Q3Electronic Measurement & Instrumentation
Question
Draw the connection diagram of a energy meter and explain its working. How do you correct it if it is found to be moving fast.
Answer
A 3-phase energy meter uses two induction elements on a common shaft to integrate total power. Fast meters are corrected by moving the brake magnet outward.
A 3-phase, 3-wire energy meter operates on Blondel's two-wattmeter principle. It consists of two separate single-phase induction meter elements driving a common aluminum disc (or two discs mounted on a single spindle).
Connection Diagram Principles: - Element 1: Current coil is in series with Line R. Potential coil is connected between Line R and Line Y. - Element 2: Current coil is in series with Line B. Potential coil is connected between Line B and Line Y. - Line Y acts as the common reference point.
Working: Each element produces an alternating magnetic flux from its voltage and current coils, which induce eddy currents in the aluminum disc. The interaction of these fluxes and eddy currents generates a driving torque. The driving torque of Element 1 is proportional to , and Element 2 is proportional to . Since they share the same spindle, the total driving torque on the disc is proportional to the total 3-phase active power (). A geared mechanism integrates the disc's revolutions over time to display Energy (kWh).
If the meter is moving too fast, it is over-registering energy. The speed of the disc is governed by a permanent braking magnet that induces opposing eddy currents. To slow the meter down, the braking magnet must be adjusted to move outwards, towards the edge (periphery) of the disc. Moving it outward increases the effective radius at which the braking force acts, increasing the braking torque and slowing the disc to the correct speed.