EI2202 ELECTRICAL MEASUREMENTS UNIT I MEASUREMENT...

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EI2202 ELECTRICAL MEASUREMENTS

UNIT – I :MEASUREMENT OF VOLTAGE AND CURRENT PART-A

A PMMC instrument has a 0.12T magnetic flux density in its air gaps. The coil dimensions are D = 1.5 cm and l =2.25 cm. Determine the number of coil turns required to give a torque of 4.5 when the coil current is 100mA. Td = NBldI N = Td / BldI = 4.5 x 10-6 / (0.12 x 2.25 x 10-2 x 1.5 x 10-2 x 100 x 10-6) N = 1111

What is the use of swamping resistor? Swamping resistor is connected in series with meter resistance to reduce the error caused by temperature rise (i.e. temperature error). Swamping resistance to meter resistance ratio should be high.

What is the electrical current effect used to produce deflecting torque in a PMMC instrument? When a current(I) carrying conductor of size l and d and N turns, cuts a magnetic field of flux density B and if the field is radial then the deflecting torque Td = NbldI = GI

Explain the terms resolution and sensitivity. Resolution : Smallest increment in input which can be detected with certainity by an instrument is its resolution or discrimination. So resolution defines the smallest measurable input change. Sensitivity : It is the ratio of the magnitude of the output signal or response to the magnitude of input signal or the quantity being measured.

Compare internal resistance of an ammeter and a voltmeter. Give reason. The internal resistance of an ammeter is very low and the internal resistance of a voltmeter is very high. What causes errors in moving iron instruments? Temperature coefficient of spring, slf heating of coils in voltmeters, Stray magnetic fields, changes of reactance of working coils, changes of magnitudes of eddy currents cause errors in moving iron instruments.

State two merits of PMMC ammeter. The scale is uniformly divided. The power consumption is very low as 25 to 200 The torque weight ratio is high which gives a high accuracy. The errors due to stray magnetic fields are very small. A single instrument may be used for many different voltage and current ranges.

State two sources of error in moving iron instrument. Hysterisis Error Temperature error Stray magnetic fields Frequency errors Eddy currents

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EI2202 – Electrical Measurements

9. Define current sensitivity of a galvanometer. The current sensitivity of a galvanometer is defined as the deflection produced by unit current. Current sensitivity = Si = F / I = Gi / K rad / A or G / 500K mm / 10. List the various methods of providing control torque. Gravity control Spring control

11. What are the disadvantages of thermal instruments? Over load capacity of thermal instruments is small as compared with other instruments is about 150 percent of the full-scale value.

12. What are the limitations of rectifier type instruments? . Effects of waveform Effect of rectifier resistance Effect of temperature changes Effect of rectifier capacitance Decrease in sensitivity

13. What are the requirements of materials used in making shunts for extending range of instruments? The temperature co-efficient of shunt and instrument should be low and should be as nearly as possibly the same. The resistance of shunts should not vary with time. They should carry the current without excessive temperature rise. They should have a low thermal electromotive force with copper.

14. List the various types of errors in electro dynamometer type instrument Low torque to weight ratio. Frequency error Eddy currents External magnetic fields Temperature change

15. What are the factors effecting the performance of rectifier type instrument Refer the answer for Q-No. 12.

16. A thermocouple element reaches 10A at full scale. Determine the current when the meter reaches 1/3 of full scale. The deflection of a thermo couple element ammeter: = K I2 Let F is the full-scale deflection. So F = k (10)2 F / k= 100. The deflection at1/3 full scale is F /3 Therefore F /3 = k I2 Current required to give 1/3 scale deflectionI = (qF /3 K) 1/2 = (100/3) 1/2 = 5.77A

17. How is the current range of PMMC instrument extended with the help of shunts? To extend the Ammeter range shunt of low resistance is connected in parallel with meter resistance. When heavy currents are to be measured, the major part of the current is bypassed through this shunt resistance. Shunt resistance can be calculated using the formula. Rsh= Rm/(m-1) Rm – meter resistance m -Multiplying power = I/Im.

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18. List out commonly used damping methods? (O-96). Air friction damping Fluid friction damping Eddy current damping Electromagnetic damping

19. What is the use of thermal instruments? (O-98). It’s used for measurement of radio frequency a.c. signals. They are very useful when used as transfer instruments to calibrate d.c instruments by potentiometer and standard cell. These instruments are suitable for both a.c and d.c.

20. How to extend the range of voltmeters? (A-98). Series resistance(multiplier) is connected in series with basic meter and this combination is connected across the voltage to be measured. Rs=(m-1)Rm Rs=series resistance Rm=meter resistance m=multiplier =V/v V=voltage to be measured v=voltage across the meter movement.

21. What is shunt? Explain briefly? (O-97). When heavy currents are to be measured or to extend the range of an ammeter, the major part of the current is bypassed through a low resistance called a shunt. It will be connected across a basic meter.

22. What is the need for control torque and state the methods to provide it in Analog indicating instruments? Control torque is needed to produce a torque equal and opposite to deflectingtorque at the final steady position of pointer in order to make the deflection of the pointer definite. Controlling torque is achieved in analog instruments by two methods. They are

(i) Gravity control (ii) spring control.

23.State the errors in PMMC instruments. Weakening of permanent magnets due to aging and temperature effects. Weakening of springs due to aging and temperature effects. Change of resistance of moving coil with temperature.

24. Compare the merits and demerits of attraction and repulsion type MI instruments? Attraction Type Merits: Lower value of inductance Accurate over a wider range of frequency and Greater possibility of using shunts with ammeters. Repulsion type Merits: Suitable for economical production. Uniform scale.

