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Induction disk motor is a low-power, low-speed
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
Induction disk motor is a low-power, low-speed AC motor
An AC motor is an electric motor driven by an alternating current (AC). The AC motor commonly consists of two basic parts, an outside stator having coils supplied with alternating current to produce a rotating magnetic field, and an inside roto ...
that is primarily known for its use in electrical meters. It is also called a Ferraris disk after the inventor, Galileo Ferraris
Galileo Ferraris (31 October 1847 – 7 February 1897) was an Italian university professor, physicist and electrical engineer, one of the pioneers of AC power system and inventor of the induction motor although he never patented his work. Many ne ...
.
Construction
The rotor of the motor consists of a conductive disk, through which a shiftingmagnetic flux
In physics, specifically electromagnetism, the magnetic flux through a surface is the surface integral of the normal component of the magnetic field B over that surface. It is usually denoted or . The SI unit of magnetic flux is the we ...
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.
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.
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 electromechanicaltime switch
A time switch (also called a timer switch, or simply timer) is a device that operates an electric switch controlled by a timer.
Intermatic introduced its first time switch in 1945, which was used for "electric signs, store window lighting, apartm ...
es.
In the 1920s, the motors briefly found a niche in phonograph
A phonograph, later called a gramophone, and since the 1940s a record player, or more recently a turntable, is a device for the mechanical and analogue reproduction of sound. The sound vibration Waveform, waveforms are recorded as correspond ...
s, 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 watt
The watt (symbol: W) is the unit of Power (physics), power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantification (science), quantify the rate of Work ...
s) was vertical and directly connected to the turntable
A phonograph, later called a gramophone, and since the 1940s a record player, or more recently a turntable, is a device for the mechanical and analogue reproduction of sound. The sound vibration waveforms are recorded as corresponding phys ...
with no speed reducing gears. The motor had high torque
In physics and mechanics, torque is the rotational analogue of linear force. It is also referred to as the moment of force (also abbreviated to moment). The symbol for torque is typically \boldsymbol\tau, the lowercase Greek letter ''tau''. Wh ...
(6 inch-ounces), 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. 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
Three-phase electric power (abbreviated 3ϕ) is a common type of alternating current (AC) used in electricity generation, transmission, and distribution. It is a type of polyphase system employing three wires (or four including an optional n ...
currents become unbalanced (for example, due to an electrical fault
In an electric power system, a fault is a defect that results in abnormality of electric current. A fault current is any abnormal electric current. For example, a short circuit in which a live wire touches a neutral or ground wire is a fault. An op ...
), the disk rotates and a protective action is initiated.
References
Sources
* * * Induction motors {{electric-stub