Transcript AC/DC and Stepper Motors

AC/DC, Stepper Motors and
Gearing
CSC 338 – Robotics and Intelligent Agents
Theodore Trotz
Magnetism  Electricity
• The link between magnetism and electricity
was first discovered during an experiment
– Hans Christian Ørsted discovered that a compass needle deflected from true
north when electric current flowing through a wire was switched on and off
• Expanded on by Albert Einstein in 1905
with his theory of Special Relativity
– Connects Electricity and Magnetism
War of Currents
• Alternating Current – Direction between load and
generator reverses cyclically
– Promoted in late 1800s by Nikola Tesla and George
Westinghouse
• Direct Current – Direction between load and
generator is constant in direction
– Promoted by Thomas Edison
• AC eventually won as the better way to distribute
power.
– AC cables could transmit much further than DC
– AC cables were smaller as well
Generators and Motors
• All contemporary Power Plants work
essentially the same way
– Mechanical energy is converted into electrical
energy
– A coil of wire is spun inside a magnetic field
exciting the electrons
• The reverse conversion is done by an electric
motor
AC Motors
• Two Main Types of AC Motors
– Synchronous – Rotates at exactly the supply
frequency or submultiples of that frequency
– Induction – Rotates slightly slower than the supply
frequency
Synchronous Motors
• Power Plants use synchronous generators
– It’s important to keep the supply frequency
constant
• Used where high precision is required
• Seen in Stepper Motors
• Clocks also employ synchronous motors
Induction Motors
• Induction motors use a time varying magnetic
field to move the rotor
• The rotor is carried around the magnetic field,
but at a slightly slower rate
– Induction motors are also known as asynchronous
motors
– The difference between the rotors output and the
magnetic field is called slip which increases with load
• Induction motors often use a squirrel cage design
– http://www.youtube.com/watch?v=PYesQZ20Kwc
DC Motors
• Two Main Types of DC Motors
– Brushed
– Brushless
• Brushed Motors have two brushes which physically
contact rotors split ring
– This ring supplies the charge onto the rotors coil, which is
typically suspended between permanent magnets
• Brushless Motors require a motor controller to convert
DC to AC
– The design is essentially the opposite of a brushed motor;
permanent magnet on the inside and a varying
electromagnetic field stator.
Stepper Motors
• A brushless, synchronous motor that can divide a
full rotation into a large number of steps
• Require motor controller or drive circuits
• Consist of multiple toothed electromagnets
around a central gear-shaped piece
of iron
• Can be viewed as a synchronous
AC motor with the number of
stators increased
Gear Ratios
• It may be necessary to increase or reduce the speed at
which the output shaft turns. For that we need to
understand gear ratios
• This is easy if we understand the circumference of a
circle C = 2 * pi * R
– If two gears are meshed together, one with twice the
circumference, it will yield a 2:1 ratio
• Teeth are put on gears for precision and to prevent
slippage
– To determine the ratio, count the number of teeth on each
gear and divide
• Consider one gear with 60 teeth and another with 20. 60/20 = 3
which produces a 3:1 ratio