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

Enameled or silk covered copper wire is used for coil

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

8

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

• To increase sensitivity, the diameter of the hair spring should be reduced, but it is limited, so Taut band is used

• It is thin, metallic, ribbon like structure

• It improves 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) is used

• Usually three times the coil resistance

• It is called Swamping resistance

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Reason for placing hair springs in opposite direction

F

x = 0 x

F

x

CASE-1

H

x=H

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Reason for placing hair springs in opposite direction

F

x = 0x

F

x

CASE-2

x=H

H

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Reason for placing hair springs in opposite direction

F

x

F

x = 0 x

CASE-3

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Multi Range Ammeter

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Make before Break

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Multi Range Ammeter (alternative design)

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Multi Range Voltmeter

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Break before Make

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Advantages• Uniform scale

• Power consumption can be made very low (25 µW to 200 µW)

• Torque to weight ratio can be made high with a view to achieve high accuracy (typically 2%)

• Single instrument can be used for multi range ammeters and voltmeters

• Error due to stray magnetic field is very small

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Limitations

• They are suitable for direct current only

• The instrument cost is high

• Variation of magnet strength with time

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Errors can be reduced by• Proper pivoting and balancing weight may reduce the frictional error

• Considering the aging can reduce errors due to magnetic decay

• Manganin in series with coil reduces temperature effects

• Maintaining nominal temperature. The stiffness of spring, permeability of magnetic core decreases with increase in temperature

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Sensitivity

Ammeter Sensitivity:

Full scale current (A)

Voltmeter Sensitivity:

Total resistance of the meter /Full scale reading (V)

k

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Assignment

Q1) A PMMC instrument has a coil resistance of 100Ω and gives a full-scale deflection (FSD) for a current of 500μA. Determine the value of shunt resistance required if the instrument is to be employed as an ammeter with a FSD of 5 A.

Ans: 0.01 Ω

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Assignment

Q2) A PMMC meter with a coil resistance 100Ω and a full scale deflection current of 100μA is to be used as a voltmeter. The voltmeter ranges are to be 50 V and 100 V. Determine the required value of resistances for each range.

Ans: 0.4999 MΩ, 0.9999 MΩ