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The advantages of moving coil permanent magnet type instrument are
Low power consumption
No hysteresis loss
Efficiency eddy current damping
All of the above
All of the above
Quick Summary: 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.
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.
TdтАЛ=NBIld тАФ The deflecting torque developed in the PMMC instrument
TcтАЛ=K╬╕ тАФ The restoring torque provided by the control springs
When a direct current passes through the moving coil, it experiences a torque TdтАЛ=NBIld 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.
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 (TdтАЛтИЭI).
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.
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.
Limited to DC measurements only.
Control springs are subject to fatigue and temperature aging.
Higher cost compared to Moving Iron (MI) instruments.
Used as DC Ammeters and Voltmeters.
Used in galvanometers for detecting minute DC currents.
Basis for many electronic test equipment measurements.
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.
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.
Always remember that PMMC = DC only; if you see AC measurement, think Moving Iron (MI) or Electrodynamometer types instead.