Eddy Current Drives:
Eddy Current Drives consists of an eddy current clutch placed between an induction motor running at a fixed speed and the variable speed load. Speed is controlled by controlling dc excitation to magnetic circuit of the clutch. Since motor itself runs at a fixed speed it can be fed directly from ac mains.
An eddy-current clutch is identical in principle to an induction motor in which both stator and rotor are allowed to rotate. Stator, which is coupled to driving induction motor, has dc winding which produces magnetic field rotating at the speed of stator. Rotor has a metal drum coupled to the load.
Eddy Current Drives are induced in rotor drum by stator magnetic field. Interaction between the stator field and eddy currents produces a torque which causes rotor to move with stator with a slip. Slip, and therefore, the load speed, can be controlled by controlling dc current through stator winding. Speed-torque characteristics are identical to an induction motor. Slip is given by
where ωms and ωmr are respectively the stator and rotor speeds. Since torque on either side of eddy current clutch is the same, ratio of output power Pm to input power Pin is given by
Equation (6.86) suggests that efficiency falls with speed. According to (6.87), speed reduction is obtained by wasting a power equal to sPin in the rotor drum. Minimum speed is usually restricted to 30% below the synchronous speed, because efficiency becomes too low and cooling of the rotor drum becomes difficult below this speed.
Load can be decoupled from induction motor by setting dc winding current to zero. Motor can now be started on no load. Load can be smoothly started by slowly increasing dc winding excitation.
Eddy current clutch is available in different constructions and sizes ranging from fraction of a kW to MW.
Advantages of Eddy Current Drives are: rugged, easy to maintain, reliable, stepless speed control with good speed regulation, controlled acceleration and soft start, high starting torque, high overload capacity and ability to handle impact loads. They have been widely used in blowers, compressors, conveyers, cranes, dredges, elevators, winders, line shafts and paper machines. However, they are rarely used in new installations, because of poor efficiency and cooling.