Which of the following is the cause of the speed error in the induction type energy meter?

Question

Examoogle · Accepted Answer

Option A: Incorrect Position of brake magnets

Explanation & Concepts:
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.

Working Principle: 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.

Key Formulas: $T_b = k \phi^2 N r^2$ — Braking torque in induction energy meter; $N \propto \frac{T_d}{T_b}$ — Relation between speed, driving torque, and braking torque

Key Points: 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.

Advantages: High accuracy in steady-state measurement; Provides linear damping effect at low speeds

Disadvantages: Sensitivity to external magnetic fields; Torque is dependent on disc temperature and conductivity

Applications: Standard AC energy measurement; Calibration of electromechanical induction meters

Additional Info: 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.

Answer: A is correct — 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.

Exam Tip: Always remember: 'Shading bands' are for phase adjustment (power factor), while 'Brake magnets' are strictly for speed (damping) adjustment.

Examoogle · Answer

Option B: Incorrect adjustment of the position of shading bands

Examoogle · Answer

Option C: Slow but continuous rotation of an aluminum disc

Examoogle · Answer

Option D: Temperature variations

Correct Answer

Discussion (0)

Correct Answer

Related Questions

Discussion (0)