Ch14 Intro to Motor Controls

transcript

Bridging Theory in PracticeTransferring Technical Knowledgeto Practical Applications

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

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

• 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

• Reversing the current direction, reverses the polarity

Current

NORTHSOUTH

• 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

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

• 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.

STATOR

• Electrical energy creates a rotating magnetic field inside the motor causing the rotor to rotate, creating mechanical motion

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

• 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

• 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

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

• 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

• 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?

Current

Left Hand Rule

• Start with two opposite ends of a magnet

Left Hand Rule:Magnetic Field

• 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

• Current flows in a wire through the magnetic field from left to right

• The middle finger follows I1 right, or I2 left

Left Hand Rule:Force

• The force, F, acting on each wire is in the direction of the thumb

• The wire with I1 is pushed up, I2 down

Left Hand Rule:Force

• The magnitude of F is give by:

| F | = | I | * * | B |

where is the length of the wire in B

Left Hand Rule:Current Loop

• If the current flows in a loop, the force(s) will cause the loop to rotate

NORTHSOUTH

• Magnetic field is from right to left• Imagine current flows out of the screen in this cross

section+ -

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

NORTHSOUTH

• At some point, the commutator halves will rotate away from the brushes

• Momentum keeps the electromagnet and the commutator ring rotating

NORTHSOUTH

• When the commutator halves reconnect with the other brush, the current in the windings is reversed

NORTHSOUTH

• 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

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

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

• 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

• This example brushless DC motor has:– An external, electromagnet stator, with magnetic

field sensors

Brushless DC Motor Construction

Brushless DC Motor Operation

A1 B1 C1

A2 B2 C2

Brushless DC Motor Control Circuit

A1 B1 C1

A2 B2 C2

A1 B1 C1

A2 B2 C2

A1 B1 C1

A2 B2 C2

A1 B1 C1

A2 B2 C2

A1 B1 C1

A2 B2 C2

A1 B1 C1

A2 B2 C2

A1 B1 C1

A2 B2 C2

• An electric motor converts electric energy into mechanical motion

Electric Motor

ElectricEnergy

• Electric motors are used to perform a mechanical task by using electricity– Open a sunroof– Lift a power antenna– Control windshield wiper

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

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Ch14 Intro to Motor Controls

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