What is the cogging phenomenon in Hysteresis Brakes or Clutches?

Under certain operating conditions, it is possible to set up a salient pole condition on the brake rotor that can result in a cogging phenomenon referred to as torque ripple. The cogging condition, is often incorrectly diagnosed as a defective brake. Cogging torque in a hysteresis brake or clutch is the result of having established salient poles on the rotors magnetic material. The salient pole effect which causes the cogging phenomenon is a result of having removed the current from the device coil while its rotor was in a stationary position. The residual impression of alternating north-south polarities around the circumference of the magnetic element such as the rotor, cup, or disk will manifest themselves as numerous evenly spaced bumps, which can be felt throughout the rotation of the brake or clutch shaft. Cogging torque can be as high as about 20% to 30% of the full rated torque. The bumps will be most prominently noted when no power is applied to the brake or clutch coil, and ...  more