PURPOSE: To improve efficiency and wear resistance by providing at least one associated disc located at a right angle to an axis for shape connecting and/or friction connecting joint to a rotor.

CONSTITUTION: An associated disc 9 located at right angles to an axis 15 is covered with an insulating layer 8 similarly to a rotor seat 28. The insulating layer 8 is joined to the associated disc 9 in shape-connecting relation via a cutout 12 of the associated disc 9. A ridge 21 and a groove 22 are provided for fixing a ball bearing and a groove 14 is provided in a transmission connection section 13. An associated projection 10, projecting beyond the associated disc 9, is manufactured of an insulating material and arranged in a star form. The groove 14 works for storing the shape connecting joint portion of the transmission. In this arrangement, flow transmission and efficiency are improved, an insulating gap is held thin throughout the contact range of a brush, and the brush is reduced in wear.