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

Instrument transformers are used in conjunction with ammeter and voltmeter to extend the range of meters. In dc circuit shunt and multipliers are used to extend the range of measuring instruments. The shunt is used to extend the range of ammeter whereas multiplier is used to extend the range of voltmeters

This type of ammeter is shown in Figure. The primary of the transformer is connected in series with the load, and the ammeter is connected to the secondary of the transformer. Notice that the range of the meter is changed by selecting different taps on the secondary of the current transformer. The different taps on the transformer provide different turns-ratios between the primary and secondary of the transformer. The working is explained in detail.

• The current transformer is used to step down the current to a lower value so that current can be measured with a normal range ammeter.
• The current transformer has a primary coil of one or more turns of thick wire having high cross-sectional area and it is connected in series.
• The primary of the current transformer is connected to the load or feeder while the secondary of the current transformer is connected to an ammeter.
• The impedance of the primary is very low, and the currents very high. The primary current is dependent on the load on the line rather than the load on the secondary circuit.
• Current drawn by the secondary has little effect on line current.
• The secondary of the transformer contains many turns of fine wire having a smaller cross-section area and has a much higher impedance. If the secondary is not loaded, this transformer acts to step-up the voltage to a dangerous level, due to the high turns ratio. Because of this, a current transformer should always have a short-circuit placed across its secondary winding when connecting or removing any device from its output. By heavily loading the secondary, the high voltage is reduced to the safe level.
• The nominal current rating of the secondary winding of the CT is 5A to 1 A.
• To illustrate the operation of a current transformer, assume that the current ratio of the primary winding is 100 A. The secondary winding has the standard rating of 5A.
• The primary winding consists of three turns of wire, and the secondary winding consists of 60 turns.
• The ratio between the primary and the secondary currents is 100 A/5 A, or 20:1
• In other words, the primary current is 20 times greater than the secondary current.
• Note that the number of turns and the current in the primary and secondary windings are related by an inverse proportion. i.e I₁ / I₂ = N₂ / N_1.
• By increasing the number of secondary windings, N₂, the secondary current can be made much smaller than the current in the primary circuit being measured. In other words, as N2 increases, I₂ goes down by a proportional amount.

Induction-type Single-phase Energy Meter

An induction-type instrument can be used as an ammeter, voltmeter, or wattmeter, the induction-type energy meters are more popular. Induction-type single-phase energy meter is used invariably to measure the energy consumed in an AC circuit in a prescribed period where supply voltage and frequency are constant. The energy meter is an integrating instrument that measures the total quantity of electrical energy supplied to the circuit in a given period.

Principle

The basic principle of induction-type energy meter is electromagnetic induction. When AC flows through two suitably located coils (current coil and potential coil), they produce the rotating magnetic field that is cut by the metallic disc suspended between the coils, and thus, an emf is induced in the disc that circulates eddy currents in it. By the interaction of rotating magnetic field and eddy currents, electromagnetic torque is developed that causes the disc to rotate. This is the same principle that is applied in single-phase induction motors.

A single phase energy meter has four essential parts:

1. Operating system
2. Moving system
3. Braking system
4. Registering system

Operating System

The operating system consists of two electromagnets. The cores of these electromagnets are made of silicon steel laminations. The coils of one of these electromagnets (series magnet) are connected in series with the load and are called the current coil. The other electromagnet (shunt magnet) is wound with a coil that is connected across the supply, called the pressure coil. The pressure coil, thus, carries a current that is proportional to supply voltage.

Shading bands made of copper are provided on the central limb of the shunt magnet. Shading bands, as will be described later, are used to bring the flux produced by a shunt magnet exactly in quadrature with the applied voltage.

Moving System

The moving system consists of a light aluminum disc mounted on a spindle. The disc is placed in the space between the series and shunt magnets. The disc is so positioned that it intersects the flux produced by both the magnets. The deflecting torque on the disc is produced by an interaction between these fluxes and the eddy current they induce in the disc. In energy meters, there is no control spring as such, so that there is the continuous rotation of the disc.

Braking System

The braking system consists of a braking device which is usually a permanent magnet positioned near the edge of the aluminum disc. The arrangement is shown in Figure.

The emf induced in the aluminum disc due to relative motion between the rotating disc and the fixed permanent magnet (brake magnet) induces an eddy current in the disc. 

When disc rotates in the air gap, eddy currents are induced in the disc which opposes the cause producing them i.e. relative motion of disc with respect to the magnet. This eddy current, while interacting with the brake magnet flux, produces a retarding or braking torque. This braking torque is proportional to the speed of the rotating disc. When the braking torque becomes equal to the operating torque, the disc rotates at a steady speed.

The braking torque is proportional to the flux of the braking magnet and the eddy current induced in the moving system due to its rotation in the field of the braking magnet.

