Introduction to

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Introduction to
NSF SPIRIT Workshop 2006
DC ELECTRIC MOTORS

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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 mini-vac
• The electric toothbrush
• The hair dryer

Source http//electronics.howstuffworks.com/motor
2.htm
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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!
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DC Electric Motors

• What They Are
• How They Work
• Their Components
• Motor Ratings
• Torque, Speed, Voltage, Current, Gear Ratio
• Motor Control

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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.

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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!

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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.

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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!

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DC Motor Operation Principles

• In a motor, the permanent magnetic field of the
  Stator surrounds the Armature field like shown
  here

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Brushed DC Motor Components

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(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

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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.

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Inside a Toy Motor(Similar to TekBot Motor)
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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

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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.

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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.

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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.

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TekBot Motor Ratings

• GM8 - Gear Motor 8 - 1431 Offset Shaft
• 1431 gear motor
• spins at 70RPM at 5V,
• drawing 670mA at stall
• generating 43 inoz torque (free running at
  57.6mA).
• Manufactured by Solarbotics
• http//www.solarbotics.com/

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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.

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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 motors
  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.

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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

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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

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For Further Information

• Basics of Design Engineering - DC Motors
  http//www.electricmotors.machinedesign.com/guiEdi
  ts/Content/bdeee3/bdeee3_5.aspx
• Overview of Motor Types Tutorial
• http//www.oddparts.com/acsi/motortut.htmDC_MOTOR
  
• How Stuff Works
• http//electronics.howstuffworks.com/motor4.htm