ch14 intro to motor controls
TRANSCRIPT
Bridging Theory in PracticeTransferring Technical Knowledgeto Practical Applications
Introduction to Motor Control
Introduction to Motor Control
Intended Audience:• Individuals with an interest in learning about electric motors and how they are
controlled• A simple understanding of magnetics is assumed
Topics Covered:• What is an electric motor?• What are some common types of electric motors?• How do these electric motors work?• How these motors are controlled.
Expected Time: • Approximately 90 minutes
Introduction to Motor Control
Agenda• Introduction to Electromagnets and Electric Motors• What Is Motor Control? • What Are Some Common Types of Motors? • Permanent Magnet DC Motors• Stepper Motors• Brushless DC Motors• Summary of Motors and Motor Control Circuits
Agenda
• Introduction to Electromagnets and Electric Motors• What Is Motor Control? • What Are Some Common Types of Motors? • Permanent Magnet DC Motors• Stepper Motors• Brushless DC Motors• Summary of Motors and Motor Control Circuits
What Is a Permanent Magnet?• A piece of iron or steel which produces a magnetic field
• Found in nature as magnetite (Fe3O4) lodestones
• Magnetic field causes the permanent magnet to attract iron and some other materials
• Two ends of the permanent magnet are usually designated North and South
• Opposite magnet ends attract and like magnet ends repel
What Is an Electromagnet?
• Electromagnets behave like permanent magnets…
… but their magnetic field is not permanent
• Magnetic field is temporarily induced by an electric current
How Do You Make an Electromagnet?
• Start with an iron bar
How Do You Make an Electromagnet?
• Start with an iron bar• Wrap a wire around the iron bar
How Do You Make an Electromagnet?• Start with an iron bar• Wrap a wire around the iron bar• Connecting a battery causes a current to flow
in the wire
Current
+ -
How Do You Make an Electromagnet?• Start with an iron bar• Wrap a wire around the iron bar• Connecting a battery causes a current to flow in the wire• The current induces a magnetic field creating an
electromagnet
Current
SOUTHNORTH
+ -
How Do You Make an Electromagnet?
• Reversing the current direction, reverses the polarity
Current
+-
NORTHSOUTH
How Do You Make an Electromagnet?
• Reversing the current direction, reverses the polarity
• If the current is stopped, the induced magnetic field decays to 0
Current
+-
NORTHSOUTH
Electromagnets andElectric Motors• We can use electromagnets in electric motors to convert
electrical energy to mechanical work…
+
Electric Motor
12V-
ElectricEnergy
• Electric motors are used to perform a mechanical task by using electricity– Open a sunroof– Lift a power antenna– Control windshield wiper
What Is an Electric Motor?
STATOR
ROTOR
• An electric motor has two basic parts:– The stationary part is called the stator. – The rotating part of the electric motor is called the
rotor.
What Is an Electric Motor?
STATOR
• Electrical energy creates a rotating magnetic field inside the motor causing the rotor to rotate, creating mechanical motion
ROTOR
Where Are Electric Motors Used?
Electric motors are used in many different automotive applications:
Power windowsPower seatsPower mirrorsFansWindshield wipersWindshield washer pumpsStarter motorElectric radio antennaeDoor locksInformation gauges
SunroofBrakesPower steeringFuel pumpWater pumpHybrid and electric vehicles Cruise controlThrottle plate controlAir ventsOthers
Agenda
• Introduction to Electromagnets and Electric Motors• What Is Motor Control? • What Are Some Common Types of Motors? • Permanent Magnet DC Motors• Stepper Motors• Brushless DC Motors• Summary of Motors and Motor Control Circuits
• The controlled application of electrical energy to a motor to elicit a desired mechanical response
– Start / Stop– Speed– Torque– Position
• Significant amount of electronics may be required to control the operation of some electric motors
What Is Motor Control ?
Control of Electromagnetics• Much of the physical design of an electric motor and its control system
are related to the switching of the electromagnetic field
• There is a mechanical force which acts on a current carrying wire within a magnetic field
• The mechanical force is perpendicular to the wire and the magnetic field
• The relative magnetic fields between the rotor and stator are arranged so that a torque is created, causing the rotor to rotate about its axis
Agenda
• Introduction to Electromagnets and Electric Motors• What Is Motor Control? • What Are Some Common Types of Motors? • Permanent Magnet DC Motors• Stepper Motors• Brushless DC Motors• Summary of Motors and Motor Control Circuits
• There are many different types and classifications of electric motors:
Permanent magnet DC motorStepper motorBrushless DC motorWound field motorUniversal motorsThree phase induction motorThree-phase AC synchronous motors Two-phase AC Servo motors torque motorsShaded-pole motor split-phase induction motor capacitor start motor Permanent Split-Capacitor (PSC) motor Repulsion-start induction-run (RS-IR) motor
Repulsion motorLinear motorVariable reluctance motorUnipolar stepper motorBipolar stepperFull step stepper motorHalf step stepper motorMicro step stepper motorSwitched reluctance motorShaded-pole synchronous motor Induction motorCoreless DC motor Others......
Types of Electric Motors
Permanent Magnet DC Motor• Similar in construction to the introductory example• Metallic contacts (brushes) are used to deliver electrical energy• Rotational speed proportional to the applied voltage • Torque proportional to the current flowing through the motor• Advantages:
+ Low cost (high volume demand)+ Simple operation
• Disadvantages:– Medium efficiency – Poor reliability (brush, commutator wear out)– Strong potential source of electromagnetic interference
Stepper Motor• Full rotation of electric motor divided into a number of "steps" • For example, 200 steps provides a 1.8o step angle• A stepper motor controller can move the electric motor one step (in
either direction) by applying a voltage pulse• Rotational speed is controlled by changing the frequency of the voltage
pulses• Advantages:
+ Low cost position control (instrument gauges)+ Easy to hold position
• Disadvantages:– Poor efficiency– Requires digital control interface – High motor cost
Brushless DC Motor• Similar to a permanent magnet DC motor• Rotor is always the permanent magnet (internal or external)• Design eliminates the need for brushes by using a more complex drive
circuit• Advantages:
+ High efficiency+ High reliability+ Low EMI+ Good speed control
• Disadvantages:– May be more expensive than "brushed" DC motors– More complex and expensive drive circuit than "brushed" DC
motors
Agenda
• Introduction to Electromagnets and Electric Motors• What Is Motor Control? • What Are Some Common Types of Motors? • Permanent Magnet DC Motors• Stepper Motors• Brushless DC Motors• Summary of Motors and Motor Control Circuits
How Does a Permanent Magnet DC Motor Work?
• "DC Motors" use magnets to produce motion– Permanent magnets
NORTHSOUTH
• "DC Motors" use magnets to produce motion– Permanent magnets– An electromagnet armature
How Does a Permanent Magnet DC Motor Work?
NORTHSOUTH
NORTHSOUTH
• Electromagnet armature is mounted on axle so that it can rotate
Permanent Magnet DC Motor Rotating Armature
NORTHSOUTH
• Electromagnet armature is mounted on axle so that it can rotate
• A commutator makes an electrical contact with the motor's brushes
Permanent Magnet DC Motor Commutator and Brushes
Permanent Magnet DC Motor Commutator Structure
• Commutator is comprised of two "near-halves" of a ring
• Commutator is comprised of two "near-halves" of a ring• Mounted on the armature's axle to rotate with the rotor
Armature
Permanent Magnet DC Motor Commutator Structure
Permanent Magnet DC Motor Commutator Structure
• Armature's windings are connected to the commutator
Permanent Magnet DC Motor Commutator and Brushes
• Armature's windings are connected to the commutator• Brushes connect the commutator to the battery
NORTHSOUTH
• Current flows through the armature's windings, which polarizes the electromagnet
+ -
Permanent Magnet DC Motor Electromagnet Polarization
NORTHSOUTH
• The like magnets (NORTH-NORTH and SOUTH-SOUTH) repel• As the like magnets repel, the armature rotates, creating mechanical
motion
+ -
Permanent Magnet DC Motor Rotation
NORTHSOUTH
• What direction will the armature spin?
• Clockwise? Counterclockwise?
Clockwise ?
Counterclockwise ?
Permanent Magnet DC Motor Rotation Direction?
+ -
• To determine the direction of the motor's rotation, we need to use the "Left Hand Rule"
Permanent Magnet DC Motor Rotation Direction?
Mag
netic
Fiel
d
Current
Forc
e
Left Hand Rule
• Start with two opposite ends of a magnet
NORTH
SOUTH
Left Hand Rule:Magnetic Field
NORTH
SOUTH
• The magnetic field (B) is from the NORTH pole to the opposite SOUTH pole
• The pointing finger follows B into screen B
Left Hand Rule:Current Flow
NORTH
SOUTH
• Current flows in a wire through the magnetic field from left to right
• The middle finger follows I1 right, or I2 left
I1
I2
Left Hand Rule:Force
NORTH
SOUTH
• The force, F, acting on each wire is in the direction of the thumb
• The wire with I1 is pushed up, I2 down
I1
I2
F1
F2
Left Hand Rule:Force
NORTH
SOUTH
• The magnitude of F is give by:
| F | = | I | * * | B |
where is the length of the wire in B
I1
I2
F1
F2
Left Hand Rule:Current Loop
NORTH
SOUTH
• If the current flows in a loop, the force(s) will cause the loop to rotate
I
F
F
NORTHSOUTH
• Magnetic field is from right to left• Imagine current flows out of the screen in this cross
section+ -
Permanent Magnet DC Motor Rotation
NORTHSOUTH
• Magnetic field is from right to left• Imagine current flows out of the screen in this cross section• The force causes the armature to rotate clockwise
+ -
Permanent Magnet DC Motor Rotation
NORTHSOUTH
+ -
Permanent Magnet DC Motor Rotation
• At some point, the commutator halves will rotate away from the brushes
• Momentum keeps the electromagnet and the commutator ring rotating
NORTHSOUTH
+ -
Permanent Magnet DC Motor Rotation
• When the commutator halves reconnect with the other brush, the current in the windings is reversed
NORTHSOUTH
+ -
Permanent Magnet DC Motor Rotation
• When the commutator halves reconnect with the other brush, the current in the windings is reversed
• The polarity is reversed and the armature continues to rotate
+ -
NORTHSOUTH
• Magnetic field is from right to left• Imagine current flows out of the screen in this cross section• The force causes the armature to rotate clockwise
+ -
Permanent Magnet DC Motor Rotation
Controlling a Permanent Magnet DC (PMDC) Motor• Bi-directional PM DC motors are controlled with an "H-Bridge"
circuit consisting of the motor and four power switches
Current
Turning On a PMDC Motor
• One switch is closed in each leg of the "H"• One switch is open in each leg of the "H"
Current
• One switch is closed in each leg of the "H"• One switch is open in each leg of the "H”
Turning On a PMDC Motorin the Other Direction
Current
• Unidirectional motors are controlled by a “half-H” bridge circuit
Controlling a Permanent Magnet DC (PMDC) Motor
Controlling a PMDC Motor Options• DC operation
– Rotational speed of the DC motor is fixed at a given voltage and load
• PWM Operation– Average voltage (and rotational speed) can be controlled
by opening/closing the switches quickly
• Braking– Shorting the terminals or momentarily reversing the drive
• Others
Agenda• Introduction to Electromagnets and Electric Motors• What Is Motor Control? • What Are Some Common Types of Motors? • Permanent Magnet DC Motors• Stepper Motors• Brushless DC Motors• Summary of Motors and Motor Control Circuits
Why a Stepper Motor ?• Unlike the permanent magnet DC motor, stepper motors move
in discrete steps as commanded by the stepper motor controller
• Because of their discrete step operation, stepper motors can easily be rotated a finite fraction of a rotation
• Another key feature of stepper motors is their ability to hold their load steady once the require position is achieved
• An example application for stepper motors is for implementing traditional "analog" instrumentation gauges on a dashboard
How Does a Stepper Motor Work ?
• A stepper motor often has an internal rotor with a large number of permanent magnet “teeth”
• A large number of electromagnet "teeth" are mounted on an external stator
• Electromagnets are polarized and depolarized sequentially, causing the rotor to spin one "step"
• Full step motors spin 360o/(# of teeth) in each step
• Half step motors spin 180o/(# of teeth) in each step
• Microstep motors further decrease the rotation in each step
Full Step Motor Operation
`
Half Rotateand Hold
`
Half Step Motor Operation
Half Rotateand Hold
Stepper Motor Control• The stepper motor driver receives square wave pulse
train signals from a controller and converts the signals into the electrical pulses to step the motor
• This simple operation leads stepper motors to sometimes be called "digital motors"
• To achieve microstepping, however, the stepper motor must be driven by a (quasi) sinusoidal current that is expensive to implement
Agenda• Introduction to Electromagnets and Electric Motors• What Is Motor Control? • What Are Some Common Types of Motors? • Permanent Magnet DC Motors• Stepper Motors• Brushless DC Motors• Summary of Motors and Motor Control Circuits
• Many of the limitations of the classic permanent magnet "brushed" DC motor are caused by the brushes pressing against the rotating commutator creating friction– As the motor speed is increased, brushes may not remain in
contact with the rotating commutator– At higher speeds, brushes have increasing difficulty in maintaining
contact– Sparks and electric noise may be created as the brushes
encounter flaws in the commutator surface or as the commutator is moving away from the just energized rotor segment
– Brushes eventually wear out and require replacement, and the commutator itself is subject to wear and maintenance
• Brushless DC motors avoid these problems with a modified design, but require a more complex control system
Why a Brushless DC Motor ?
How Does a Brushless DC Motor Work ?
• A brushless DC motor uses electronic sensors to detect the position of the rotor without using a metallic contact
• Using the sensor's signals, the polarity of the electromagnets’ is switched by the motor control drive circuitry
• The motor can be easily synchronized to a clock signal, providing precise speed control
• Brushless DC motors may have:– An external PM rotor and internal electromagnet stator– An internal PM rotor and external electromagnet stator
• This example brushless DC motor has:– An internal, permanent magnet rotor
Example Brushless DC Motor Operation
• This example brushless DC motor has:– An external, electromagnet stator
Example Brushless DC Motor Operation
• This example brushless DC motor has:– An external, electromagnet stator, with magnetic
field sensors
Example Brushless DC Motor Operation
Brushless DC Motor Construction
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Agenda• Introduction to Electromagnets and Electric Motors• What Is Motor Control? • What Are Some Common Types of Motors? • Permanent Magnet DC Motors• Stepper Motors• Brushless DC Motors• Summary of Motors and Motor Control Circuits
+
• An electric motor converts electric energy into mechanical motion
Electric Motor
12V-
ElectricEnergy
• Electric motors are used to perform a mechanical task by using electricity– Open a sunroof– Lift a power antenna– Control windshield wiper
What Is an Electric Motor?
Permanent Magnet Stepper Brushless DC DC Motor Motor Motor
Advantages: + Low cost + Position control + High efficiency(high volume) (low cost + High reliability
+ Simple operation control circuits) + Low EMI+ Speed control
Disadvantages: - Medium efficiency - Poor efficiency - Maybe higher cost
- Poor reliability - Digital interface - Complex control- Bad EMI - High cost
Types of Electric Motors
Agenda
• Introduction to Electromagnets and Electric Motors• What Is Motor Control? • What Are Some Common Types of Motors? • Permanent Magnet DC Motors• Stepper Motors• Brushless DC Motors• Summary of Motors and Motor Control Circuits
Introduction to Motor Control
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