Rotor Assembly

The Rotor Assembly consists of the rotor shaft, a winding around an iron core, two pole pieces and slip rings.

The rotor shaft is pressed into the core, then six-fingered malleable iron pole pieces are assembled on the shaft against each end of the winding core.

The pole pieces are placed so that the fingers mesh but do not touch. When direct current is passed through the field coil winding, the fingers become alternately North and South poles.

As a result of this arrangement of poles, the magnetic flux lines will move in opposite directions between adjacent poles.

NOTE: Flux lines always move from North to South.

This arrangement provides for several alternating magnetic fields to intersect the stator as the rotor is turning.

Fig 1: Field Coil Windings And Six-Fingerpole Pieces

Courtesy of BMW OF NORTH AMERICA, INC.

Fig 2: Field Coil Magnetic Flux Lines

Courtesy of BMW OF NORTH AMERICA, INC.

A slip ring pressed on to the rotor shaft is connected to the two ends of the field winding. Two brushes are held by springs against the slip rings. One brush is connected through a switch to the battery B+, the other to the voltage regulator. The brushes conduct only the field current (2 to 5 amps).

Fig 3: Rotor Function Diagram

Courtesy of BMW OF NORTH AMERICA, INC.