Newnes Audio and Hi-Fi Engineer's Pocket Book, 3rd Edition

139

Motors and speed control

Mains driven Mains recorder motors are of the synchronous

type as these are controlled by the mains frequency and run to a

constant speed. The rotor is a permanent magnet which follows

a rotating field set up by the stator windings. This is achieved

by splitting the phase to alternate poles, or by shading one pole

with a copper ring. Two-pole motors run at 3000 r.p.m. and

four-pole at 1500 r.p.m. at 50 Hz. Bearings are usually made of

sintered bronze (powdered metal compressed under heat)

which acquire lubricant from a surrounding felt pad by

capillary attraction. Being of spherical shape and retained by a

sprin ...