Automating Manufacturing Systems - Process Control and ...

continuous actuators - 24.7
ASIDE: When a motor is to be controlled
with PWM in two directions
the H-bridge circuit (shown below)
is a popular choice. These can be
built with individual components, or
purchased as integrated circuits for
smaller motors. To turn the motor in
one direction the PWM signal is
applied to the Va inputs, while the
Vb inputs are held low. In this
arrangement the positive voltage is
at the left side of the motor. To
reverse the direction the PWM signal
is applied to the Vb inputs, while
the Va inputs are held low. This
applies the positive voltage to the
right side of the motor.
Va
Vb
+Vs
-Vs
Vb
Va
Figure 24.6
PWM Bidirectional Motor Control Circuit
24.2.2 AC Motors
• Power is normally generated as 3-phase AC, so using this increases the efficiency
of electrical drives.
• In AC motors the AC current is used to create changing fields in the motor.
• Typically AC motors have windings on the stator with multiple poles. Each pole
is a pair of windings. As the AC current reverses, the magnetic field in the rotor appears to
rotate.
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