Ramson & sons supplies ac variable speed commutator motors that are essentially 3-phase induction motors with an emf injected into the secondary circuit in order to give speed control. The injected emf is obtained from a commutator mounted on the rotor. The important construction difference between this motor and the ordinary induction motor is that the primary winding of the former is on the rotor and is fed through the slip rings while the secondary winding is on the stator. Each secondary phase is connected to a pair of movable brushes [rockers], which collects from the commutator the required emf for injection into the secondary circuit. Two-brush rockers are fitted with toothed racks over a portion of their periphery. The teeth projecting inwards and engaging respectively with the two sides of a pinion mounted between them on a vertical shaft. This pinion movement determines the brush separation. Control of speed is obtained by turning the pinion by means of a hand wheel. For remote control operations, the pinion can be motorized. Industrial applications elude cranes and hoists, fans and centrifugal pumps, paper making machinery, ring spinning frames, etc.
