Permanent Magnet Moving Coil
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Transcript of Permanent Magnet Moving Coil
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
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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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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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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