Download Introduction to DC Electric Motors

NSF SPIRIT Workshop
2006
Introduction to
DC ELECTRIC MOTORS
Motors Everywhere!
• The fan over the stove and in the microwave
oven
• The dispose-all under the sink
• The blender
• The can opener
• The washer
• The electric screwdriver
• The vacuum cleaner and the Dustbuster minivac
• The electric toothbrush
• The hair dryer
Source: http://electronics.howstuffworks.com/motor2.htm
More Motors . . .
•
•
•
•
•
•
•
•
•
•
The electric razor
Power windows (a motor in each window)
Power seats (up to seven motors per seat)
Fans for the heater and the radiator
Windshield wipers
Most toys that move have at least one motor (including
Tickle-me-Elmo for its vibrations)
Electric clocks
The garage door opener
Aquarium pumps
TEKBOT
In Short, EVERYTHING THAT MOVES
uses some type of motor!
DC Electric Motors
•
•
•
•
What They Are
How They Work
Their Components
Motor Ratings
– Torque, Speed, Voltage, Current, Gear Ratio
• Motor Control
DC Electric Motors
• Electric Motors convert electrical energy to
mechanical motion
• DC Electric Motors use Direct Current (DC)
sources as their source of electrical energy
• Mechanical motion results when electrical
current in a wire produces a magnetic field
which in turn, produces a force that moves the
shaft of the motor in rotational motion.
How DC Electric Motors Work
• The motion of a DC motor is caused by the interaction of
2 magnetic fields housed inside the motor.
• A permanent magnetic field exists in the stationary portion
of the motor, called the Stator.
• The opposing armature magnetic field exists on the
rotating portion of the motor, called the Rotor.
• The magnetic poles of the armature field will attempt to
line up with the opposite magnetic poles on the stator.
(Opposites ATTRACT).
• Once opposite poles align, the movement of the motor
would stop.
• However, to ensure continuous movement of the motor,
the poles of the armature field are electronically reversed
as it reaches this point, so it keeps turning to keep the
motor shaft moving along in the same direction!
Electromagnet
• How do we change the magnetic poles of
the armature magnetic field?
– We can control the poles of the armature
magnetic field because it is created using
electromagnetic windings.
– An electromagnet is a magnet created when
electricity flows through a coil. It requires an
DC power source (such as a Battery) to set up
the magnet.
– This contrasts to a permanent magnet (on
household refrigerators) that exists all the
time.
A Simple Electromagnet
• A Nail with a Coil of Wire
• Q - How do we reverse the poles of this
electromagnet?
• A – By reversing polarity of the battery!
DC Motor Operation Principles
• In a motor, the permanent magnetic field
of the Stator surrounds the Armature field
like shown here:
Brushed DC Motor Components
(Permanent Magnet) Brushed DC
Motor Components
• Stator is a Permanent Field Magnet
• Armature
– An electromagnet comprised of coils wound around 2 or more
poles of the metal rotor core
• Commutator
– Attached to the rotor and turns with the rotor to mechanically
switch direction of current going to the armature coils
• Brushes
– Stationary attached to battery leads. These metal brushes
touch the Commutator terminals as it rotates delivering electric
current to the commutator terminals.
• Axle or Shaft
– Moves in rotational motion
How the Commutator Works
• As the rotor turns, the commutator terminals also
turn and continuously reverse polarity of the
current it gets from the stationary brushes
attached to the battery.
Inside a Toy Motor
(Similar to TekBot Motor)
Toy DC Motor, cont.
• End Views of Motor
– Axle
– Battery Leads
• Axle will turn if connect
battery leads to a 9V
battery
• Reverse battery leads
and axle will turn the
Opposite direction!
• The white nylon cap on
the motor can be
removed to reveal…
A View of the Brushes
• Inside the Nylon cap
are the Brushes
• Brushes can be made
of various types of
metal.
• Their purpose is to
transfer power to the
commutator as it
spins.
Inside the Motor, cont.
• The Axle is the rotating part of
the motor that holds the
armature and commutator.
• This armature is comprised of
3 electromagnets.
• Each electromagnet is a set of
stacked metal plates with thin
copper wire wound around
each.
• The two ends of each coil wire
is terminated and wired to a
contact on the commutator.
• Thus, there are 3 commutator
contacts in all.
Inside the Motor, cont.
• The final piece is the
stator, a permanent
field magnet.
• It is formed by the
motor enclosure and
two curved
permanent magnets
(2 pole) shown.
TekBot Motor Ratings
•
•
•
•
•
“GM8 - Gear Motor 8 - 143:1 Offset Shaft”
143:1 gear motor
spins at 70RPM at 5V,
drawing 670mA at stall
generating 43 in*oz torque (free running at
57.6mA).
• Manufactured by Solarbotics
• http://www.solarbotics.com/
Torque Concepts
• The movement of the motor comes from the
interaction of magnetic fields.
• A magnetic force that is perpendicular to the
magnetic field and the current in the coils
delivers a rotational force or torque that turns the
axle of the motor.
• Intuitively, the higher the torque the greater the
force of rotational movement.
• The higher the motor input current, the greater
the torque on the output.
Speed Concepts
• Speed or rotation of the output shaft is
measured in rpm – revolutions per minute.
• The speed of rotation is directly proportional to
the voltage applied to the armature windings.
– This is a linear relationship up to the motor’s max
speed.
• These motors produce high speed, low torque
axle rotation, which is improved by a gear
reduction to reduce speed and increase torque
on the output shaft.
Characteristics of
Brushed DC Motors
• Very commonly used in everything from
toys to toothbrushes, electric toys to
mobile robots.
• Easy to control using simple control
circuitry
DC Motor Varieties
• Brush-type DC Motor
– Used for RPM under 5,000
– Simpliest to control
– Very common choice for hobby use
• Brushless DC Motor –
– Better suited for applications that require a large speed range
– Extra electronics and position sensors are required
•
Wound-field DC Motor
– Common in industrial applications
– Allows for wide range of precision speed control & torque control
• Permanent Magnet DC Motor
– The field magnet is a permanent magnet and does not need to be
activated by a current
• Intermittent vs. Continuous Duty
– Continuous Duty motors can operate without an off period.
• Electric motor power rating
– hp = (torque X rpm)/5,250
For Further Information
• Basics of Design Engineering - DC Motors
http://www.electricmotors.machinedesign.com/g
uiEdits/Content/bdeee3/bdeee3_5.aspx
• Overview of Motor Types Tutorial
http://www.oddparts.com/acsi/motortut.htm#DC_MOTO
R
• How Stuff Works
http://electronics.howstuffworks.com/motor4.htm