CHAPTER-7-ELECTRIC-AND-ELECTRONICS-DEVICES-ELECTRIC-MOTOR.pdf

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CHAPTER 7

ELECTRIC MOTOR



Generators, Motors and How We

Get Electricity



What is Electricity?

Electricity is energy transported by

the motion of electrons

**We do not make electricity, we CONVERT

other energy sources into electrical

energy**

Conversion is the name of the game



Energy Conversion Options for ElectricityNon-Thermal Paths

• Source to Electrical

Source Converter Sun Photovoltaic (photon to electron)

Chemical Fuel Cell

• Source to Potential/Kinetic to Mechanical to Electrical

Source Converter Kinetic to Mechanical Mech to

Electrical Dam Penstocks Turbine (water) Generator

Tides Machine Turbine (air or water) Generator Wind N/A Turbine (air) Generator



Energy Conversion Options for ElectricityThermal Paths

• Heat to Mechanical to Electrical

Source Heat to Mechanical Mech to Electrical Geothermal Turbine (vapor) Generator OTEC Turbine (vapor) Generator

• Stored Energy to Heat to Mechanical to Electrical

Source Reactor Heat to Mechanical Mech to Electrical Fuel Combustor Turbine (gas or vapor) Generator

U, Pu Reactor Turbine (gas or vapor) Generator

Sun Collector* Turbine (gas or vapor) Generator H, H2, H3Reactor Turbine (gas or vapor) Generator

* More a modifier or concentrator than a reactor



GENERATORS AND MOTOR

• These can be divided into:

– generators – which convert mechanical energy

into electrical energy

– motors – which convert electrical energy into

mechanical energy

• Both types operate through the interaction

between a magnetic field and a set of

windings



Faraday Effect

• Faraday Effect

• Basic Concepts

• Voltage – V – Potential to Move Charge (volts)

• Current – I – Charge Movement (amperes or amps)

• Resistance – R – V = IxR (R in =ohms)

• Power – P = IxV = I2xR (watts)



Basic Electromagnet

• Electromagnets are very useful tools. They

have the ability to gain a magnetic field with

the introduction of current and to lose it once

the current ceases



Basic Electromagnet – Right Hand Rules



Basic Electromagnet – Left Hand Rules



ELECTRIC MOTOR

• An electric motor is an electric machine thatconverts electrical energy into mechanical energy

• Found in applications as diverse as industrial fans,

blowers and pumps, machine tools, householdappliances, power tools, and disk drives, electricmotors can be powered by direct current(DC) sources, such as from batteries, motor

vehicles or rectifiers, or by alternating current(AC) sources, such as from the powergrid, inverters or generators.



DC MOTOR

• A DC motor relies on the fact that like magnetpoles repel and unlike magnetic poles attracteach other.

• A coil of wire with a current running through itgenerates an electromagnetic field aligned withthe centre of the coil.

• By switching the current on or off in a coil itsmagnet field can be switched on or off or by

switching the direction of the current in the coilthe direction of the generated magnetic field canbe switched 180°.



DC MOTOR-PRINCIPLES





DC MOTOR - EFFICIENCY

• Increasing the number of turns in the coil

• Increasing the strength of the current

•

Increasing the area of cross-section of the coil• Increasing the strength of the radial magnetic

field



BRUSHED DC MOTOR

• BRUSHED – The brushes of dc motor are made with carbon or

graphite structures, making sliding contact over therotating commutator.

– The brushes are used to physically relay the electriccurrent from external circuit to the rotatingcommutator form where it flows into the armaturewinding.

– So, the commutator and brush unit of the dc motor is

concerned with transmitting the power from the staticelectrical circuit to the mechanically rotating region orthe rotor.



BRUSHLESS DC MOTOR-

CONSTRUCTION



BRUSHLESS DC MOTOR –

OPERATING PRINCIPLES



BRUSHLESS vs. BRUSHLESS DC MOTOR

BRUSHED BRUSHLESSPROS •Two wire control

•Replaceable brushes for extended life

•Low cost of construction

•Simple and inexpensive control

•No controller is required for fixed

speeds•Operates in extreme environments

due to lack of electronics

•Less maintenance due to absence of

brushes

•Higher speed and torque

•High efficiency, no voltage drop

across brushes

•Low noise

CONS •Periodic maintenance is required

•Speed/torque is moderately flat. At

higher speeds, brush frictionincreases, thus reducing useful

torque

•Poor heat dissipation due to internal

rotor construction

•Brush Arcing will generate noise

causing EMI

•Higher cost of construction

•Control is complex and expensive

•

Electric Controller is required to keepthe motor running. It offers double

the price of the motor.