T_B ∝ φi

where φ is the flux of the braking magnet and ‘i’ the induced current. Now i = e/R where e is the induced e.m.f. and R the resistance of the eddy current path. Also, e ∝ φn where n is the speed of the moving part of the instrument.

∴ T_B ∝ φ × φn/R

T_B ∝ φ²n/R

The position of the permanent magnet with respect to the rotating disc is adjustable. Therefore, braking torque can be adjusted by shifting the permanent magnet to different radial positions with respect to the disc. By changing the distance between braking magnet and magnetic shunt, the braking torque can be adjusted. The braking torque increases when the distance between them is increased because of moving the magnetic shunt away from the magnet, it will bypass a less amount of flux and vice versa. 

Hence the Braking torque provided by a permanent magnet is proportional to

• Square of the flux of the permanent magnet
• The speed of the meter
• The distance of permanent magnet from the center the revolving disc

Registering System

The function of a registering or counting system is to continuously record a numerical value that is proportional to the number of revolutions made by the rotating system.

Material handling is an integral part of any manufacturing activity. A material handling system is used for handling, controlling and storage of materials. Here, the material has a very broad meaning, covering all kinds of raw materials, work-in-process, subassemblies, and finished assemblies. The primary objective of using a material handling system is to ensure that the material in the right amount is safely delivered to the desired destination at the right time and at minimum cost.

Layout Type

The activities involved in production or service operations may be grouped and arranged in a variety of ways. Several popular types of layout are product layout, process layout, and fixed-position layout or a combination of them. In the product layout, the product and its flow dictate the design. In the process layout, the basic organization of technology is around processes In the fixed-position layout, the construction of a large product is accomplished in one place.

PRODUCT LAYOUT or LINE LAYOUT

Product layout, also called mass production layout, line layout, and straight-line layout is employed when the product is produced through a continuous process. Here the raw material enters the production line, and operations are performed on it in sequence until it goes out as a finished product. The product moves continuously, perhaps down a conveyor line past successive workstations where men and machines perform the necessary operations in sequence. Automobile and home appliance lines are examples of this type of layout. 

When the conditions for product layout are met, we have a low-cost, highly specialized, very productive system. The requirements follow:

• An adequate volume that makes possible reasonable equipment use.
• Stable product demand.
• Product Standardization.
• Part interchangeability.
• Continuous material supply.

The main advantage of product layout is its low production cost per unit Production control is relatively simple because product designs are stable and routes through the system are standardized.

The disadvantages of product layout are that the investment in single-purpose equipment is high, and the system is, therefore, open to obsolescence if product designs change. Also, if a machine in the sequence breaks down or men are absent, not only is the process immediately affected delayed but perhaps the entire line is delayed. Thus, seemingly minor stoppages or material shortages could be very costly. Also, the highly repetitive nature of the work has resulted in low job satisfaction and morale in man’s instances.

Product layout is most commonly used with assembly operation Fabrication lines are less common because of the difficulty in attaining ha ance between operations. A common combination is for fabrication operations to be organized on a process basis and assembly on a product fixed position basis.

PROCESS LAYOUT

In process layout, often called Functional and job lot layout, all machines or processes of the same generic type are grouped together in what is commonly called machine centers or departments. The departments are technologically specialized in such processes as turning, grinding, heat treating, painting, assembly, castings, accounts receivable, and typing. This layout is suitable for batch or intermittent production. Workshops producing small quantities of products or factories producing batches of made-to-order products are examples of this type of plant layout.

GROUP LAYOUT

Group layout is a hybrid approach combining the features of product layout m a process layout context. It is used in the more structured job shop

Current Dependent Voltage Source: It produces a voltage as a function of current elsewhere in the given circuit. This is called CDVS and it is given as

v_a= βi_a = 0.8 × 9.5

v_a= 7.6 mV

The small pressure difference which causes a flow of gas to take place is termed as a draught. The function of the draught, in the case of a boiler, is to force air to the fire and to carry away the gaseous products of combustion. In a boiler furnace, proper combustion takes place only when a sufficient quantity of air is supplied to the burning fuel.

Natural Draught (chimney)

Natural draught is obtained by the use of a chimney. The chimney in a boiler installation performs one or more of the following functions:

(i) It produces the draught whereby the air and gas are forced through the fuel bed, furnace, boiler passes, and settings;

(ii) It carries the products of combustion to such a height before discharging them that they will not be objectionable or injurious to surroundings.

A chimney is a vertical tubular structure built either of masonry, concrete or steel. The draught produced by the chimney is due to the density difference between the column of hot gases inside the chimney and the cold air outside.

