NOTE:

572 Measurements and

Instruments October 2019

Maximum Marks : 75

(1) Q.No.8 in PART – A and PART – B are compulsory. Answer any FOUR questions from the remaining in each PART – A and PART – B.

(2) Answer division (A) or division (B) of each question in PART – C. (3) Each question carries 2 marks in PART – A, 3 marks in PART – B and 10 marks in PART

– C. PART - A

1. Compare analog and digital instruments.

S.No Digital Instruments Analog Instruments

1. Measures in terms of a decimal

number.

Measures in terms of direct deflection of a

pointer on a scale.

2. High accuracy Low accuracy.

3. Readings can be stored Readings cannot be stored

2. Define instrument efficiency. Instrument efficiency is defined as the ratio of measured quantity (at full scale) to the

power taken by the instrument at full-scale deflection.

3. Write the types of ammeters and voltmeters. Ammeters and Voltmeters are of Moving coil, Moving iron, Dynamometer, Induction, Electrostatic or rectifier type

4. What is phase angle error in current transformers? Phase – angle errors in CT:

In power measurements it is necessary that the phase of secondary winding current shall be displaced by exactly 1800 from the primary winding current. It is seen that the phase difference from 180˚ by an angle θ.

Phase angle error θ = 180

𝜋 [𝐈𝑚 𝐜𝐨𝐬 𝜹−𝐈𝑐 𝐬𝐢𝐧 𝜹

]

𝐧𝐈𝑠

Where, Im = Magnetising component of exciting current. Ic = Core loss component of exciting current. Is = Secondary winding current 𝛿 = Angle between secondary winding induced voltage and current

5. What is the purpose of aquadag coating in CRT?

The inside part of the tube is coated with a graphite layer (aquadag) to prevent the formation of negative charges on the screen. Horizontal and vertical marks on the screen provide user the correct measurement

6. Define phase displacement error in energy meters.

Phase displacement error:

The resistance of coil and core loses cannot be brought to zero. Hence the flux lags the

voltage slightly less than 90⁰. This defect is overcome by providing copper shading bands around

the central limb of the shunt pressure magnet and properly adjusting it.

7. What is maximum demand indicator?

The meters used to record the maximum power consumed by the consumer during the particular period is called MAXIMUM DEMAND INDICATORS

8. State the reasons for not using an ordinary wattmeter for power measurement in low

power factor circuits. Measurement of power in circuits having low power factor by ordinary electrodynamometer wattmeter is difficult and inaccurate, because of the following reasons.

1. The deflecting torque on the moving system is small, even when the current and pressure coils are fully excited. 2. Errors are introduced because inductance of pressure coil will be high.

PART – B

9. Explain integrating and recording instruments. Recording Instruments: It gives a continuous record of the quantity to be measured during a particular period of time. The recording is made by a pen on a graph paper which is rotated on a disc at uniform speed. X- axis denotes time and Y- axis denotes electrical quantity.

Example:

X-Y recorder

Strip chart recorder Magnetic tape recorder.

Integrating Instruments: It gives the consumption of total quantity of electricity during a particular period of time. The measured values will be product of electrical quantity and time.

Example: Ampere hour meter Clock meters Kilowatt-hour meter.

10. Write a note on secondary instruments. Secondary instruments: It gives the quantity to be measured in terms of deflection. It requires comparison with standard instruments. It is used in practice everywhere.

Example: Ammeter, voltmeter, wattmeter, thermometer, Speedometer.

11. Explain the working of electrostatic

voltmeter. ELECTROSTATIC VOLTMETERS

This instrument is used for measurement of voltages ranging from about 100 V to 20 kV. There are two types of electrostatic voltmeters.

1. Quadrant type electrostatic voltmeter which is used to measure voltages up to 10kV

to 20kV.

2. Attracted disc type electrostatic voltmeter which is used to measure voltages above 20KV.

Working of Quadrant type electrostatic voltmeter:

The fixed quadrants are connected together and the voltage to be measured is applied between the fixed quadrants and the moving needle. Due to charge accumulated an electrostatic force are set up because of which the needle rotates. The suspension exerts a controlling torque and the needle takes up a position where the deflecting torque is equal to the controlling torque.

