Another essential part of the electromechanical meter is a magnetic brake. Torque on the disk caused by interaction of fluxes tends to cause constant acceleration.

Without a brake, the speed of rotation would only be limited by the supply frequency, friction, and certain counter torques at higher speeds (discussed in later paragraphs concerning overload compensations).

Therefore, some method of limiting the rotor speed and making it proportional to power is needed. A permanent magnet performs these functions. As the disk moves through the field of the permanent magnet, eddy currents result in much the same manner as though the magnetic field were changing as previously described.

These eddy currents remain fixed in space with respect to the magnet pole face as the rotor turns. Again, as in the case of eddy currents caused by fluxes from the voltage and current coils, the eddy currents are maximum when the rate of cutting flux lines is greatest.

In this case the cutting of flux lines is caused by the motion of the disk, so the eddy currents are proportional to the rotational speed of the disk. They react with the permanent-magnet flux, causing a retarding torque which is also proportional to the speed of the disk.

This balances the driving torque from the stator so that the speed of the disk is proportional to the driving torque, which in turn is proportional to the power flowing through the meter. The number of revolutions made by the disk in any given time is proportional to the total energy flowing through the meter during that time interval.

The strength of the permanent magnet is chosen so that the retarding torque will balance the driving torque at a certain speed. In this way the number of watthours represented by each revolution of the disk is established.

This is known as the watthour constant (Kh) of the meter.

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