Page 690 - Mechatronics with Experiments

676 MECHATRONICS
Torque
Series connection
winding

Parallel connection
winding

Speed
FIGURE 8.52: Steady-state torque and speed curves for series and parallel
connected windings.

operation, two windings are connected in series and center tapped to ground. For bipolar
operation, two windings can be either parallel or serial connected and the current direction
is controlled by the drive (Figure 8.51b,c). The implications of connecting the windings in
series or in parallel in terms of the steady-state torque-speed characteristics of the motor
are shown in Figure 8.52.
Step loss and resonance are two fundamental problems inherent in stepper motors. If
the phase currents are switched so fast that the rotor cannot keep up with it, then step loss
occurs. The only way to correct the problem is to use a position sensor to detect the step loss
and command additional steps to compensate for it. Therefore, the maximum switching fre-
quency is limited. Higher switching frequencies can be used only if the rotor is accelerated to
high rotational speeds under controlled acceleration profiles (Figure 8.53). As the switching
frequency increases, there is less time for the phase current to develop due to the electrical
time constant of phase winding ( = L∕R). Therefore, at higher switching frequency the
torque capacity of the motor drops compared to the torque capacity at lower speeds.

Torque
Holding
torque Start-stop
curve

Slew curve

Start-stop Slew
range
range

Step / second
FIGURE 8.53: Torque capacity and step rate of stepper motors.