Transcript Introduction to DC Electric Motors

NSF S.P.I.R.I.T. Workshop
2007
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 mini-vac
 The electric toothbrush
 The hair dryer
Source: http://electronics.howstuffworks.com/motor2.htm
More Motors . . .
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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!
Important Concepts
 How Motors Work
 The Components of a DC Electric Motor
 What’s 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
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.
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.
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.
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).
A Simple Electromagnet
 A Nail with a Coil of Wire
S
N
+
-
 Q - How do we reverse the poles of this
electromagnet?
 A – By reversing the polarity of the battery!
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.
OPPOSITE
POLES
ATTRACT!
 The rotating motion is transmitted to the shaft, providing
useful mechanical work. This is how DC motors work!
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!
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.
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.
Brushed DC Motor Components
Brushed DC Motor Component
Descriptions
 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
Brushed DC Motor Components
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.
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
+
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.
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. (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.
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.
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.
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 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.
TekBot Motor Ratings
 “GM8 - Gear Motor 8 - 143:1 Offset Shaft”
 143:1 gear motor (“gear ratio”)
 spins at 70RPM at 5V, (maximum speed)
 drawing 670mA at stall (“stall current”)
 generating 43 in*oz torque (free running at
57.6mA).
 Manufactured by Solarbotics
 http://www.solarbotics.com/
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
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
References / For Further Info
 Basics of Design Engineering - DC Motors
http://www.electricmotors.machinedesign.com/guiEd
its/Content/bdeee3/bdeee3_5.aspx
 Overview of Motor Types Tutorial
http://www.oddparts.com/acsi/motortut.htm#DC_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
3 Pole DC Motor
http://www.solarbotics.net/starting/200111_
dcmotor/200111_dcmotor2.html