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A Measured Approach Towards Measurement

Hi Guys,

In this post we will deal with a subject that most of the Electrical Engineers neglect and do not give the due importance. The one mistake that everyone does is not realizing that this subject has got a good weight-age of around 3-5 marks in the GATE exam. So guys my first recommendation will be to not neglect this subject as this is narrow and easy as well, so spare at least one week for its preparation also.
The books that can be referred for this subject are “Electrical Measurement and Instrumentation by AK Sawhney” but i still would not recommend using a book as that book is very vast and still if you want to read the book then you can better highlight the topics that need to be read from each chapter.
This subject includes the following topics:
  • Measurement and Error

    • This chapter includes basic error analysis in terms of Guaranteed Accuracy Error, Limiting Errors for various arithmetic operations like addition, subtraction etc. Also errors in terms of Standard Deviations so here this much is sufficient and no need to study the Standards of Length and time as those are not required for GATE.
  • Electro-mechanical Instruments

    • This is the most important topic from this subject.
    • First you need to study about the different types of torques in Deflecting Type Instruments such as Deflecting Torque, Damping Torque and Controlling Torque.
    • The basic principle of instruments and the expressions for deflecting torque in various instruments like PMMC, Moving Iron and Dynamometer type Instruments is the most important topic in this chapter.
    • Always remember that PMMC measures average value of output and other two measure the RMS values.
    • The applications of PMMC like ammeter, voltmeter and rectifier type instruments are also important and range extension of these instruments is the most important using Shunt and Series Resistances.
    • In Dynamometer the most important application is the wattmeter. For that you need to be clear with the three phase circuits phasor diagram as if you know the phasors well then you do need to remember any formulas and you can easily crack the non standard cases as well. Also remember the formula for calculating the power factor angle in case of two wattmeter method as that is most commonly asked.
    • Also, you can understand the various errors in Wattmeter reading such as Error due to self inductance of Pressure Coil, Error due to resistance of Pressure Coil etc.
    • Energy meter is the next important instrument and here the only important concept is the error in reading and that can be computed using the energy constant, other than that the working principle and other things are not that important from GATE perspective.
    • Electrostatic and Thermal instruments are not that important but try to remember the expressions for deflecting torques for both these instruments. Also, remember that these instruments measure RMS Value of a quantity.
Electrostatic and Thermal instruments
  • Instrument Transformers

    • The current Transformer is important from here though not many questions are asked. But this is relatively simple as we just need to study about the ratio and phase angle error. Also you can remember the equivalent circuit of Current Transformer as  it is same as the Transformers from Electrical Machines so it can be easily understood.
  • Bridges

    •  The Inductance and Capacitance Measurement come under the AC Bridges and these are very simple as they are just based on a simple phenomenon that product of impedances in opposite branches are equal. This concept can be applied to simple bridges like Maxwell,s Inductance Bridge, Maxwell’s LC Bridge and Owen’s Bridge but still expressions must be remembered for Hay’s Bridge and Anderson’s Bridge  as they are not that easy to derive.
    • The Capacitance Measurement bridges include De Sauty and Schering Bridge and those are also simple but one thing must be remembered in all bridges is the circuit of each bridge against its name.
    • The last is Wien Bridge Oscillator for measurement of Frequency.
  • Measurement of Resistance

    • Wheatstone Bridge is the most important from this topic and mostly questions are based on the condition when bridge is unbalanced and we need to find the voltage across the detector.
    • Other methods include Kelvin’s Double Bridge, Voltmeter-Ammeter Method, Ohmmeter, Carey-Foster Slide Wire Bridge.
    • You do not need to remember any formulas for this topic because if you remember the circuits well then you can just deal with it as Network Analysis.
  • Cathode Ray Oscilloscope (CRO)

    • This is second most important topic from this subject.
    • The first and most important thing in this topic is to remember the purpose of each and every part in the functionality of CRO like Horizontal Deflection Plates, Vertical Deflection Plates etc.
    • The other important thing is the Lissajous Figure and calculating ratio of frequency of x and y signal from the figure, calculating phase difference between the signals.
    • Other than this there are small concepts such as Probes in a CRO, bandwidth of a CRO etc.
  • Digital Meters-

    • In this topic all type of ADC must be studied but they are also covered under Digital Electronics and other than that parameters of Digital Instruments like Resolution etc must be studied and those are more important but this topic is not very important from GATE perspective.
So guys, since this subject has good weightage and is small also so do complete this subject and score as much as you can in this subject. Also, if you find the article helpful please share it with your friends and do comment with your queries.

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