Electricity meter#Electromechanical meters

The Ferraris meters, named after Galileo Ferraris, is an electromechanical device for measuring electric energy, colloquially electricity meter, which is used to display the consumed, rarely also the injected electrical energy in single-or multi-phase AC voltage in the range of low voltage systems. It consists of a special form of induction motor, the rotor Ferrari, which has the shape of a circular aluminum plate, in conjunction with a mechanical counter.

Construction

Ferrari rotor consists of a rotatably mounted aluminum disc, which runs through the two alternating fields of the exciting coils. A coil is performed with a few turns, and denotes the so-called current path, the other is performed at a high impedance, and denotes the voltage path. The current flowing through the load electric current flows through the coil in the current path, the voltage ( grid voltage ) is applied to the coil in the voltage path. The force exerted by the magnetic fields on the disk torque is proportional to the product of current and voltage at any instant. In multi- phase systems a separate coil in the current and voltage path is because the possibility of an asymmetric load is, for each outer conductor necessary, their fields add.

The cores of the coils of the current or the voltage are arranged on the path Alumiumscheibe so that together they produce a rotating magnetic field that drives the disc into its induced eddy currents, such as in an asynchronous motor. The geometrical arrangement of the coils and the fact that the phase angle is shifted by 90 ° in the path of voltage due to the inductance, the torque at any point is proportional to the product of current and voltage, that the electrical active power. To adjust the correct phase shift in the voltage path in the calibration on the meter can be adjusted. Then the reactive power results in a time average to no torque and will not be counted. Next shorting of resistance wire are often present with which the torque can be adjusted for various power levels.

The Ferraris meter operates correctly only in so far as the network frequency is constant and the speed of the disk is much smaller than that of the traveling field. Furthermore, there must be a speed-proportional braking torque so that the integral of the instantaneous power (torque ) results over time as the number of revolutions per amount of energy. The braking torque is achieved by an eddy current brake with a permanent magnet, whose field through the disc also moves. The friction caused by the counter and the bearing must be negligible compared to the braking torque of the magnet. Only then the speed of the disk is proportional to the driving torque of the traveling field or the electrical active power. This is only possible above a certain minimum performance - including the counter is.

The Ferrari rotor drives a worm gear, a roller counter indicating the number of disk revolutions as energy (kilowatt hours).

In back-feed electricity into the grid Ferrari counter runs backwards normally, unless mechanical locks are available.

Ferraris meters are position sensitive and can be operated correctly only in the horizontal position of the aluminum disc.

For higher currents, the current coil is connected via a voltage converter, for higher voltages, the power coil connected to a voltage transformer. For the measurement of reactive power, the phase of the current can be rotated by the tension of the coil, for example by a bumblebee circuit by 90 °.

The electromechanical Ferraris meters are increasingly being replaced by electronic energy meter with the possibility for remote reading.