Page 684 - Mechatronics with Experiments

670 MECHATRONICS

bs

br
q

Field frame
i q
d
i s
Rotor frame
ar
i d
θ s
θ r
Stator frame
as

FIGURE 8.44: Coordinate frames used to describe the dynamics of an AC induction motor.

for a DC motor and the same desired torque–current relationship for a vector controlled
AC motor,

T DC = K ⋅ i (8.263)
m T
T AC = K (i ) ⋅ i (8.264)
m T ds qs
Notice that the same linear relationship of the DC motor between torque and current can
be obtained. Let the “dq”-coordinate frame be fixed to the field vector of the rotor. Hence,
it rotates relative to the rotor with slip frequency (Figure 8.44). The torque gain K is a
T
function of the direct (parallel) component of the current in “dq” coordinate frame, and the
torque producing current is the “q” (perpendicular) component of the current.

8.6 STEP MOTORS

The step motor, also called a stepper motor, electromechanical construction is such that it
moves in discrete mechanical steps. A change in phase current from one state to another
creates a single step change in the rotor position. If the phase current state is not changed,
the rotor position stays in that stable position. In contrast, a brush-type DC motor keeps
accelerating for a fixed supply voltage condition until the back EMF voltage due a top
speed balances the supply voltage.
The basic position control of a stepper motor does not require a position sensor. It
can be position controlled in open loop (Figure 8.45). Whereas a DC motor must use a
position sensor in order to be controlled in position mode.

Controller Drive Enc Motor
FIGURE 8.45: Stepper motor control system
components. Position sensor feedback is
(Optional feedback)
optional.