Classification of Measuring Instruments

• Quantity measured• Principle of operation

•Principle of operation1) Electromagnetic (Force due to B & I)2) Electrostatic (Force due to E & Q)3) Electro thermal (due to heating effect)

Electromagnetic-Electromechanical Indicating Instruments

Electromechanical Indicating InstrumentsInput is electrical signalOutput is mechanical forceAnalog (Output is a continuous

function of time)Example: PMMC Instruments

Permanent Magnet Moving Coil (PMMC) Instruments

• To measure DC current or DC voltage • Can be used for measuring AC currents and voltages by introducing additional circuit and proper calibration

Constructional Features of PMMC

Constructional Features of PMMC

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Various Forces/ Torques in Measuring Instruments

• The movement of the pointer in PPMC is governed by three different torques

1) Deflection Torque2) Control Torque3) Damping Torque

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Deflection Torque

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Deflection Torque

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Deflection Torque

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Control Torque

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Damping Torque

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Damping Torque

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Two methods of supporting the moving system

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Two methods of supporting the moving system

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• Examination of torque equation• Reason for placing hair springs in opposite direction• Why taut band is used• How to use PMMC for AC• Effect of temp, Swamping resistance • Multi range ammeters and voltmeters

•What switches should be used• Advantages, Limitations and sources of error• Ammeter and voltmeter sensitivity

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Effect of temperature and methods to compensate

• Hair springs, Magnetic field, Coil resistance are effected• Series resistance with temperature coefficient nearly zero (manganin)• Usually three the coil resistance•It is called Swamping resistance

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