THE UNIVERSAL
MOTOR
by Fred Eady
Electric motors come in a seemingly
endless variety of shapes and sizes. If you’re
into robots and mechanical devices that
move about freely, DC (Direct Current)
motors capable of operating on battery
power are almost always your most
practical motor choice. However, not every
robot created by man or alien is a fully
mobile Robby running around on forbidden
planets. If your robot is a stationary
collection of nuts and volts that’s at home
working next to a wall outlet, you may
be able to use the advantages of an AC
(Alternating Current) power source to drive
your mechanical animal’s motors.
It is often desirable to be able to control a motor’s speed
and direction. I’ll bet that most of you have experience
with controlling the speed of a standard brushed DC motor.
The first brushed DC motor speed control circuit that comes
to my mind is shown in Schematic 1A and consists of a PIC
microcontroller squirting a PWM (Pulse Width Modulation)
signal into the gate of a MOSFET whose job it is to switch
current to a brushed DC motor. Adding a trio of MOSFETs
in Schematic 1B forms an H-bridge configuration that allows
us to change both the speed and the direction of the
brushed DC motor with a few bits of PIC I/O.
Can you conjure up a similar circuit in your mind’s eye
for a simple AC motor speed control? I see a PIC microcontroller punching an optoisolated DIAC (short for the words
DIode AC Switch) triggering a TRIAC, which is controlling
the flow of AC voltage to an AC motor (Schematic 1C). The
problem with the DIAC/TRIAC motor circuit is that the DIAC
and a series resistor have all of the “control” and that
control is very limited. Another problem with the AC motor
control circuit I’ve envisioned lies in the need to provide a
separate DC power supply for the PIC microcontroller. What
VDD
VDD
DRIVE A+
DRIVE B+
BRUSHED DC MOTOR
BRUSHED DC MOTOR
Schematic 1A. The duty cycle of the PWM signal that
is applied to the gate of the MOSFET determines the
speed of the motor. In this case, the duty cycle
percentage (0% to 100%) is directly proportional to
the speed of the motor shaft.
PWM IN
DRIVE B-
DRIVE A-
SCHEMATIC 1A
V+
SCHEMA TIC 1B
Schematic 1B. By selectively energizing diagonally
opposing MOSFETs, this H-bridge configuration
allows for both speed and direction control of the
DC motor. The PWM signal is applied to one of the
diagonally opposing MOSFETs while the other
associated diagonally opposing MOSFET is held in
an energized state.
FROM MICRO
MT2
MT1 ~
AC MOTOR
Schematic 1C. This is about as simple as it gets for
AC motor control. Fact is, we don’t have much
“control” here as the DIAC break over voltage
characteristics and the value of the resistor between
MT2 and the DIAC determine the TRIAC’s
triggering point.
SCHEMA TIC 1C
40 SERVO 11.2008