When the electric energy meter is connected to the circuit under test, alternating current flows through the current coil and voltage coil. These two alternating currents generate alternating magnetic flux in their iron core respectively. The alternating magnetic flux passes through the aluminum plate and induces eddy current in the aluminum disc. The eddy current is acted by force in the magnetic field, so that the aluminum plate can get the torque (active torque) and rotate. The greater the power consumed by the load, the greater the current passing through the current coil, the greater the eddy current induced by the aluminum disc, and the greater the torque of the aluminum disc. That is, the torque is proportional to the power consumed by the load. The greater the power, the greater the torque, and the faster the aluminum disc rotates. When the aluminum disc rotates, it is also affected by the braking torque generated by the permanent magnet, which is opposite to the active torque; the brake torque is directly proportional to the rotational speed of the aluminum disc, and the faster the aluminum disc rotates, the greater the braking torque. When the active torque and braking torque reach a temporary balance, the aluminum disc will rotate at a constant speed. The power consumed by the load is proportional to the number of revolutions of the aluminum disc. When the aluminum plate rotates, it drives the counter to indicate the consumed electric energy. This is the simple process of electricity meter work.
Although there are many types and models of mechanical electric energy meters (also called induction electric energy meters), their structures are basically similar. They are composed of measuring mechanism, compensation and adjustment device and auxiliary parts (shell, frame, end button box, and name plate). The following are several commonly used mechanical energy meters.