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Electrical Measurement

EI 34-ELECTRICAL MEASUREMENTSUnit: I

1. Define Ballastic sensitivity?Ballastic sensitivity sq =1/Q

2. State the advantage and disadvantage of rectifier type instruments?

Advantage

The frequency range extends from about 20 hz to high audio frequency With the introduction of germanium and silicon rectifier rectifier instrument may be built in half wave configuration with a sensitive PMMC instrument at frequency in the region of 3000 MHZ.Disadvantagei) Effect of waveform

ii) Effect of rectifier Resistance3. Why is the MI meter has non uniform scale?

The deflection in a MI instrument is given by

=1/2 *I2 /K*dL/d

Thus the angular deflection is proportional to the square of the operating current and the instrument has a square law response. The deflection is in terms of rms value of current or voltage as the deflection is proportional to square of the current it is evident that the scale of such instrument is non-uniform.4. What is residual error?The happenings or disturbances about which we are unaware are lumped together and called Random or Residual .Here the errors caused by these happenings are called Random error .Since these errors remain even after the systematic errors have been taken care of we all these errors as residual.5. Why electrostatic instruments cannot be used for the measurement of low voltage while electromagnetic instrument can be?When the voltage being measured is small the two discs should be very near together in order to get an appreciable force. In such cases the measurement of distance between the plates is difficult to carry out. The solution lies in increasing the voltage between fixed and moving plates by using hetrostatic connection.6. Define loading effect of an instrument?

The incapability of the system to faithfully measure record or control the input signal in undistorted form is called the loading effect.7. Define the term current sensitivity and voltage sensitivity?

The current sensitivity of a galvanometer is defined as the deflection produced by unit current

Current sensitivity Si=Gi/KVoltage sensitivity

Voltage sensitivity is the deflection in scale division per unit voltage impressed on the galvanometer.

Sv=d/i*Rg*106 8. What precautions are to be observed when using an ammeter?

Ammeter which is connected in series with the circuit carrying the current under measurement must be very low resistance. So that the voltage drop across the ammeter and power absorbed from the circuit are as low as possible.9. State the advantage disadvantage of PMMC instrument?Advantage

i) The scale is uniformly divided.

ii)The power consumption is very low as 25 w to 200 w.

Disadvantage

i) There instrument useful only for dc

ii) The cost of this instrument is higher than that of MI instrument.10. Compare Ammeter and Voltmeter.Ammeter

a. The ammeter carries the current to be measured or a definite fraction of it and the current or its definite fraction produces the deflecting torque.b. Connected in series.

Voltmeter

a. Carries the current proportional to the voltage to be measured which produces the deflecting torque

b. Connected in parallel11. What is the basic principle of PMMC instrument?Working principle is same as the dArsonoval type of galvanometer the difference being that a direct reading instrument is provided with a pointer and scale.12. Compare and contrast moving iron and Electro dynamometer type meters?MI

a. Suitable for DC or AC

b. Inexpensive type used for rough indication of currents and voltages widely used in indicator type applications such as on panels

Electrodynamometer type instrument

a. Suitable for DC or AC

b. Widely used for precise ac current or voltage measurement current frequencies Used as standard meter for calibration and also as transfer instrument.13. A PMMC instrument has a 0.12T magnetic flux density in its airgaps. The coil dimensions are D=1.5 cm and l= 2.25 cm .Determine the no of coil turns required to give a torque of 4.5N-m when the coil current is 100A.N= K/BldI= 1111 turns14. What is the use of Swamping resistor?When swamping resistance of maganin of resistance 20 to 30 times the coil resistance is connected in series with the coil and a shunt of manganin is connected across this combination. Since copper forms a small fraction of the series combination the proportion in which the currents would divide between the mater and the shunt would not change appreciably with the change in temperature.15. Explain the term instrumentation and instruments?An instrumentation system is a physical system which is a an collection of physical objects connected in such a way to give the desired output response.

