EDDY CURRENT VARIABLE SPEED DRIVE
	Eddy Current drive is a combination of a constant speed AC induction motor, coupled to an eddy current clutch. The function of an eddy current clutch is to transmit a controlled torque from the induction motor, (prime mover) to the load (driven machine). The medium of torque transmission is a controlled magnetic field, generated by a stationary field coil. A thyristor base electronic controller is used to convert a relatively small amount of AC power into DC power in order to excite the field coil.
WORKING PRINCIPLE
The Drum-fan assembly, forming the input member, is driven at a constant speed, by a fixed speed prime-mover which, in this case, is a flanged AC motor. The output member consists of a low inertia pole assembly, connected to the output shaft of the drive. Thus there is no mechanical linkage between the driving and driven members to cause any mechanical wear and tear. When the stationary field coil is excited by a low voltage, (typically 45 volts DC, from the solid state electronic controller) a strong magnetic field is developed around the coil. The magnetic lines of force from the coil are guided to the pole pieces, through the working air gaps, into the drum assembly and back to the coil thus forming a closed magnetic circuit. The flux path through the working gaps between the pole and the drum is shown in the adjoining figure. The flux lines are seen concentrated in the air gap near the pole faces, resulting in a varying magnetic field around the periphery of the drum. Since the drum rotates in relation to the pole assembly, the surface of the drum experiences a time rate of change of magnetic field. Depending on the relative speeds between these members, the induced cross currents (eddy currents), flow in the drum material. The direction of flow of these induced currents is perpendicular to the magnetic flow lines, as shown with conventional 0 and + signs. The reactive magnetic field caused by these cross currents is responsible for the torque transmission from the input to the output members, in these forms of torque transmitters. The extent of torque transmission, depends both on the magnetic field strength and the relative speeds (slip speed) between the drum and the pole. An increase in excitation, therefore, can correspondingly reduce the slip required to transmit a desired level of torque, thus providing an easy electrical means to control the drive speed.