Page 661 - Mechatronics with Experiments

ELECTRIC ACTUATORS: MOTOR AND DRIVE TECHNOLOGY 647
F

N S N + S

F

F

N + S N S

+
Detent position
T = 0

No detent position when
N S
there are many conductor pairs

FIGURE 8.27: DC motor operating principles: obtaining a continuous torque by winding a coil
around a rotor.

̇
the strength of the magnetic field. This is represented by the k (t) term in Equation 8.176.
e
Therefore, both the motor and generator actions are at work at the same time during the
operation of a DC motor.
Figure 8.28 shows the brush and commutator arrangement and torque as a function
of rotor position for a different number of commutator segments. Ideally, the larger the
number of commutators, the smaller the torque ripple is. However, there is a practical
limit on how small the brush-commutator assembly can be sectioned. If we neglect the
torque ripple due to commutation resolution, torque is proportional to the armature current
for a given permanent magnet field and independent of the rotor angular position. In a
PMDC motor, the magnetic field strength is fixed, and current is controlled by a drive (the
term drive is used to describe the amplifier and power supply components together as one
component).
A brushless permanent magnet DC (BPMDC) motor is basically an “inside-out”
version of the brush-type PMDC motor (Figure 8.29). The rotor has the permanent magnets,
and the stator has the conductor windings, usually in three electrically independent phases.
The stator winding of brushless servo motors is similar to the stator winding of traditional