Eddy current
Eddy current drives are slip-controlled systems the slip energy of which is necessarily all dissipated as heat. Such drives are therefore generally less efficient than AC/DC-AC conversion based drives. The motor develops the torque required by the load and operates at full speed. The output shaft transmits the same torque to the load, but turns at a slower speed. Since power is proportional to torque multiplied by speed, the input power is proportional to motor speed times operating torque while the output power is output speed times operating torque. The difference between the motor speed and the output speed is called the "slip speed". Power proportional to the slip speed times operating torque is dissipated as heat in the clutch. While it has been surpassed by the variable-frequency drive in most variable-speed applications, the eddy current clutch is still often used to couple motors to high-inertia loads that are frequently stopped and started, such as stamping presses, conveyors, hoisting machinery, and some larger machine tools, allowing gradual starting, with less maintenance than a mechanical clutch or hydraulic transmission.