Page 655 - Mechatronics with Experiments

ELECTRIC ACTUATORS: MOTOR AND DRIVE TECHNOLOGY 641
name field-wound DC motor). Field-wound DC motors are used in high power applications
(i.e., 20 HP and above) where the use of permanent magnets are no longer cost effective.
Permanent magnet (PM) DC motors are used in applications below 20 HP.
Coil winding (either on the stator in the case of brushless DC motors, AC induction
motors, stepper motors or on the rotor in the case of brush-type DC motors) determines one
of the magnetic fields essential to the operation of a motor. The coil design question is the
question of how to distribute the coil around the perimeter of the stator or rotor. The design
parameters are [16]
1. the number of electrical phases,
2. the number of coils in each phase,
3. the number of turns on each coil,
4. the wire diameter, and
5. the number of slots and how each coil is distributed over these slots.
There are two types of windings in terms of the spatial distribution of wire on the stator
(Figure 8.20):
1. distributed winding, where each phase winding is distributed over multiple slots and
one phase winding has overlaps with the other windings (i.e., AC induction motors,
DC brushless motors),
2. concentrated winding where a particular winding is wound around a single pole (i.e.,
stepper motors).
Most common step motors have concentrated winding, whereas AC and DC motors have
distributed winding. In concentrated winding, one coil is placed around a single tooth. By
controlling the current direction in that particular coil, magnetic polarity (N or S) of that
tooth is controlled. Hence, a desired N and S pole pattern can be generated by controlling
each coil current direction and magnitude. In distributed winding, there are many variations
on how to distribute the coils. The most common type is a three-phase winding. The coil
can be distributed to generate two poles, four poles, eight poles, and so on the stator at any
given current commutation condition. By controlling the current in each phase, both the
magnitude and direction of the magnetic field pattern are controlled. It is common to view
the coil distribution in slots in a linear diagram by considering the unrolled version of the
motor stator and rotor.

A A A
B B
B B
A A
A A
Phase A B B
A Phase A
Phase B B A A B
B Phase B
Phase C
(a) (b)
FIGURE 8.20: Winding types on the stator: (a) distributed winding (i.e., AC induction motors,
brushless DC motors), (b) concentrated winding (i.e., stepper motors).