STEPPER MOTOR - OVERVIEW

• A stepper motor convert electrical pulses into rotatingmechanical energy. These pulses are directed by a stepmotor controller. The rotor rotates according to discreteincrements, measured in degrees and called steps. This iswhere the name "step motor" comes from.

• The speed and direction in which the rotor rotates in thestep motor is dependent upon the action of the step motorcontroller on the motor’s windings.

• The direction of the motor’s rotation is dependent on howthe controller sequences the electrical pulses. The motor’sspeed is dependent on the frequency of the controllerpulses. And the length of the rotation is dependent on howlong the controller keeps sending electrical pulses to themotor.



STEPPER MOTOR - BENEFIT

• a step motor with a step motor controller is itsaccuracy. There is a direct relationship between therotation angle and the input pulse, so you can "stop ona dime." In other words, you can get the motor to stop

almost exactly where you want at any time, or you canget it to reverse direction at exactly where you want.For applications that require great precision, stepmotors are therefore ideal.

• these motors work well with open loop control. Open

loop control means that there is no feedback from thestep motor to the step motor controller.

• the fact that it is brushless.



STEPPER MOTOR - OPERATION



DC MOTOR – SPEED CONTROL

• Pulse widthmodulation (PWM) – It is a technique of

speed control by giving

on and off signal(power) to the motor.

– To avoid stutter, itmust be fast enough(high frequency) but

varying the duty cycle(higher for morespeed).





SERVO DC MOTOR - INTRO

• The servo has a 3 wire connection: power, ground, andcontrol. The power source must be constantly applied; theservo has its own drive electronics that draw current fromthe power lead to drive the motor.

• The control signal is pulse width modulated (PWM), buthere the duration of the positive-going pulse determinesthe position of the servo shaft. For instance, a 1.520millisecond pulse is the centre position for a Futaba S148servo. A longer pulse makes the servo turn to a clockwise-from-centre position, and a shorter pulse makes the servo

turn to a counter-clockwise-from-centre position.• The servo control pulse is repeated every 20 milliseconds.

In essence, every 20 milliseconds you are telling the servo,“go here.”



DC MOTOR – SERVO

• To recap, there are two important differences betweenthe control pulse of the servo motor versus the DCmotor. First, on the servo motor, duty cycle (on-timevs. off-time) has no meaning whatsoever—all that

matters is the absolute duration of the positive-goingpulse, which corresponds to a commanded outputposition of the servo shaft.

• Second, the servo has its own power electronics,

so very little power flows over the control signal. Allpower is draw from its power lead, which must besimply hooked up to a high-current source of 5 volts.



AC MOTOR

• The difference between AC and DC motor:

– Power source AC motor uses alternating current

while DC motor uses direct current

– DC motor construction either brushed or

brushless, but AC motor can only b brushless.

– Speed control: DC motor uses current while AC

motor uses frequency



AC/DC(not the band)

• Alternating Current

– Large-scale generators

produce AC

– Follows sine wave with ncycles per second

– 1, 2, 3-phase?

– US:120 V,60 Hz

– Europe: 240 V,50Hz

– Transforming ability

• Direct Current

– Batteries, Photovoltaics,

fuel cells, small DC

generators – Charge in ONE direction

– Negative, Positive

terminals

– Easy conversion AC to DC,not DC to AC



AC MOTOR - INTRO

AC motors can be divided into two main forms:

– synchronous motors

– induction motors

High-power versions of either type invariably operatefrom a three-phase supply, but single-phase versions

of each are also widely used – particularly in a

domestic setting



AC MOTOR - INTRO

Synchronous motors

– just as a DC generator can be used as a DC motor, so

AC generators (or alternators) can be used as

synchronous AC motors – three phase motors use three sets of stator coils

the rotating magnetic field drags the rotor around with it

– single phase motors require some starting mechanism

– torque is only produced when the rotor is in sync withthe rotating magnetic field

not self-starting – may be configured as an induction motor

until its gets up to speed, then becomes a synchronous motor



AC MOTOR – INDUCTION MOTOR

Induction motors

– these are perhaps the most important form of AC motor

– rather than use slip rings to pass current to the field

coils in the rotor, current is induced in the rotor by

transformer action

– the stator is similar to that in a synchronous motor

–

the rotor is simply a set of parallel conductors shortedtogether at either end by two conducting rings