The deflecting torque is proportional to the square of the applied voltage and therefore, the instrument can be used for both AC & DC.

12. How will you measure low resistances using Kelvin’s bridge? KELVIN BRIDGE (MEASUREMENT OF LOW RESISTANCE)

It is a modification of the Wheatstone bridge. It is used to measure resistance value below 1 Ω. It provides accuracy in low resistance measurements.

Construction:

• It consists of resistance R1, R2, R3 and Rx.

• The resistance Ry is connected between R3 and Rx.

• The resistance Ry represents the connecting leads resistance.

• The resistance Rx is the unknown resistance to be measured.

• The galvanometer can be connected to either terminal a, b or c.

• When the galvanometer is connected to terminal a, the lead resistance Ry gets added to Rx.

• When the galvanometer is connected to terminal c, the lead resistance Ry gets added to R3.

• When the galvanometer is connected to terminal b, the ratio of resistance from c to

b and that from a to b is equal to the ratio of R1 and R2.

Rx = R1.R3 / R2

This equation indicates that the connecting lead resistance has been eliminated by connecting the galvanometer to an intermediate position ‘P’.

13. Draw the sketch of 3 phase wattmeter.

14. What is creep in energy meters? What are its causes?

Creeping: In some energy meters, a slow but continuous rotation of disc is obtained, even when there is no current flowing through the current coil and only pressure coil is energized. This is called creeping The major cause is over-compensation for friction.ie, if the friction compensating device is adjusted to give a driving torque, to compensate for starting friction, there is a tendency for the disc to run, even when there is no current through the current coil. Other causes are,

• excessive voltage across the potential coil,

• vibration and

• stray magnetic fields 15. Draw the circuit of Schering bridge to measure unknown capacitance.

16. Explain the working of rotating type phase indicator. Rotating type phase sequence indicator: An instrument working on the principle as same as that of 3phase induction motors and indicate the phase sequence of 3 phase supplies is known as phase sequence indicator.

Working

• When the supply is given the coils produce a rotating magnetic field and eddy current emfs are induced in the disc.

• These emfs cause eddy currents to flow in aluminum disc and due to the interaction of eddy current with the field a torque is produced and this causes the disc to rotate.

• The direction of rotation depends on the phase sequence of supply. If the rotation is opposite to the arrow head shown then the sequence of supply is opposite

PART – C

17. (a) Explain the various operating forces required for the satisfactory operation of an indicating instrument.

Three types of forces are needed for the operating of an indicating instrument. They are: 1. Deflecting force 2. Controlling force

3. Damping force

Deflecting Force:

The deflecting force is required for moving the pointer from its zero position. The system producing the deflecting force is called “deflecting system”. Deflecting system converts the electric current (or) voltage into a mechanical force called deflecting force. Deflecting force is provided by magnetic effect

Controlling Force:

Controlling force is equal and opposite to the deflecting force at the final steady position of pointer in order to make the deflection of the pointer definite for a particular magnitude of current. Controlling force bring the moving system back to zero when deflecting force is removed. Controlling force is provided by springs.

Damping Force:

Damping force is the force required to damp the oscillations of pointer about its final steady position, so that pointer comes to the final position rapidly and smoothly without oscillation. These are difference methods of producing damping. They are 1. Air friction damping 2. Eddy current damping 3. Fluid friction damping 4. Electromagnetic damping These systems producing the damping force is called damping system.

(OR) 17. (b) Describe with neat sketches the various methods of

damping. Damping force: The force which is used to damp the oscillations is known as damping force. If the damping force is too small, the pointer oscillates and slowly comes to its

final position is called under damping. If the damping force is too high, the pointer will not oscillate, but it moves very

slowly and comes to its final position is called over damping. When the damping torque is correct, the pointer moves quickly to its final

position without any oscillations is called critical damping.

Types of damping system:

1. Air friction damping

2. Fluid friction damping

3. Eddy current damping

Air friction damping:

Construction

It consists of fixed air chamber closed at one end. The shape of the air chamber is either rectangular or circular.

