particular setting of its resonant tank circuit.

Applications:-

Heterodyne frequency meters are employed for such purposes

• Producing known frequencies
• Measuring an unknown frequency by comparison with the controllable an unknown frequency of the heterodyne frequency meter
• For interpolation between known fixed frequencies.
• Superheterodyne circuit found in almost all radio and television receivers, it is used in radio transmitters, modems, satellite communications and set-top boxes, radar, radio telescopes, telemetry systems, cell phones, cable television converter boxes and headends, microwave relays, metal detectors, atomic clocks, and military electronic countermeasures (jamming) systems.

The conductivity of a metal is determined by the electronic concentration and the mobility of the free electrons.

Mobility and Conductivity

Considering a metal, there are a large number of electrons that are free and are not bound to an atom or molecule. Since there is no energy gap in metals, each atom contributes an electron or more for conduction. When an electron is removed from an atom, the remaining portion of the atom is a positively charged ion. So a large number of electrons roam around in the material where the positive heavy masses of ions are stationary. This is known as the electron gas theory of a metal. 

These free electrons in the metal are not stationary but are moving randomly in the material. The electrons moving in a given direction collide with the stationary ions and change their direction and proceed further. 

Let us now see how the situation is changed if a constant electric field e (volts per meter) is applied to the metal. As a result of this electrostatic force, the electrons would be accelerated and the velocity would increase indefinitely with time.

Due to the presence of these static ions, collisions take place and the electron loses energy. When a steady state is achieved, the velocity of the electrons becomes finite. This velocity, referred to as drift velocity, is in opposite direction to that of the applied field and its magnitude is proportional to the ε. Thus

V =µε

Where

μ = Mobility of electrons

Also, Conductivity is proportional to the product of mobility and carrier concentration. Hence the conductivity of the metal is determined by

σ =nµe

Where

n= number of valence electrons per unit volume ( electron concentration)

μ = Mobility of electrons

The frequency of oscillation is given as

F_n = 1/2π√LC

Given Reactance = 3 ohms

Capacitance = 0.01 µF

Frequency = 50 Hz

Inductive reactance X_L = 2.π.f.L

3 = 2 × 50 × π × L

L = 0.009 Henry

∴ F_n = 1/2π√0.009 × 0.01 × 10−6

F_n = 16,328 Hz

Out of the three transistor connections, the common emitter circuit is the most efficient. It is used in about 90 to 95 percent of all-transistor applications. The main reasons for this are :

• The transistor circuit will have a moderately high input impedance
• Low output impedance
• Moderately high current gain
• Moderately high voltage gain and very good and wide range of frequency response and hence find dominant applications in voltage, current and power amplifiers.

High current gain

In common emitter connection, I_c is the output current and I_B is the input current. The collector current here is expressed as:

Ic = βI_B + I_CEO
Where β = Current Gain
I_CEO = Collector-Emitter current

As the value of β is very large, therefore, the output current I_C is much more than the input current I_B. This increases the current gain effectively. The current gain of CE arrangement ranges from 20 to
500.

High Voltage and Power Gain

As we have seen above, CE arrangement has a high current gain. This is turn, increases the voltage and power gain of CE circuit. In comparison to CB and CC circuits, the common emitter connection has the highest voltage and power gain. For this reason, the CE transistor connection is often used for amplifying purposes.

Welding electrode. A component of the welding circuit through which current is conducted and that terminates at the arc, molten conductive slag, or base metal. 

Electrode (arc welding)—a “stick” or wire” from which an arc is struck. It may include the bare metal electrode, carbon electrode, composite electrode, covered electrode, lightly coated electrode, and the heavy coated electrode.

Electrode (resistance welding)—that part or parts of a machine through which the welding current and in most cases, pressure, are applied directly to the work. The electrode may be in the form of a rotating wheel, bar, cylinder, plate, clamp.

Spot welding is one of the oldest welding processes. Spot welding is one form of resistance welding, which is a method of welding two or more metal sheets together without using any filler material by applying pressure and heat to the area to be welded. When similar materials are spot welded, then the highest resistance is between the interfaces of the two sheets, provided the correct welding conditions are used. When welding dissimilar metals, the maximum resistance may not exist at the interface; instead, it may be located within one of the metals if it has a higher specific resistance than the other metal.

In spot welding, it is possible that the electrodes themselves may get welded to the materials, or it may result in making a hole in the material.

In order to prevent the electrodes from getting welded to the sheet, the electrodes are made of metals having high electrical and thermal conductivity and are generally water-cooled. It results in much less production of heat in the electrodes and at the same time, it helps to remove heat from the outer surface of the metal sheets. Hence, the maximum temperature rise takes place only at the weld. 

Apart from high thermal and electrical conductivity, the electrode material should have the very high strength to keep the wear and tear at the tip of the electrode to a minimum.

A hard-drawn copper alloy is generally used as electrode material so that it can withstand the high contact pressure, keep the temperature low, and prevent deformation of electrodes. Electrodes, however, can be made of the following materials to suit different duties.

A forklift is a powered industrial truck used to lifting, lowering and moving materials over short distances.

Load capacity of conveyor belt in Tonnes per hour is given as:-

CAPACITY (TPH) = .03 x Belt Speed (FPM) x material weight (lb. per cu. ft.) x load cross section (sq. ft.)

And its capacity is around 20-100 tons/hour