this leads us to conclude that the modified DC motor will
run in one direction only with an AC power source, as well.
Imagine your right hand detached from your body and
floating in 3D space. Reverse the direction of your hand to
point towards you (reverse the polarity of the stator’s
magnetic field) while rotating your thumb towards the floor
(reverse the polarity of the rotor winding current). The force
(your palm) ends up in the same position as when your
hand was pointed away and your thumb pointed skyward.
Our modified AC/DC-capable motor is the a universal
motor. Most of your AC-powered kitchen appliance motors
and most vacuum cleaner motors are universal. I’ve led you
down this path because that’s the type of motor we are going
to drive with our PIC microcontroller-based controller. Our
target universal motor is a product of Ametek Lamb Electric.
The one shown in Photo 1 is an Ametek 116207-00, two-stage tangential bypass discharge 120 VAC vacuum motor.
The 116207-00 is rated at 1,000 watts. Typical input current
draw for this motor is quoted as 8. 6 amperes when pulling
a respectable vacuum through a 19 mm orifice. If you have
a robotic application that requires creating a vacuum, that’s
great. However, the universal motor controller circuitry
we’re about to design can be adapted to drive most any
motor you may wish to employ in your robotic applications.
The Universal Motor
I realize that many of you work with potentially
hazardous mechanical tools and devices on a daily basis.
Every metal head I’ve ever had the good fortune to meet
knows that safety is paramount when working with and
around metal-forming tools. For you electron heads, safety
should also be at the top of your list when working with
the universal motor controller electronics. The controller circuitry you see in Schematic 2 is driven by potentially lethal
120 VAC mains power. Do not under any circumstances
handle the universal motor controller electronics when the
120 VAC mains power is applied to it. If you don’t want to
release the magic smoke from your electronic tools, never
attach a PIC programmer or debugger device to a universal
motor controller when it is powered directly from the 120
VAC mains. If you want to probe the controller circuitry, get
the mains power, behind an
isolation transformer first. Like
anything else, the universal
motor controller circuitry is
best understood when
digested in small chunks. So,
as you study the components
in Photo 2, think of the
controller electronics as three
cooperative subsystems: an AC control subsystem; a DC
power subsystem; and a microcontroller subsystem. The AC
control subsystem is responsible for interfacing the motor
with the 120 VAC. The PIC18F2620 microcontroller depends
upon the DC power subsystem for its power supply needs.
The STMicroelectronics BTA16-600CW snubberless TRIAC —
which is the heart of the AC control subsystem — is under the
control of the PIC18F2620-based microcontroller subsystem.
The AC control subsystem provides overall protection
for the controller circuitry with a 10 ampere fuse and a zinc
oxide varistor placed in series with and across the 120 VAC
mains supply, respectively. A 470 nF X2-type capacitor (C1)
also traverses the 120 VAC supply to limit electromagnetic
interference (EMI). Believe it or not, C1 is not there for the
controller circuitry. It’s there to restrict harmonic pollution
of the 120 VAC mains. Take another look at Photo 2 and
you’ll see that the entire AC control subsystem is contained
to the left of the area delineated by the TRIAC’s heatsink.
In this instance, we are driving an inductive load and
one would normally see a resistor/capacitor snubber
network placed across the TRIAC’s MT1 and MT2 terminals
to reduce the possibility of false triggering of the TRIAC.
The BTA16-600CW is designed to work against the high
instantaneous voltages that occur during TRIAC
commutation that can force a TRIAC to conduct
unexpectedly. In the case of a TRIAC, commutation is the
switching from an ON state (TRIAC conducting) to an OFF
state (TRIAC not conducting). When an AC signal is applied
to a TRIAC and a trigger is applied to the TRIAC gate during
a time when the AC signal is not crossing zero, the TRIAC
will conduct until the next zero crossing of the AC signal. If
a trigger is not applied to the TRIAC gate following the zero
crossing event, the TRIAC will remain in a nonconductive
state until it is triggered again. The TRIAC’s ability to
reliably turn itself off (commutate) and be triggered into
conduction at will by our PIC within the time domain of an
AC cycle are the keys to controlling the amount of power
Photo 2. Although you can easily breadboard a universal
motor controller, building up your own on a printed circuit
board like this is recommended. Note that there are no
exposed printed circuit board traces or lands, which reduce
the probability of you getting your fingers fried. Be sure to
use X2-type capacitors for C1 and C2 as X2z are approved for
safe use in this type of AC circuit.
Photo 1. This universal motor
could be a blender motor or a
kitchen mixer motor. As long as
the motor is considered to be
universal, it’s a safe bet that our
core AC motor controller circuitry
can be adapted to drive it.
SERVO 11.2008 43