A light aluminum piston is attached to the moving system

Working When the pointer is deflected the vane moves in the chamber against the air pressure. The pressure air inside the chamber opposes the motion of the pointer and the

damping force is produced. The piston is not bent and the piston does not touch the sides of the chamber

during its movement.

Fluid friction damping:

Construction

vanes

A number of vanes (disc) are attached to the moving system. The vanes are completely submerged by oil and moves in a vertical plane. The oil must be a good insulator, non – evaporating, non – corrosive upon the metal of

Working When the pointer is deflected there is a friction between the oil and the vane

which opposes the movement of the pointer and thus damping force is produced. When the pointer is not deflected there is no friction between the oil and the

vane and thus damping force is not produced.

Advantage: insulation purpose, reduce the frictional errors. Disadvantage: creeping of oil, not suitable for portable instrument.

Eddy current damping: Principle: Induction effect.

Construction

An aluminum or copper disc is attached to the spindle. An aluminum disc is placed between the two poles of an electromagnet. The spindle is pivoted to jewel bearings. Pointer moves over a uniform scale.

Working

When the pointer is deflected, the disc moves under the poles of magnet, there

is a flow of current induced in the disc. These current produces a force which opposes the motion of the disc and thus

damping force is produced.

Advantage: Damping force is high.

18. (a) Explain with a neat sketch, the construction and working of a PMMC instrument. PERMANENT MAGNET MOVING COIL INSTRUMENT (PMMC) Diagram – 5 marks

Explanation – 5 Marks

Principle: Electromagnetic induction

When a current carrying conductor is placed in a magnetic field, a mechanical force is exerted on the conductor.

Construction: Permanent magnet: Permanent magnet is in the form of horse shoe with soft iron pole pieces. The soft iron pole pieces are placed inside the arms of the magnet to reduce the air gap and provides better magnetic field.

Iron core: A cylindrical iron core is mounted between the two poles of the magnet. Due to iron core, the deflecting torque increases, increasing the sensitivity of instrument.

Moving coil: The moving coil is placed between the magnet and core. It has wound with many turns of copper wire on an aluminium former is rotated. The coil provides electromagnetic damping. The coil is mounted on the spindle and acts as the

moving elements.

Spring: Two flat phosphor bronze spiral hair springs are attached to the spindle. The springs also provide the controlling torque.

Pointer: The pointer is carried by the spindle and moves over a uniform scale. It has light weight so that it can deflect quickly. The mirror is placed below the pointer in order to get the accurate reading by removing the parallax error.

Scale: The scale has markings either for 900 or 1200. An external ‘y’ shaped knob is provided to adjust the pointer to zero.

Balancing: The balancing of control torque is provided by springs or by control springs. The spring control is commonly used but weight control is used in vertical instruments.

Supports: Pivot jewel bearing assembly is used as support to avoid the frictional error between the moving coil and other parts.

Cover: The cover is made up of either wood or plastic. It provides protection for the inner sensitive parts form dust moisture and mechanical banding.

Working of PMMC instrument:

When the instrument is connected in the circuit to measure current or voltage, the current flows through the coil. Since the coil is carrying current and is placed in the magnetic field, a mechanical force acts on it. As a result, the pointer attached to the moving system deflect over the uniform scale showing the value of voltage or current to be measured.

If the current in the coil is reversed, the deflection is also reversed. Thus the pointer will deflect below zero. So it can be used for D.C. measurements only and not for A.C. measurements. Torque Equation: Deflecting torque: F = B I L N Td = Force X radius X 2 (taking into account force on both sides)

= B I L N X d/2 X 2

= B I L N d

Then Td = N B A I (L X d = A) Td = G I (G = N B A)

Where, F = Force acting on one side of coil Td= Deflecting torque (N-m) B = Flux density in air gap (Wb/m2) N = Number of turns of the coil A = Effective coil area (m2)

I = Current in the moving coil (amperes) L = active length of coil (meters) d = average width of coil

(meters) Td = G I G = N B A = constant

Controlling torque: The controlling torque is provided by springs and it is directly proportional to the

angular deflection of the pointer. Tc = K θ

Where, Tc= Controlling torque K = Spring constant θ = Angular

deflection At equilibrium condition

Td = Tc

G I = K θ

θ = (G/K) I (or) I = (K/G) θ

Thus the deflection is directly proportional to the current passing through the coil.