The draught produced by the chimney is given as

Where

ma = Mass of air supplied 

ma + 1 = Mass of flue gases,

pa = atmospheric pressure

T_a = Temperature of atmospheric air,

T_g = Average temperature of flue gases

H = Height of chimney, 

h = Draught required in mm of water

The draught of the chimney is related to the temperature difference between the flue gas and outside air as well as the height of the chimney. The higher temperature difference creates a higher draught of air-flow inside the furnace also draught is directly proportional to chimney height. In order to ensure proper draught of the chimney, its diameter and height must be calculated according to volume, average temperature, and velocity of the flue gas. 

An electrical measuring instrument depends on its operation on one of the many effects produced by current or voltage. When current flows through a wire, it produces a magnetic field as well a heating effect.

When it flows through a battery, it produces a chemical effect in the electrolyte solution. In a similar way, it produces an electrostatic field and can also induce emf in a nearby circuit due to electromagnetic induction. In a secondary instrument, one of the above effects is used to produce the deflecting torque. For example,

1. The magnetic effect is used in ammeters, voltmeters, wattmeters, and energy meters.
2. The heating effect is used in ammeters and voltmeters.
3. The chemical effect is used in dc ampere-hour meters.
4. An Electrostatic effect is used in voltmeters. electromagnetic induction effect is used in ac ammeters, voltmeters, wattmeters are energy meters.

The capacity of a storage battery is the product of the current drawn from a battery, multiplied by the number of hours this current flows.

The factors upon which the capacity of storage batteries depend may be grouped in two main classifications:

1. Design and Construction of Battery.
2. Conditions of Operation.

Design and Construction can be divided into

1. Area of the plate surface.
2. Quantity, arrangement, and porosity of active materials.
3. Quantity and strength of electrolyte.
4. Circulation of the electrolyte.

Area of Plate Surface. The chemical and electrical activity of a battery is much higher at the surface of the plates since the acid and active material are in intimate contact here.

Quantity and Strength of Electrolyte. The voltage of the battery is also dependent on the concentration of the electrolyte. The strength of the electrolyte is set high enough to provide enough sulfuric acid to satisfy the voltage and capacity limit of the battery and low enough to provide good ionic conductivity.

Temperature. Chemical reactions take place much more readily at high temperatures than at low. 

Operating a storage battery in cold weather is equivalent to using a battery of lower capacity. For example, a fully charged battery at 26°C may be capable of starting an engine twenty times. At [17.8° C|. the same battery may start the engine only three times.

Low temperatures greatly increase the time necessary for the charging & battery. A battery that could be recharged in 1 h at 26°C may require approximately 5 h of charging when the temperature is 17.8° C. These effects on a battery’s capacity are caused by the slow chemical reactions created by the cold temperatures.

Chemical constituent:- Battery such as Zinc-carbon batteries, Alkaline batteries, Lithium-ion batteries (rechargeable) are used for different purposes, their life and battery capacity are also different from each other.

Capacitor-Start Motors

The capacitor start motor is identical to the split-phase motor in both construction and operation, except that a capacitor is installed in series with the starting winding, as shown in Fig.

It has a single cage rotor, and its stator has two windings known as main winding and starting winding. Both the windings are displaced 90 degrees in space.

Also, the starting winding of the capacitor start motor is usually wound with a larger wire than that used for the starting winding of the split-phase motor. The use of a capacitor in series with the starting winding causes the current in this winding to lead the voltage, whereas the current in the running winding lags the voltage by virtue of the high inductance of that winding.

With this arrangement. the phase displacement between the two windings can be made to approach 90 electrical degrees so that the two-phase starting is achieved. For this reason, the starting torque of the capacitor start motor is very high, which makes it an ideal drive for the small compressor that must be started under full load.

So if the low value of the capacitor is used in the motor the phase angle between the starting and running winding will be reduced and the motor can’t generate enough torque to rotate the motor.

The Nyquist rate is the minimum sampling rate required to avoid aliasing. 

Aliasing can be defined as the phenomenon of a higher frequency in the spectrum of an original signal [say of x(t)] seemingly taking on the identity of a looter frequency in the spectrum of the sampled signal.

The minimum sampling rate is given as

fs = 2fm 

Where m is the maximum frequency where the signal can have non-zero energy.

The Cosine-phase equation for a single frequency signal with frequency f and constant amplitude y_max is:

y(t) = (y_max) × cos(2πft)

Now comparing the given equation with the standard equation

y(t) = 5cos400πt = 5cos[2π(200)t]

The frequency of an analog signal is f_m = 200 Hz.

Therefore, the minimum sampling rate required to avoid the aliasing is fs = 2fn = 2 x 200 = 400 Hz. 

The highest frequency is 150Hz. Therefore f_max = 150Hz
Nyquist rate = 2 f_max
= 2 * 150
= 300Hz.