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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