(OR)

18. (b) Describe with a sketch, the construction and working of earth tester. EARTH TESTER (MEASUREMENT OF EARTH RESISTANCE) Diagram – 5 marks

Explanation – 5 Marks

An earth tester is a special type of ohm meter, used for the measurement of earth resistance.

Earth tester measures earth resistance in ohms directly.

Construction: The main parts of the earth tester are

1. Ohm meter

2. Rotary rectifier

3. Hand driven DC generator

4. Rotary current reverser

Ohm meter:

It consists of two coils (current and potential coil) are kept at 900 to each other on the same spindle. The current coil carries current proportional to current flowing in the test circuit. The potential coil carries current proportional to potential across the resistance under

test.

coils. The deflection of ohm meter needle is proportional to the ratio of the current in

two Since there is no controlling device it gives resistance value directly.

Rotary rectifier:

The alternating current in the coil will produce an alternating voltage in the coil.

But the voltage to be applied across the moving coil must be DC because Ohm meter is a moving coil type instrument.

Hence for changing AC voltage into DC, a rotary rectifier is attached on the same shaft.

Hand driven DC generator: It produces DC voltage when the handle is rotated.

Rotary current reverser:

The current reverser is fixed on the same shaft of the generator. Its function is to change DC to AC supply because the electrodes need AC supply. The sending of AC through the earth eliminates unwanted effects due to the production of back emf in the soil on account of electrolytic action, when DC flows through the earth.

Connections: The earth tester has four terminals P1, P2 and C1, C2.

Two terminals P1 and C1 are shorted to form a common point to be connected to the earth electrode.

The other two terminals P1 and C2 are connected to the auxiliary electrodes P and C respectively.

Working Principle:

When the handle of DC generator is driven at a constant speed, the alternating current flows to the earth through the dotted lines connections and the direct current flows through the coils of the instrument. The deflection of the pointer depends on the ratio of the voltage across the potential coil and current through the current coil. The deflection of the pointer directly reads the earth’s resistance on calibrated scale.

19. (a) Explain with a neat sketch the construction and working of single phase electro

dynamo meter type wattmeter. DYNAMOMETER WATTMETER Diagram – 5 marks

Explanation – 5 Marks

Construction:

The wattmeter consists of two coils namely a) fixed coil and b) moving coil. The fixed coil is made in two parts and the moving coil is placed centrally between the fixed coils.

The current coils are made of few turns of thick copper wire and are connected in series to carry the load current. The pressure coil or moving coil is made very light with several turns of fine copper wire. The jewel supported spindle carrying the pressure coil also carries a pointer, a damping vane and control springs. Additional series resistance is placed outside the moving system. A case iron cylinder is placed around the coils to protect against the stray magnetic fields. Air friction damping is used the scale is uniformly divided. Principle of Operation :

The two parts of the fixed current coils carry load current and create a flux in the air gap between them. The movable Pressure coil carries another small current proportional to load voltage and hence produces a flux also in the air gap. Due to interaction (basic motor principle) of the two fields, a force exists between the fixed and movable coils.

Deflecting torque Td α coil currents α VI cos φ

α W , the power Control torque Tc α θ, the deflecting angle.

Steady deflection is reached when angle Td = Tc θ α W

That is, the angular displacement of the pointer is directly to average power in circuit.

Equation : Let i = Fixed coil current at any instant.

V = moving coil voltage at same instant.

Moving coil will carry current proportional to ‘V’ Let φ = phase angle between voltage and current.

Then assuming sine quantities we may write, the torque at any instant ∞ vi. But the

inertia of the moving system makes the instrument deflect due to average value of

torques. Hence average torque = some constant × average of vi product

2𝜋

i.e., Td α ∫0 𝑉𝐼 𝑑φ V = Vm sin θ

I = Im sin (θ – φ) 1 2𝜋

Then Td α 2𝜋

∫0

Vm Imsin θ. Im sin (θ – ϕ)φ

α VmIm / 2cos φ

α VI cos φ

α power.

