Bridging Theory in Practice

Name: Bridging Theory in Practice
Uploaded: 2017-07-19T07:03:35+00:00
Duration: PTM31S55
Channel: Evelyn Barker
Description: Bridging Theory in Practice

Bridging Theory in Practice
Transferring Technical Knowledge to 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

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

Start with an iron bar Wrap a wire around the iron bar Connecting a battery causes a current to flow in the wire + - Current

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 NORTH SOUTH + - Current

Reversing the current direction, reverses the polarity SOUTH NORTH + - Current

Reversing the current direction, reverses the polarity If the current is stopped, the induced magnetic field decays to 0 SOUTH NORTH Current + -

Electromagnets and Electric Motors
We can use electromagnets in electric motors to convert electrical energy to mechanical work… Electric Motor Electric motors are used to perform a mechanical task by using electricity Open a sunroof Lift a power antenna Control windshield wiper Electric Energy + - 12V

What Is an Electric Motor?
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. ROTOR STATOR

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

Where Are Electric Motors Used?
Electric motors are used in many different automotive applications: Sunroof Brakes Power steering Fuel pump Water pump Hybrid and electric vehicles Cruise control Throttle plate control Air vents Others Power windows Power seats Power mirrors Fans Windshield wipers Windshield washer pumps Starter motor Electric radio antennae Door locks Information gauges

What Is Motor Control ? 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

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

Types of Electric Motors
There are many different types and classifications of electric motors: Permanent magnet DC motor Stepper motor Brushless DC motor Wound field motor Universal motors Three phase induction motor Three-phase AC synchronous motors Two-phase AC Servo motors torque motors Shaded-pole motor split-phase induction motor capacitor start motor Permanent Split-Capacitor (PSC) motor Repulsion-start induction-run (RS-IR) motor Repulsion motor Linear motor Variable reluctance motor Unipolar stepper motor Bipolar stepper Full step stepper motor Half step stepper motor Micro step stepper motor Switched reluctance motor Shaded-pole synchronous motor Induction motor Coreless DC motor Others......

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

How Does a Permanent Magnet DC Motor Work?
"DC Motors" use magnets to produce motion Permanent magnets NORTH SOUTH

How Does a Permanent Magnet DC Motor Work?
"DC Motors" use magnets to produce motion Permanent magnets An electromagnet armature NORTH SOUTH

Permanent Magnet DC Motor Rotating Armature
Electromagnet armature is mounted on axle so that it can rotate NORTH SOUTH

Permanent Magnet DC Motor Commutator and Brushes
Electromagnet armature is mounted on axle so that it can rotate A commutator makes an electrical contact with the motor's brushes NORTH SOUTH

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

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

Permanent Magnet DC Motor Electromagnet Polarization
Current flows through the armature's windings, which polarizes the electromagnet + - NORTH SOUTH

Permanent Magnet DC Motor Rotation
The like magnets (NORTH-NORTH and SOUTH-SOUTH) repel As the like magnets repel, the armature rotates, creating mechanical motion + - NORTH SOUTH

Permanent Magnet DC Motor Rotation Direction?
What direction will the armature spin? Clockwise? Counterclockwise? + - Counterclockwise ? NORTH SOUTH Clockwise ?

Permanent Magnet DC Motor Rotation Direction?
To determine the direction of the motor's rotation, we need to use the "Left Hand Rule" Magnetic Field Current Force

Left Hand Rule Start with two opposite ends of a magnet SOUTH NORTH

B 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 SOUTH B NORTH

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 SOUTH I1 I2 NORTH

Left Hand Rule: Force The force, F, acting on each wire is in the direction of the thumb The wire with I is pushed up, I2 down F1 SOUTH I1 I2 NORTH F2

Left Hand Rule: Force The magnitude of F is give by:
| F | = | I | *  * | B | where  is the length of the wire in B F1 SOUTH I1 I2 NORTH F2 

Left Hand Rule: Current Loop
If the current flows in a loop, the force(s) will cause the loop to rotate F SOUTH I NORTH F

Magnetic field is from right to left Imagine current flows out of the screen in this cross section + - NORTH SOUTH

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 + - NORTH SOUTH

At some point, the commutator halves will rotate away from the brushes Momentum keeps the electromagnet and the commutator ring rotating + - NORTH SOUTH

When the commutator halves reconnect with the other brush, the current in the windings is reversed + - NORTH SOUTH

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 + - + - NORTH SOUTH

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 + - NORTH SOUTH

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

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

Turning On a PMDC Motor in the Other Direction
One switch is closed in each leg of the "H" One switch is open in each leg of the "H” Current

Controlling a Permanent Magnet DC (PMDC) Motor
Unidirectional motors are controlled by a “half-H” bridge circuit Current

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 Rotate and Hold

Half Step Motor Operation
` Half Rotate and 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

Why a Brushless DC Motor ?
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

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

Example Brushless DC Motor Operation
This example brushless DC motor has: An internal, permanent magnet rotor

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
A com com A a a c b c b c b com a B C C B com

Brushless DC Motor Operation
com com A a 1 a c b c b c b com a B C C B com

com com A a a c b c b c b com a B C C B com 2

Brushless DC Motor Control Circuit
A1 B1 C1 A a c b com B C A2 B2 C2

A1 B1 C1 A 1 a c b com B C A2 B2 C2

A1 B1 C1 A a c b com B C A2 B2 C2 2

An electric motor converts electric energy into mechanical motion Electric Motor Electric motors are used to perform a mechanical task by using electricity Open a sunroof Lift a power antenna Control windshield wiper Electric Energy + - 12V

Types of Electric Motors
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

Thank you!

Bridging Theory in Practice

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