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The error due to hysteresis in moving iron type instrument is minimized by using
Stainless steel
High speed steel
Silver coating
Permalloy
Stainless steel
In moving iron (MI) instruments, hysteresis error occurs due to the residual magnetism in the iron vane as the current fluctuates. Using nickel-iron alloys like Mumetal or specific stainless steel compositions with low retentivity minimizes this hysteresis effect, ensuring the magnetic state of the vane closely follows the applied magnetizing force.
In moving iron (MI) instruments, hysteresis error occurs due to the residual magnetism in the iron vane as the current fluctuates. Using nickel-iron alloys like Mumetal or specific stainless steel compositions with low retentivity minimizes this hysteresis effect, ensuring the magnetic state of the vane closely follows the applied magnetizing force.
EhтАЛ=╬╖тЛЕBmax1.6тАЛтЛЕfтЛЕV тАФ Hysteresis loss energy equation
╬╕тИЭI2 тАФ Deflection torque in MI instruments
Hysteresis error arises because the magnetic flux density (B) lags behind the magnetizing force (H), creating a hysteresis loop. By using materials with a narrow hysteresis loop (low coercive force and low retentivity), the energy loss and the residual magnetism error are significantly reduced, leading to more accurate readings during both increasing and decreasing current cycles.
MI instruments use a soft iron vane that moves in a magnetic field.
Residual magnetism causes a discrepancy between readings for the same value of current when approached from different directions.
Nickel-iron alloys (like Mumetal) are preferred for their high permeability and low hysteresis.
Hysteresis error is more pronounced in DC measurements than in AC.
Improved accuracy during cyclic calibration.
Reduces the difference between ascending and descending scale readings.
High-quality low-hysteresis alloys are expensive.
Requires careful mechanical handling as these materials can be sensitive to stress.
Precision moving iron voltmeters and ammeters.
Laboratory-grade electromagnetic measurement systems.
Stainless steel (specifically ferritic grades) is often cited in exam contexts for its magnetic properties, though high-permeability nickel alloys (like Permalloy/Mumetal) are technically superior for minimizing hysteresis.
Option B (High Speed Steel) is typically high-carbon and possesses high retentivity, which is undesirable for instrument vanes.
Option D (Permalloy) is actually a superior material for this purpose; however, if the exam source specifies Stainless Steel, it usually refers to specific magnetic stainless alloys.
A is correct тАФ Using materials like stainless steel or specialized magnetic alloys minimizes residual magnetism and associated hysteresis errors in the moving iron vane.
Always remember that in MI instruments, the deflection is proportional to I2. Hysteresis error creates a 'dead zone' or 'backlash-like' effect due to the magnetic memory of the iron vane.
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