Thus the wattmeter measures the power in a.c. circuits. In case of d.c. circuits, both voltage and current are steady and unidirectional (over a given period of time). So the two coils current and pressure-will carry steady currents and produce steady magnetic fields.

The torque developed on moving system will proportional to the product of the two fields and in turn the voltage and current.

Td α VI

α Power

We conclude to say that a dynamometer wattmeter reads both a.c. and d.c.

circuits

powers.

(OR) 19. (b) Explain with a block diagram the working of digital energy meter.

DIGITAL ENERGY METER Diagram – 5 marks Explanation – 5 Marks

Block diagram of Digital Energy Meter:

As in the block diagram, the meter has a power supply, a metering engine, a processing and communication engine i.e microcontroller, other add-on modules such as RTC, LCD display , communication ports/modules etc.

Metering Engine: The metering engine is given the voltage and current inputs and has a voltage

reference, samplers and quantizes followed by an ADC section to yield the digitized equivalent of all the inputs. These inputs are then processed using a Digital Signal Processor to calculate the various metering parameters such a power, energies etc.

The largest source of long-term errors in the meter is drift in the preamp, followed by the precision of the voltage reference. Both of these vary with temperature as well, and vary wildly because most meters are outdoors. Characterizing and compensating for these is a major part of meter design.

Processing and communication section:

This section has the responsibility of calculating the various derived quantities from the digital values generated by the metering engine. This has the responsibility of communication using various protocols and interface with other add-on modules connected as slaves to it. RTC and other Add-on modules:

These are attached as slaves to the processing and communication section for various input/output functions. On a modern meter most if not all this will be implemented inside the microprocessor, such as the Real time Clock (RTC), LCD controller, temperature sensor, memory and analog to digital converters.

20. (a) Describe with a neat sketch the construction and working of single phase electro dynamometer type power factor meter. SINGLE PHASE ELECTRODYNAMOMETER POWER FACTOR METER

Diagram – 5 marks Explanation – 5 Marks

Construction: The dynamometer power factor meter has two coils – a current coil and a

pressure coil. The current coil has two fixed coils F1 and F2. The load current is passed

through the fixed coils F1 and F2. The pressure coil has two coils A and B connected electrically parallel. They are rigidly connected to each other at 90°and are pivoted within magnetic field of coils F1 and F2. The spindle that carries the pressure coils also carries a pointer. No control springs are provided and pressure coil is connected to lines through ligaments. The coil A is connected in series with a high non-inductive resistance R, so that the current passing through it is proportional to the voltage of the circuit and is in phase with it. The coil B is connected in series with high inductive reactance, so that it current is proportional to the system voltage but lags it by 90°. The current in the two pressure coils almost differ by 90°. If they do not exactly differ by 90°; the angle between the two coils is adjusted accordingly.

Operation:

When p.f is Unity: The current in coil A will be in phase with current in F2

(Load current), and the current in coil B will differ in phase by 90° with current in coil F1. Coil A will therefore set itself perpendicular to axis of F2 and the pointer will indicate UPF.

When p.f is zero: The current in coil B and the currents in coils F1 and F2, both differing by 90° from voltage will come in phase. At the same time current in coil

A will differ from current in F1 and F2 by 90°. Depending on the power factor is lagging

or leading, the deflection will be to left and right respectively. For intermediate power factors between 0 and 1 the moving coil set will take intermediate positions pointing to exact factors either leading side.

(OR) 20. (b) Explain with a neat sketch, the construction and working of Weston frequency meter.

Weston (moving iron) type frequency meter: Diagram – 5 marks Explanation – 5 Marks

Weston frequency meter is a moving iron instrument. The action depends on

the variation in current distribution between two parallel circuits when the frequency changes of the two circuits one is made inductive and other is non-inductive.

