Eddy current electric motors are a type of variable speed, alternating current electric motor used for a variety of applications and frequently used in industrial applications. The structure of such eddy current motors is well known and typically includes a support assembly on one side of which is secured a power input assembly. The power input assembly typically includes an alternating current induction motor having an output shaft and a cup-shaped drum secured to the shaft. The induction motor rotates the output shaft and drum at a constant speed determined by the induction motor's design and the frequency of the supplied alternating current power. The drum rotated by the induction motor is fabricated from a magnetically soft metal such as iron.
Opposite the input assembly, typically secured to the other side of the eddy current motor's support assembly, is a variable speed output assembly. The output assembly preferably includes a star-shaped inductor which fits snugly within, but does not mechanically contact, the power input assembly's cup-shaped drum. This inductor, which is also fabricated from a magnetically soft metal such as iron, is positioned within the drum on one end of the eddy current motor's output shaft. This shaft is supported within the output assembly and is rotatable together with the star-shaped inductor about an axis coaxial with that of the input assembly's drum.
Eddy current motors also typically include a stationary field coil secured to the output assembly adjacent to the inductor and proximate the rotating drum. In operation, an electric current is applied to the field coil producing a magnetic field which envelopes the inductor and the drum. The rotation of the drum about the star-shaped inductor in the presence of this magnetic field generates eddy currents in the rotating drum. The generation of such eddy currents results in a torque being applied to the inductor that urges the eddy current motor's output shaft to rotate.
The rotational speed of the eddy current motor's output shaft can be sensed with a tachometer generator. Properly applying the tachometer generator's output voltage to an electronic circuit that controls the current flowing through the field coil provides control of the eddy current motor's output shaft including the speed of rotation of the shaft. In addition, electronic control circuitry may be designed to control speed of rotation of the eddy motor's output shaft by varying a speed control electronic signal supplied thereto.
In principle, an eddy current motor should produce no torque on its output shaft if the current flowing through the field coil is reduced to zero. However, the rotating drum and inductor, as well as the respective mechanical structures of the input, output, and support assemblies of the previously known eddy current motors described above, are all fabricated from magnetically soft materials which retain some remanent magnetism even when no current flows through the field coil. Therefore, after initial operation of such eddy current motors, the input assembly's drum and the output assembly's inductor retain significant magnetization even when no current flows through the field coil.
During energization of the induction motor of previously known eddy current motors, the remanent magnetism present at the motor's drum and inductor causes a torque to be applied to the eddy current motor's output shaft even when no current flows through the field coil. Thus, while the induction motor is running and the eddy current motor's output shaft is freed from any driving load, the output shaft continues turning even if no current flows through the field coil. Therefore, known eddy current motors only provide complete removal of torque from an unloaded shaft after the induction motor is turned off and the induction motor's output shaft stops rotating. The torque present on the output shaft of eddy current motors known to date, when the induction motor is running, has deleterious effects in many industrial applications for variable speed electrical motor drives. Accordingly, it is desirable and beneficial in a variety of applications to provide an eddy current electric motor having control circuitry which substantially eliminates all torque from the output shaft without turning off the induction motor.
An example of a class of industrial applications which benefits from a variable speed eddy current electric motor whose output torque can be substantially reduced, and preferably be reduced to zero, is winding or unwinding webs of materials such as films, textiles, metal foils, or paper. In these winding applications, a roll receives or delivers a web which typically flows through a processing apparatus at a constant linear velocity.
For example, a roll receiving a web may be stationary when the web is first attached. After the web is secured and operation begins, the roll must then be accelerated immediately to its highest speed for winding the web since the roll is at its smallest diameter. As the diameter of the roll increases, its rate of rotation proportionally decreases. When the roll is fully wound, it must then be stopped quickly so the web may be disconnected and attached to the next empty roll.
If, rather than winding the web onto a roll, the web must be unwound for further processing, the torque requirements for the variable speed motor drive are reversed from those for winding the web. That is, the electric motor must start smoothly and slowly while accelerating the mass of the web already wound onto the roll. As the roll unwinds, the motor's speed must increase proportionally until the web is completely unwound at which time the roll must be stopped. In these types of web processing applications, the quality of the product can be adversely affected if the tension in the web changes as it is wound onto or unwound from the roll.
Another class of industrial applications which may require a variable speed eddy current electric motor whose output torque can be reduced to zero is driving low friction machine tools and conveyors. At certain times, the motion of such devices must be halted after which they must be restarted with their motion being gradually increased up to full operating speed. Such operation is difficult to achieve if the output torque of the variable speed driving motor cannot be reduced to zero while it remains energized.