25. Why electrodynamometer type instrument is called transfer instruments? A transfer type instrument is one that may be calibrated with a d.c. source and Then used without modification to measure a.c. This requires the transfer type instrument to have same accuracy for both d.c. and a.c. which the electrodynamometer instruments have. .

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EI2202 – Electrical MeasurementsDepartment of ICE & EIE 26. Define thermocouple. State merits and demerits of thermocouple instruments. When two metals having different work functions are placed together, a voltage is generated at the junction, which is nearly proportional to the temperature. This junction is called a Thermocouple. This principle is used to convert heat energy to electrical energy at the junction of two conductors Merits: Thermocouple instruments correctly indicate the rms value of voltage or current irrespective. These instruments are not affected by stray magnetic fields. These instruments are free from frequency errors and hence they can be used over a very wide frequency range. Highly sensitive. They are very useful when used as transfer instruments to calibrate d.c. Instruments by potentiometer and standard cell. Demerits: Overload capacity of thermo couple instruments is small as compared with other instruments.

27. An ohmmeter is MI or MC or dynamometer type? Moving coil type.

28. Give the expression for torque in moving coil instruments. Deflecting torque Td = NBldI=GI Controlling Torque Tc=Kθ

29. What is the use of a rectifier instruments? Rectifier type instrumentsare suitable for D.C and A.C. Widely used for medium sensitivity service type voltage measurements in medium impedance circuits.

30.. Compare moving coil and moving iron Instruments used for measurement of an electrical quantity. (2011)

1. Part B Explain in detail about the working principle of D’Arsonval galvanometer and derive its torque equation.

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Explain in detail about Ballistic galvanometer.

(i) Sketch the basic construction of typical PMMC instrument. Explain its operation. (Nov 2004)

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(ii)A PMMC instrument with FSD = 100mA and Rm =1KW is to be employed as an ac voltmeter with FSD 100V(rms). Silicon diodes are used in the bridge rectifier circuit. Calculate the multiplier resistance value required. (Nov 2004) Explain the principle, Construction of (i) Attraction type, Repulsion type moving iron instruments. Discuss their merits and demerits.(10)

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(ii) The coil of a moving coil galvanometer is wound on a non – magnetic former whose height and width are both 20 mm. It moves in a constant field of 0.12 wb/m2 . The moment of inertia of its moving parts is 0.25×10-6 Kg-m2 and control spring constant is 30 × 10-6 Nm /rad. Calculate (1) the number of turns that must be wound on the coil to produce a deflection of 150 0 with a current of 10 m A. (2) The resistance of the coil to produce critical damping.(6) With neat figure, explain the construction and operation of repulsion type moving Iron instrument. Give the advantages and limitations of such instruments. (May2003)

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Radial Vane Repulsion type instrument: The figure shows the Radial vane and co-axial vane

type instruments. Out of other moving iron instruments, this is the most sensitive and has most

linear scale.

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6. (i).Explain how flux is measured using Ballistic galvanometer. (May 2003)

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(ii).Justify the need for under damping, critical damping and over damping in Measuring instruments.

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7. Explain the operating principle of PMMC type ammeter with a neat diagram. What special features are incorporated in its construction in order to reduce the errors? (Nov2003)

8. Draw diagram showing the main components of a dynamometer type ammeter and explain the working principle of this device. What steps are taken to reduce the errors in this instrument? (Nov 2003)

9. (i) Describe the working principle of an attraction type MI instrument.(May 2005) (ii)A PMMC instrument with FSD = 50mA and Rm = 1700W is to be employed as a voltmeter with ranges of 10V, 50V and 100V. Calculate the required values of multiplier resistors for circuit shown in fig.

10. (i)List the three forces involved in the moving system of deflection instrument. (ii)Explain with sketches how they are typically produced. (iii)Discuss the function of each force. (May 2005) 11. (i) Explain the working principle of induction type meter with suitable sketches. (ii) What are the methods available to get deflection torque an Induction type Instrument? Explain and derive deflection torque equation.(8)

12. Draw an illustrative diagram and explain the working of a hot wire instrument. Derive magnification of expansion equation.

13. (i) Discuss the working and constructional details of rectifier type instrument. Derive half wave and full wave rectifier equations.(10)

Rectifier type meter Rectifier type instruments are used for measurement of ac. voltages and currents by employing a rectifier element, which converts a.c. to a unidirectional d.c. and then using a meter responsive to d.c. to indicate the value of rectified a.c.

The indicating instrument is PMMC instrument, which uses a d’Arsonval movement.

This method is very attractive since PMMC instruments have a higher sensitivity than thed

electrodynamometer or the moving iron instruments. The arrangement which employs a full

wave rectifier.

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EI2202 – Electrical Measurements Department of ICE & EIE

(ii).Compare actual and ideal characteristics of rectifier instruments.

14. (i). How would you extend the range of dc ammeters and voltmeters? Explain with suitable diagrams.

Extension of range and calibration of voltmeter and ammeter:

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(ii).The moving coil ammeter has a fixed shunt of 0.02 ! with a coil resistance of R = 1000! and potential difference of 500 mv across it. Full scale deflection is obtained , Calculate (a) To what shunt current does this correspond to ?

(b) Calculate the value of R to give full scale deflection when shunted current I is 10 A (case1) and 75 A (case b).


15. Illustrate with a neat diagram the principle of operation, construction and working of moving iron instrument.(2011)

16. Write a short notes on different types of error in measuring instruments?

17. A set of independent 10 measurements were made to determine the weight of lead short. The weights in grams were: 1.570, 1.597,1.591, 1.562, 1.577, 1.580, 1.564, 1.586, 1.550, 1.575. Determine the i) Arithmetic mean, standard deviation and variance.

EI2202 ELECTRICAL MEASUREMENTS UNIT I MEASUREMENT...

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