Page 662 - Mechatronics with Experiments

648 MECHATRONICS
Torque
DC
i- constant

N S

0 1/2 rev 1 rev

Torque
DC
i- constant

N S

0 1/2 rev 1 rev
Torque
DC
i - constant

N S

0 1/2 rev 1 rev

FIGURE 8.28: Commutation and torque variation as a function of the angular position of the
rotor. Torque ripple magnitude and frequency is a function of the number of commutation
segments. At any given time, coils on one side of the line between the brushes (and the coils on
the other side) have a current in the opposite direction. The contribution of each coil to torque
production under a constant current depends on its angular position relative to the magnetic
field of the permanent magnets at that instant.

induction motors and lends itself to the same well established winding processes used in
manufacturing induction motors.
The operating goal is the same: maintain the field (stator) and armature (rotor)
magnetic fields perpendicular to each other at all times. If this can be accomplished,
the electromechanical power conversion relationship and torque generation in a BPMDC
motor would be identical to that of a brush-type PMDC motor. Of course, the difference
is in the commutation (Figure 8.29). In a brush-type motor, the magnetic flux generated
by permanent magnets (or electromagnets) of the stator is fixed in space. The magnetic
field generated by armature is also maintained fixed in space by the mechanical brush-
commutator assembly and perpendicular to that of the stator. In the case of the brushless