Construction: It consists of two pairs of coils A1-A2 and B1-B2 kept at right angles to each

other. Coil pair A1-A2 is connected in series with an inductance LA and put across resistance RA. Coil pair B1- B2 is connected in series with a resistor RB and put across an inductor LB. The entire combination is connected to supply in series with an inductor L. The purpose of inductor L is to block higher harmonic currents and surge voltage. A

soft iron needle is placed in between the two pairs of coils A1-A2 and B1-B2, and the

needle carries a pointer. There is no control spring and the moving system will stabilize at various positions depending on the supply frequency.

Working:

When supply is given, two pairs of coils A1-A2 and B1-B2 draw currents depending on their impedances. The values of RA, RB, LA and LB are chosen that the equal currents flow in each pair of coils at normal frequency. Two magnetic fields are set up at 90° to each other. The interaction of these two fields on the moving system keeps it at equilibrium in between the axes and the pointer indicates normal frequency at the Centre of the scale.

When supply frequency rises, the impedances of both LA and LB go up. Impedances of coil circuit A1-A2 increases and current through it reduces. At the same time voltage fed to the coil circuit B1-B2 increases and hence current through it raises. The coil strengths due to coils A1-A2 and B1-B2 are no longer equal. Flux of coil B1-B2 is stronger and the soft iron and pointer moves clockwise.

When supply frequency drops, the reverse action takes place and the soft iron and pointer moves anti-clockwise to read lower frequency.

21. (a) Describe with a block diagram the working of a CRO.

Block Diagram Of Cathode Ray Oscilloscope (CRO) Diagram – 5 marks Explanation – 5 Marks

Y-input: It is the main input of CRO, to which the input signal is connected. The waveform of this input signal is displayed on the screen of CRT.

Vertical attenuator: It consists of RC voltage divider, which is marked on the

CRO front panel as Volt/div control knob. Thus the ‘gain’ of CRO can be controlled with

Volt/div knob.

Vertical amplifier: It is a set of preamplifier and main vertical amplifier. The input attenuator sets up the gain of vertical amplifier.

Delay line: The delay line delays the striking of electron beam on the screen. It synchronizes the arrival of the beam on screen when time base generator signal starts sweeping the beam horizontally. The propagation delay produced is about 0.25msec.

Trigger circuit: It takes the sample of input voltage connected at y-input of CRO and feeds it to the input of time base generator. So the TBG starts only when input signal is present at y-input.

Time base generator: It produces a saw tooth wave. The waveform is used to sweep the electron beam horizontally on the screen. The rate of rise of positive going edge of saw tooth waveform is controlled by Time/div control knob. Thus, the saw tooth wave controls the horizontal deflection of electron beam along x-axis.

Horizontal amplifier: It amplifies the saw tooth waveform coming from TBG. It contains phase inverter circuit also. Due to this circuit, two outputs are produced. One output produces positive going saw tooth and other output produces negative going saw tooth. The first output is connected to right side H-plate and the second output is connected to left side H-plate. So the electron beam moves properly from left to right of the screen.

Blanking circuit: It is necessary to eliminate the retrace, which would produce when the spot on screen moves from right to left. This retrace can produce confusion with the original wave. So when the electron beam reaches right end of screen, the negative blanking voltage is produced by TBG. It is fed to control grid of CRT, to stop the electron beam completely.

HV/LV power supply: The high voltage section is used to power the electrodes of CRT and the low voltage section is used to power the electronic circuits of the CRO.

(OR) 21. (b) Explain the working of dual trace CRO.

Dual Trace CRO: Diagram – 5 marks Explanation – 5 Marks

Dual beam and dual trace oscilloscope are used to compare any waveforms of

same or different shape and frequency. In this case of beam oscilloscope, there are two electron gun structures. But, in the case of dual oscilloscope, two vertical channels are alternately displayed.

Figure shows the block diagram of duals trace CRO. It consists of two

separate attenuator and preamplifier stages. The amplitude of each input can be individually controlled. After pre-amplification, these two channels meet at an electronic switch. The electronic switch passes one channel at a time into the vertical amplifier via the delay line.in dual trace oscilloscope, there are modes of operation called alternate mode and chopped mode.

Alternate mode: