Introduction to

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Introduction to
NSF S.P.I.R.I.T. Workshop 2007
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
• Your TEKBOT

In Short, EVERYTHING THAT MOVES uses some type of
motor!
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Important Concepts

• How Motors Work
• The Components of a DC Electric Motor
• Whats inside the motor box?
• The parts of the motor and what they do!
• Motor Control
• How to change directions
• Concepts of Speed, Torque, Gear Ratio
• Practical considerations
• Noise, slippage, limits of operation speed,
  torque

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DC Electric Motors

• Electric Motors or Motors convert electrical
  energy to mechanical motion
• Motors are powered by a source of electricity
  either AC or DC.
• DC Electric Motors use Direct Current (DC)
  sources of electricity
• Batteries
• DC Power supply
• Principle of How Motors Work
• Electrical current flowing in a loop of wire will
  produce a magnetic field across the loop.
  
• When this loop is surrounded by the field of
  another magnet, the loop will turn, producing a
  force (called torque) that results in mechanical
  motion.

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Motor Basics

• Motors are powered by electricity, but rely on
  principles of magnetism to produce mechanical
  motion.
• Inside a motor we find
• Permanent magnets,
• Electro-magnets,
• Or a combination of the two.

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Magnets

• A magnet is an object that possesses a magnetic
  field, characterized by a North and South pole
  pair.
• A permanent magnet (such as this bar magnet)
  stays magnetized for a long time.
• An electromagnet is a magnet that is created when
  electricity flows through a coil of wire. It
  requires a power source (such as a battery) to
  set up a magnetic field.

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A Simple Electromagnet

• A Nail with a Coil of Wire
• Q How do we set up a magnet?
• A The battery feeds current through the coil of
  wire. Current in the coil of wire produces a
  magnetic field (as long as the battery is
  connected).

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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 the polarity of the battery!

S
N

-
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The Electromagnet in a Stationary Magnetic Field

• If we surround the electromagnet with a
  stationary magnetic field, the poles of the
  electromagnet will attempt to line up with the
  poles of the stationary magnet.
• The rotating motion is transmitted to the shaft,
  providing useful mechanical work. This is how DC
  motors work!

OPPOSITE POLES ATTRACT!
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DC Motor Operation Principles

• Once the poles align, the nail (and shaft) stops
  rotating.
• How do we make the rotation continue?
• By switching the poles of the electromagnet. When
  they line up again, switch the poles the other
  way, and so on.
• This way, the shaft will rotate in one direction
  continuously!

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Motor Terminology

• Thus, the motion of a DC motor is caused by the
  interaction of two magnetic fields housed inside
  the motor.
• These two magnetic fields can be described by
  where they are located inside the motor.
• The stationary parts of the motor make up the
  STATOR.
• The Stator Stays Put!
• The rotating parts of the motor constitute the
  ROTOR.
• The Rotor Rotates!
• The Stator houses the Permanent Field Magnet.
• The electronically-controlled magnet, called the
  Armature, resides on the Rotor.

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Motor Terminology

• 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!
• This electronic switching of the Armature poles
  is accomplished using Brushes and Commutators.

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Brushed DC Motor Components

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Brushed DC Motor ComponentDescriptions

• The Stator is a Permanent Field Magnet
• The Armature
• Is 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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Brushed DC Motor Components

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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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Controlling Motor Direction

• To change the direction of rotation
• Simply switch the polarity of the battery leads
  going to the motor (that is, switch the and
  battery leads)

Direction of Rotation
CW
CCW
-
-
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Controlling Motor Direction

• In the TekBot, this switching is done using an
  H-bridge motor control circuit.
• A signal is sent from your hand-held tether to
  the TekBot when you tell each wheel to go forward
  or reverse.
• This signal goes to the H-bridge circuitry on the
  TekBot which sends the correct polarity to the
  battery leads wired to the TekBot motors to
  accomplish the desired rotation.

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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.
  (3-Pole DC Motor)
• 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 1 North, 1
  South) shown.

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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 - 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 of 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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TekBot Motor Ratings

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

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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
• Small, Cheap
• Generally not used in industrial applications

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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 (a.k.a AC Synchronous Motor)
• Better suited for applications that require a
  large range precise speed
• Extra electronics for control 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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References / For Further Info

• 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 - Motors
• http//electronics.howstuffworks.com/motor4.htm
  
• Magnets Defined
• http//en.wikipedia.org/wiki/Magnet
• Presentation Created by Alisa N. Gilmore, P.E.,
  University of Nebraska-Lincoln, NSF SPIRIT, July
  2007

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3 Pole DC Motor
http//www.solarbotics.net/starting/200111_dcmotor
/200111_dcmotor2.html