Question Detail
The advantages of moving coil permanent magnet type instrument are
Options
- Option A: Low power consumption
- Option B: No hysteresis loss
- Option C: Efficiency eddy current damping
- Option D: All of the above
Correct Answer
All of the above
Solution & Explanation
{"type":"technical","methodBadge":"Concept & Principle","explanation":"Permanent Magnet Moving Coil (PMMC) instruments are highly efficient DC measuring devices that operate on the principle of the Lorentz force acting on a current-carrying coil in a static magnetic field. They are characterized by linear scales, high sensitivity, and excellent damping properties, making them the standard choice for precision DC measurements.","workingPrinciple":"When a direct current passes through the moving coil, it experiences a torque $T_d = N B I l d$ due to the interaction between the coil's magnetic field and the static field produced by the permanent magnet. The coil is wound on a light aluminum former, which acts as a short-circuited loop; as the coil moves, eddy currents are induced in this former, creating a braking torque that provides highly effective electromagnetic damping.","diagramSvg":"Torque Equation: PMMC$T_d = N B I l d$$T_d$: Deflecting Torque (N-m)$N$: No. of turns, $B$: Flux Density (Wb/$m^2$)$I$: Current (A), $l \\times d$: Coil Area ($m^2$)","keyFormulas":["$T_d = N B I l d$ тАФ The deflecting torque developed in the PMMC instrument","$T_c = K \\theta$ тАФ The restoring torque provided by the control springs"],"keyPoints":["PMMC instruments only work on DC supplies because the torque direction reverses with polarity.","The scale is linear because the deflecting torque is directly proportional to the current ($T_d \\propto I$).","They provide the highest torque-to-weight ratio among analog instruments.","Hysteresis loss is negligible as there is no iron core within the moving coil itself."],"advantages":["Very low power consumption due to high sensitivity.","Absence of hysteresis loss ensures high accuracy.","Effective eddy current damping provides quick, dead-beat settling.","Uniform scale distribution allows for precise readings."],"disadvantages":["Limited to DC measurements only.","Control springs are subject to fatigue and temperature aging.","Higher cost compared to Moving Iron (MI) instruments."],"applications":["Used as DC Ammeters and Voltmeters.","Used in galvanometers for detecting minute DC currents.","Basis for many electronic test equipment measurements."],"comparisonTable":[],"additionalInfo":["Low power consumption: The high torque-to-weight ratio allows for a low number of turns and minimal current required for full-scale deflection.","No hysteresis loss: Since the magnetic circuit involves only an air gap and no soft iron core inside the moving coil, magnetic reversal losses are absent.","Eddy current damping: The aluminum former moving within the permanent magnetic field induces an EMF, resulting in circulating currents that oppose motion."],"answer":"D is correct тАФ PMMC instruments offer low power consumption, zero hysteresis loss, and superior eddy current damping, making them ideal for high-precision DC measurements.","correctedOptions":["Low power consumption","No hysteresis loss","Efficiency eddy current damping","All of the above"],"coreConcepts":["Lorentz Force","Eddy Current Damping","Linear Scale Characteristics"],"crossTopicTip":"Always remember that PMMC = DC only; if you see AC measurement, think Moving Iron (MI) or Electrodynamometer types instead."}
