Description

The eddy-current coupling is an electromechanical torque-transmitting device installed between a constant-speed prime mover and a load to obtain adjustable-speed operation. Generally ac motors are the most commonly used pump drives, and they inherently operate at a fixed speed. Insertion of an eddy-current slip coupling into the drive train will allow desired adjustments of load speeds.

In most cases, the eddy-current coupling has an appearance very similar to that of its driving motor except that the coupling has two shaft extensions. One shaft, which operates at constant speed, is connected to the motor, whereas the other shaft, providing the adjustable-speed output, is connected to the load. A typical self-contained air-cooled eddy-current slip coupling is shown in Figure 1.

The input and output members are mechanically independent, with the output magnet member revolving freely within the input ring or drum member. An air gap separates the two members, and a pair of antifriction bearings maintain their proper relative position. The magnet member has a field winding that is excited by direct current, usually from a static power supply. Application of this field current to the magnet induces eddy currents in the ring. The interaction between these currents and magnetic flux develops a tangential force tending to turn the magnet in the same direction as the rotating ring. The net result is a torque available at the output shaft for driving a load. An increase or decrease in field current will change the value of torque developed, thereby allowing adjustment of the load speed. Field current, output torque, and load speed are usually not proportional. Therefore load torque characteristics and speed range must be known for proper adjustable-speed drive size selection.

Most eddy-current couplings have load speed control. An integral part of this system is a magnetic pickup that provides an indication of exact output speed and enables control of load speeds within relatively close tolerances regardless of reasonable variations in load

torque requirements. Slip-type adjustable-speed drives without load speed control will experience fluctuations in load speed when load torque requirements vary.

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