DC Motors
DC Motors are widely used in
robotics because of their
small size and high energy
output.
Key characteristics of DC
motors include:
1. High Speed
2. Low Torque
3. Reversibility
Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.
DC Motor - How it Works
• A DC Motor has 4 major
components:
– A permanent magnet that
doesn’t move, called the
stator.
– An electromagnet (usually
wound bare wire)
– A frame on which the
electromagnet is wound,
called the armature
– A set of brushes for
transferring voltage to the
armature
Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.
DC Motor - How it Works
• If a loop of wire were placed within a magnetic field, and
current were applied to it, a magnetic field would be
induced in the loop and it would try to rotate until both
magnetic fields lined up.
• As the loop of wire rotates, its connection with the
brushes is broken, but because of momentum, the loop
will continue to rotate until, after 1800, the other sides of
the loop are in contact with the brushes.
• As long as current is applied to the brushes, the loop of
wire will continue to rotate again and again and the rotor
will continue rotation.
Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.
DC Motor - Label
Permanent Magnet
Armature
(Rotor)
Electromagnet
(Wound Wire)
Brushes
Voltage Source
Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.
Power, Torque and Speed
• The power of a DC Motor is proportional to
the product of its torque and its speed.
• As the speed of the motor decreases, the
torque increases proportionally until
maximum torque is achieved. At this point,
the motor is stalled, meaning that the
motor is not turning even though power is
being supplied to it. This is known,
appropriately, as the “Stall Torque.”
Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.
Power, Torque and Speed
• The DC Motor has two “End States”- the
Unloaded Speed (maximum speed, no
torque) and Stalled Speed (zero speed,
maximum torque).
• Current ratings of DC Motors are given at
the stall torque, since this is the point of
maximum current.
Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.
Power, Torque, and Speed
• Maximum power is achieved at a point
between Stall and Unloaded Speed, where
the speed and torque curves intersect.
• The speed of the DC Motor is directly
proportional to the applied voltage.
Increase the voltage for higher speeds and
decrease the voltage for lower speeds.
Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.
Direction of Rotation
• Direction of rotation depends upon the
polarity of the applied voltage.
• To change the direction of a DC Motor,
simply change the polarity of the applied
voltage.
Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.
Output Gears
• DC Motors run at very high speed with
very low torque – too low to be able to do
any useful work directly.
• If you were to connect a DC Motor directly
to your robot, you wouldn’t have enough
torque to overcome starting friction and
your robot wouldn’t move.
Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.
Output Gears
• To make a DC Motor do usable work, the speed
needs to be reduced and the torque needs to be
increased. Speed can be reduced by
decreasing the input voltage, but this also
decreases the power from the motor.
• The solution to this problem is to use a train of
gears. If you put a small gear on the shaft of the
motor to drive a larger gear, the larger gear will
rotate slower and, because it has a larger
diameter, it will have a larger torque.
Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.
Output Gears
• Even though this train of gears has a very
high gear ratio (the ratio of the number of
teeth on the driven gear to the number of
teeth on the driving gear), it is called a
Gear Reducer.
Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.
Pulse Width Modulation (PWM)
• Pulse Width Modulation is used when the
amount of power delivered to the DC Motor is to
be reduced without decreasing the input voltage.
• Full power to the Motor is switched on and off
rapidly, usually 60 times per second.
• To decrease the power output to 50%, the motor
has full voltage to it 50% of the time and zero
voltage to it 50% of the time for each cycle.
Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.