AC MOTOR – INDUCTION MOTOR

A squirrel-cage induction motor



AC MOTOR – THREE PHASE

INDUCTION MOTOR In a three-phase induction motor the three phasesproduce a rotating magnetic field (as in a three-phasesynchronous motor)

–

a stationary conductor will see a varying magnetic fieldand this will induce a current

– current is induced in the field coils in the same waythat current is induced in the secondary of atransformer

– this current turns the rotor into an electromagnet whichis dragged around by the rotating magnetic field

– the rotor always goes slightly slower than the magneticfield – this is the slip of the motor



AC MOTOR – SINGLE PHASE

INDUCTION MOTOR

• In single-phase induction motors other

techniques must be used to produce the

rotating magnetic field

– various techniques are used leading to various

forms of motor such as

• capacitor motors

•

shaded-pole motors – such motors are inexpensive and are widely used

in domestic applications



UNIVERSAL MOTOR

• While most motors operate from either AC or DC,

some can operate from either

• These are universal motors and resemble series-

wound DC motors, but are designed for both AC

and DC operation

– typically operate at high speed (usually > 10,000 rpm)

– offer high power-to-weight ratio

– ideal for portable equipment such as hand drills and

vacuum cleaners



Electrical Machines – A Summary

• Power generation is dominated by AC machines

– range from automotive alternators to the synchronous

generators used in power stations

– efficiency increases with size (up to 98%)

• Both DC and AC motors are used

– high-power motors are usually AC, three-phase

– domestic applications often use single-phase inductionmotors

– DC motors are useful in control applications



Key Points

• Electrical machines include both generators and motors

• Motors can usually function as generators, and vice versa

• Electrical machines can be divided into AC and DC forms

•

The rotation of a coil in a uniform magnetic field produces asinusoidal e.m.f. This is the basis of an AC generator

• A commutator can be used to produce a DC generator

• The magnetic field in an electrical machine is normally producedelectrically using field coils

• DC motors are often similar in form to DC generators

• Some forms of AC generator can also be used as motors

• The most widely used form of AC motor is the induction motor



GOING GREEN



Where do we get our Electricity?

• Fossil – Coal, Natural Gas, Oil –

550 Gigawatts (GW)

• Nuclear – 200 GW

•

Hydro–

75 GW• Geothermal – 2.3 GW

• Other Renewable – Wind, Solar,

OTEC – 13.6 GW



0

1

2

3

4

5

6

7

8

9

U S A

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R u s s

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J a p a

n U .

K .

M e x

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C h i n

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Energy Usage Per Capita (1999)

T O E / p e r s o n - y e a r

*TOE - Tons of Oil Equivalent (~40 Million Btus)



Oil Resources

Saudi Arabia 26%

Iraq 11%

Kuwait 10%

Iran 9%

UAE 8%Venezuela 6%

Russia 5%

Libya 3%

Mexico 3%

China 3%Nigeria 2%

U.S. 2%

U.S. 26%

Japan 7%

China 6%

Germany 4%

Canada 4%Russia 3%

Brazil 3%

S. Korea 3%

France 3%

India 3%Mexico 3%

Italy 2%

Have Oil… Use Oil…

The U.S. uses more than the next 5 highest

consuming nations combined.



U.S. Renewable Energy Resource Assessment

WindSolar

Geothermal

Temperature <90C

Temperature >90C

Geopressured resources

o

o

10

10

12

12

14

14

16

16

18

1820

20

2224

26

22 2426

16

14

14

1614

12

10

10

12 <10

10-12

12-14

14-16

16-18

18-20

20-22

22-24

24-26

26-28

>28

6.0-6.5 m/s13.4-14.6 mph

6.5-70 m/s14.6-15.7 mph

>7.0 m/s15.7+ mph

Megajoules/m2

Biomass

Agricultural resources& residues Woodresources & residuesAgricultural & woodresidues Lowinventory



US energy infrastructure is large and deeply entrenched

• 400,000+ miles of gas and oil pipelines

• 160,000+ of high voltage transmission lines

• 176,000 gasoline stations

• 1000’s of oil and gas wells drilled annually in the US and Canada

Barriers to Change



oil and gas are readily available as a world commodity at lowcost -- equivalent to $ 4 to 5 / million Btu

US coal is even more abundant and cheaper – approximately$1/million Btu

US electricity prices remain low relative to other commodities

The average American family spends only 3 to 4% of theirincome on energy!!

Barriers to Change

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