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The design of an induction electrical meter uses pairs of coils, one using the current to be measured, one voltage on the terminals. The design of coils makes the magnetic fields in each pair out-of-phase, inducing the torque proportional to the current and voltage. A permanent magnet works as a brake ("retarding element") making the speed proportional to the torque.
In the 1920s, the motors briefly found a niche in , displacing the spring power due to the advantages of silent operation, constant speed, lack of vibration, and reliability. The shaft of a relatively powerful (35 ) was vertical and directly connected to the turntable with no speed reducing gears. The motor had high torque (6 ), giving it rapid acceleration while starting. The 78 to 80 revolutions per minute speed was controlled by a fly-ball governor.
The Ferraris disks were used in the electromechanical relays used for power system protection. The accompanying illustration shows a device where the disk rotation triggers the action for a circuit breaker (not shown). One of the three coils is wired in opposition to the other two, but uses twice the turns. This produces a situation in which the forces applied to the disk are roughly balanced and the disk does not rotate, held in place by adjustable force. If the three phase currents become unbalanced (for example, due to an electrical fault), the disk rotates and a protective action is initiated.
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