Abstract: The electromagnetic clutch 1 includes a driven shaft 3, an armature assembly 4, a rotor and a coil assembly. The armature assembly 4 is mounted on the driven shaft 3. The rotor 7 is driven by a belt that engages the poly V-groove 7d. The coil assembly is supported in a groove 7a in the rotor. When the coil assembly is energized the armature 17 of the armature assembly 4 is pulled axially into contact with the rotor 7 and torque is transmitted from the rotor to the driven shaft 3. The coil assembly includes a magnetizing coil 21 enclosed in a bobbin 19 and a cover 20 and secured in a yoke 18. The bobbin 19 includes a terminal holder 19j. A terminal holder 19j receives a terminal assembly 23 and is held in position by a terminal housing 28. The terminal assembly 23 includes two terminals 24 connected by a diode 25. The diode 25 and a portion of each terminal is embedded in an enclosing member 27.

Abstract: Rotation slip between the rotor and the armature of an electromagnetic clutch results in the exciting current I in the electromagnetic coil decreasing locally abruptly to form a unique waveform. The locally abruptly rising variations in the current is taken out as a locally abruptly variation (an alternating current component) in an inverted and amplified voltage, and the alternating current component is applied to a comparing circuit (400). The instantaneous value of the alternating current component is compared with a threshold which rises as the rotational speed of the rotor becomes higher, and while the instantaneous value exceeds the threshold, a comparison output is outputted. The comparison output is applied to a continuous waveform confirming circuit (500), and if the predetermined number of comparison outputs are detected with a predetermined time after a first comparison output has been generated, it is determined that a rotation slip has occurred.