Measurement involves the use of instruments as a physical means of determining quantities or variables.16. What is the importance of static characteristics of systems?It is used to check how good the measurements under steady state conditions.17. What is sensitivity of voltmeter?

Sdc=1/Ifs18. What are the necessary forces to operate an indicating instrument?Deflecting ,controlling ,Damping19.Compare moving iron and moving coil type instruments?

Moving iron

For AC and DC

Inexpensive type used for rough indication of current and voltages . Widely used in indicator type applications such as on panels .

Moving coil

a. Suitable for DC

b. Most widely used meter for current and voltage and resistance measurement in low and medium impedance circuit.20.What is meant by measurement?

Measurement is an act or the result of comparison between the quantity and a predefined standard. 21 Mention the basic requirements of measurement.

The standard used for comparison purpose must be accurately defined and should be commonly accepted. The apparatus used and the method adopted must be provable. 22.What are the 2 methods for measurement?

Direct method and

Indirect method. 23.Explain the function of measurement system.

The measurement system consists of a transducing element

which converts the quantity to be measured in an analogous form. The analogous signal is then processed by some intermediate means and is then fed to the end device which presents the results of the measurement.24.List the types of instruments.

The 3 types of instruments are

Mechanical Instruments

Electrical Instruments and

Electronic Instruments.

25.Classify instruments based on their functions.

Indicating instruments

Integrating instruments

Recording instrumentsUNIT: II1. List the various types of registering mechanism used in single phase energy meter? Pointer type of register.

Dycometer register.2. Derive the average deflecting torque equation for electrodynamometer type of wattmeter?

Td= 1/TT0T1d(wt) =VI/Rp*cos .dM/d

3. Two wattmeter are connected to measure the input of a balance three circuit indicates 1000w(+ve) and 125 (+ve ) respectively. Find the power factor of the circuit?

Pf cos = cos [tan-1(3(1000-125/1000+125)]=0.59604. What is the need for light load compensation in watt hour meter?

Despite every care taken in the design of both the jeweled pivot bearing which forms the lower bearing for the spindle and of the simple sleeve pin type bearing at the top of the spindle friction errors are liable to be serious particularly at light loads .In order to ensure accurate registration at low loads it is therefore necessary to arrange for small torque practically independent of the load on the meter which acts in the direction of rotation and which is nearly as possible equal in magnatitude to the friction torque. This is usually obtained by means of small shading loop.

5. Write a short note on adjustments required in energy meter? Lag adjustment (LPF Adjustment)

The pressure coil is connected across rated supply voltage and rated full load current is passed through the current coil at 0.5 pf lagging .The lag device is adjusted till the meter runs at correct speed.6. Define creep?

In some meters a slow but continuous rotation even when there is no current flowing through the current coil and only pressure coil is energized. This is called creeping.7. List the different errors that occur in a Electro dynamometer type wattmeter.i) Pressure coil inductance

ii) Pressure coil capacitance

iii)Error due to Mutual Inductance Effect.8. An induction type energy meter runs fast. How the speed can be reduced?

The current coil flux s is made small as compared pressure with coil flux p9.List out the special features incorporated in an electrodynamometer type of wattmeter for low power factor applications?i) Pressure coil current

ii) Compensation for pressure coil current

iii) Compensation for Inductance of pressure coil.10. What is phantom loading?When the capacity of the meter to be tested is high considerable power will be wasted if ordinary loading arrangement is made. For testing of meter of higher capacities phantom or fictitious loading arrangement is made.11. State the disadvantage of wattmeter?

i) Even a small error in measurement of voltages causes serious errors

ii) Supply voltage higher than normal voltage is required.12. What is the basic principle of energy meter?

Induction principle13. What does systematic error of instrument mean?

Such errors that remain constant or change according to a definite law on repeated measurement of the given quantity.14. A three phase 500 v motor load has p.f 0.4 .Two wattmeters connected to measure the input. They show the input to be 30 kw .Find the reading of each instrume