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Which of the following is the cause of the speed error in the induction type energy meter?
Incorrect Position of brake magnets
Incorrect adjustment of the position of shading bands
Slow but continuous rotation of an aluminum disc
Temperature variations
Incorrect Position of brake magnets
In an induction-type energy meter, the speed of the aluminum disc is controlled by the braking torque produced by permanent magnets. If the position of these magnets relative to the center of the disc is incorrect, the damping torque will be improper, leading to a speed error in the registration of energy.
In an induction-type energy meter, the speed of the aluminum disc is controlled by the braking torque produced by permanent magnets. If the position of these magnets relative to the center of the disc is incorrect, the damping torque will be improper, leading to a speed error in the registration of energy.
TbтАЛ=k╧Х2Nr2 тАФ Braking torque in induction energy meter
NтИЭTbтАЛTdтАЛтАЛ тАФ Relation between speed, driving torque, and braking torque
The brake magnet creates eddy currents in the rotating disc. The interaction between these currents and the magnetic field of the permanent magnet creates a braking torque proportional to the disc speed. If the magnet is moved closer to the outer periphery, the braking torque increases, slowing the disc; moving it toward the center decreases the braking torque, increasing the speed. An incorrect radial position shifts this balance, causing the meter to run too fast or too slow.
The permanent magnet acts as a mechanical damper for the rotating disc.
Braking torque is directly proportional to the square of the distance from the disc center to the magnet pole.
Speed error adjustment is achieved by sliding the brake magnet radially.
High accuracy in steady-state measurement
Provides linear damping effect at low speeds
Sensitivity to external magnetic fields
Torque is dependent on disc temperature and conductivity
Standard AC energy measurement
Calibration of electromechanical induction meters
Option B (Shading bands) is used for phase angle compensation (lag adjustment), not speed error.
Option C (Creeping) refers to the slow rotation of the disc with no load, caused by excessive friction compensation or voltage surges.
Option D (Temperature) causes variations in resistance, but is typically corrected via compensation circuits rather than being the primary cause of inherent speed error.
A is correct тАФ The radial position of the brake magnet dictates the braking torque, and any misalignment directly impacts the intended speed of the disc relative to the load current.
Always remember: 'Shading bands' are for phase adjustment (power factor), while 'Brake magnets' are strictly for speed (damping) adjustment.
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