we need is a microcontroller-based AC motor control circuit
that gives us effective control of the AC power that is
being applied to the motor without having to pamper the
microcontroller with additional power supply circuitry. We
have our challenge. Let’s deliver the solution.
The Universal Motor
A universal motor is a variation of a standard DC
motor. However, the universal motor can spin its rotor shaft
using AC or DC power. In that this motor spins so well with
AC power, you’ll find that most of them are powered with
AC rather than DC. If the universal motor application
requires slower speeds and quieter operation, you’ll most
likely see it powered with DC.
The stator of an everyday DC motor is composed of a
stationary group of permanent magnet poles. A common
DC motor rotor uses it rotor windings in conjunction with
a commutator and brushes to work with and against the
stator’s magnetic field to rotate the motor shaft. DC motor
brushes are made up of a carbon/graphite material and are
placed in direct physical contact with the commutator. The
motor brushes are also electrically connected directly to the
motor’s power source.
The brushes and commutator are considered part of
the DC motor’s stator assembly as the brushes and
commutator are permanently mounted in a stationary
position. The commutator is designed in such a manner
as to act as a switch that is fed by the brushes. The
commutator switches incoming current between the coils
within the DC motor. This switching of current between the
motor coils is called commutation.
A DC motor’s rotor windings are energized in such a
fashion as to force the rotor winding to attract to a stator
magnetic pole. In other words, the rotor winding is given
an opposite polarity than that of the stator pole that it is
moving towards. Opposite polarities attract and the rotor
moves to align itself with the attracting stator pole. When
the rotor winding aligns with the attracting stator pole, the
commutator is positioned in relation to the brushes to provide the stator pole with an attracting polarity to the next
rotor winding. The newly attractive rotor winding forces the
rotation of the rotor shaft as it is now being forced to move
and align with the stator pole that was previously the
attraction of the rotor winding before it. The commutator
contacts are angularly arranged in order to insure the
correct electrical and magnetic positioning of the
commutator contact versus the angular position of the
rotor winding and the stator magnetic pole. Since the DC
motor’s stator magnetic poles never change their polarity,
we can force the DC motor to run in the opposite direction
by simply reversing the polarity of the power being applied
to the commutator by the brushes.
To understand the forces that cause a DC motor to run
in a particular direction, point your right hand away from
your body while holding your thumb skyward. Your fingers
represent the polarity of the stator’s magnetic field with the
tips of your fingers representing the stator’s South magnetic
pole and your wrist standing in for the stator’s North
magnetic pole. The direction of your thumb is the direction
of the current flow through the stator’s magnetic field. The
palm of your hand is the direction of the resultant force.
Rotate your hand while observing the position of your palm
versus your thumb. It will be obvious to the most casual
observer that when your thumb is pointing towards the
floor (opposite current flow), the force will be opposite to
when your thumb was reaching for the sky. This simple
hand twisting technique illustrates what scientists call the
Lorentz force. Applying an AC voltage to a DC motor such
as the one we’ve just discussed will result in the rotor
simply thumping back and forth as the polarity of the
rotor winding current is constantly changing due to the
continual reversal of power polarity applied to the brushes
and commutator on every AC half cycle. To experience
this phenomenon, just hook up one of those 12 VDC
RadioShack DC hobby motors to a 6. 3 VAC transformer’s
secondary windings. The DC hobby motor will whip its
shaft back and forth at a 60 Hz rate.
Conversely, a true AC motor will not run with the
application of a DC power source. That’s because the
rotation of a true AC motor’s output shaft depends on the
very thing that won’t allow our DC motor shaft to spin: the
continual rhythmic reversal of power source polarity.
Let’s take that DC motor we’ve been discussing and
modify it by replacing the permanent magnets in the stator
with electromagnets. We’ll rewire that same modified DC
motor to put all of its major components in series. That is,
the stator electromagnets are wired in series with the rotor
windings. Our modified DC motor will retain the original
brushes and commutator, which are also participants in the
series wiring scheme.
Now that we have replaced the stationary permanent
magnets with physically stationary electromagnets and
wired them in series with the rotor windings, our new DC
motor will no longer react to the changes in power source
polarity as we have come to expect. That’s because every
magnetic pole in our modified DC motor will change
polarity in step and negate any changes in the magnetic
force between the poles.
If we apply a DC power source to our modified DC
motor, the stator electromagnets will retain a fixed
magnetic polarity and function just as if they were
permanent magnets. The modified DC motor thinks it has
permanent stator magnets and everything will run just fine.
The Lorentz force right hand rule proves this out as the
North and South stator poles are fixed.
Changing the modified DC motor’s power supply polarity
will cause every magnetic pole to change its polarity. Thus,
reversing the current through the rotor windings also causes a reversal of the polarity of the stator electromagnets.
This simultaneous rotor winding and stator electromagnet
polarity reversal means that the modified DC motor will
always turn in the same direction no matter the polarity of
the power source applied to the motor brushes. Expanding
on the observation that the modified DC motor will always
turn in the same direction despite the input power polarity,
SERVO 11.2008 41
