Induction disk motor

Induction disk motor is a low-power, low-speed AC motor that is primarily known for its use in electrical meters.^[1] It is also called a Ferraris disk after the inventor, Galileo Ferraris.

Induction disk from an electrical meter

Construction

The rotor of the motor consists of a conductive disk, through which a shifting magnetic flux is applied by one or more split phase electromagnetic coils. Eddy currents are generated in the disk and the reaction between the eddy currents and the flux produces the driving torque.^[1]

The design of an induction electrical meter uses two coils, one using the current to be measured, one voltage on the terminals. The design of coils makes the magnetic fields in them out-of-phase, inducing the torque proportional to the current. A permanent magnet works as a brake ("retarding element") making the speed proportional to the torque.^[2]

Applications

Due to its low efficiency, the induction disk motor applications are limited. The motors are best known for their use in the common household induction-type electrical meters. The motors were also used in electromechanical time switches.^[1]

In the 1920s, the motors briefly found a niche in phonographs, 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 watts) was vertical and directly connected to the turntable with no speed reducing gears. The motor had high torque (6 inch-ounces), giving it rapid acceleration while starting. The 78 to 80 revolutions per minute speed was controlled by a fly-ball governor.^[1]

Protective relay checking the balancing of three phases based on the Ferraris disk

The Ferraris disks were used in the electromechanical relays used for power system protection. An illustration on the right shows a device where the disk rotation triggers the action for a curcuit 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.^[3]