Lecture First and Second

8/9/2019 Lecture First and Second

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SCHOOL OF ENGINEERING

Electrical Machines and DrivesIntroduction to Electric Motors

Pr. Prashant Ja !al


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Pr. Prashant

D.C. Motors Alternating Current

Motors

Stepper Motors

Series DC motors Single phase and polyphase

Variable reluctance

stepperShunt DC motors Single-phase

squirrel-cageinduction motor

Permanent magnet

stepper

Compound DC motors

Three-phase

induction motor

Hybrid stepper

Separately excited

DC motors

Synchronous AC

motorsBrushless

permanent magnet

DC motors

Basic Classification of Electric Motors


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Electrical Actuation Systems(motors)

• Motors classified into two main categories , d.c. and a.c. m otors• The basic principles involved in the action of a motor are:

1. A force is exerted on a conductor in a magnetic field hen acurrent passes through it !right hand rule". #or a conductor of lengthL carr$ing a current I in a magnetic field of flux densit$ B at rightangles to the conductor, the force F e%uals BIL

Pr. Prashant


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Electrical Actuation Systems(motors)

&. 'hen a conductor moves in a magnetic field then an e.m.f. is

induced across. The e.m.f. e is e%ual to the rate at hich themagnetic flux Φ !product of the flux densit$ and the area" s eptthrough b$ the conductor changes !#arada$(s la ", i.e. e=-d Φ /dt .

). The minus sign is because the e.m.f. is in such a direction as tooppose the change producing it !*en+(s la ", i.e. the direction of theinduced e.m.f. is such that the current produced b$ it sets up amagnetic field hich tend to neutrali+e the change in the magneticflux lin ed b$ the coil responsible for producing e.m.f.

#or this reason the induced e.m.f is often called a back e.m.f.

Pr. Prashant


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Electrical Actuation Systems(DC motors)

Basic principle of the d.c. motor :

-. 'hen a current is passed through the coil or a loop of irehich is free to rotate in the field of a permanent magnet, the

resulting forces acting on its sides at right angles to the fieldcause forces to act on those sides to give rotation

. /o ever, for the rotation to continue , hen the coil passesthrough the vertical position the current direction through the coilhas to be re ersed

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• 0n conventional d.c. motor, coils of ire are mounted in slots on ac$linder of magnetic material called armature

• The armature is mounted on bearings and is free to rotate• 0t is mounted in the magnetic field produced b$ the field poles• #or small motors, these ma$ be permanent magnets or

e lectromagnets in the form of field coils! for big motors• The figure sho s four-pole d.c. motor with field coils

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Co "onents O# $n Electric Motor%cont&'

!1otating"Commutator

2tator

Brushes

Armature


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• The ends of each armature coil are connected to ad3acent segmentsof a segmented ring called the commutator ith electrical contactsmade to the segments through carbon contacts call brushes

• As the armature rotates, the commutator reverses the current in eachcoil as it moves bet een the field poles

• This is necessar$ if the forces acting on the coil are to remain acting in

the same direction and so the rotation continue• The direction of rotation of the d.c. motor can be re ersed b$

reversing either the armature current or the field current in the fieldcoils

Pr. Prashant


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Electrical Actuation Systems(DC motors): How they look like

Pr. Prashant


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()

A 4C motor is designed to run on 4C electric po er.B$ far the most common 4C motor t$pes arethe brushed and brushless t$pes, hich use internal andexternal commutation respectivel$ to reverse the currentin the indings in s$nchronism ith rotation.


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dc otor ((

A T o 5ole 4C Motor


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dc otor (*

A #our 5ole 4C Motor


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dc otor (+

Armature of a 4C Motor


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(,

Action of a Commutator


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In the figure abo e! the wa eform shown is the oltage across the motor" In thiscase the motor ha# $ %oles! so the %erio# measure was between si& commutations%ikes" All the s%ikes are clearly isible an# looking the oltage wa eform it'susually easy to erify the number of %oles since e ery * s%ikes the wa eformre%eats itself"S%ee# of the e&am%le shown + ,- . -"--/0 + ,//- *2

3he commutation s%ikes can beuse# to measure the motor s%ee#"3hey can be %icke# u% by anoscillosco%e! rea#ing them with a%ulse counter"

4nowing the fre5uency of thes%ikes ( f s ) an# the number of %oles(*) of the motor! it's easy tocalculate the motor s%ee# as:

S%ee# + ,- f s . * 6r%m7

In a real case! it's %referable tomeasure the %erio# (3) between *(number of s%ikes) an# calculate thes%ee# as:S%ee# + ,- . 3 6r%m7


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(-

6enerated 7oltage in a 4C Machine


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Electrical Actuation Systems(DC motors with fiel# coils)

Pr. Prashant

• 0n series wound motor the field indings and armature inding areconnected in series

• 2uch motor exerts the highest starting tor'ue and has the greatest no-loadspeed

• /o ever, ith light loads there is a danger that a series ound motor mightrun at extremel& high speed

• (e ersing the polarit& of the suppl$ to the coils has no effect on the direction

of rotation of the motor8 it ill continue rotating in the same direction sinceboth the field and armature currents ill re erse simultaneousl&


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Electrical Actuation 2$stems!4C motors ith field coils"

$""lications o# series !ound DCotor

• Electric traction• Cranes• Elevators• $ir co "ressor•

acuu cleaner• Hair drier• Se!in/ achine

El i l A i S


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• 0n shunt wound motor the field indings and armature inding areconnected in parallel

• 2uch motor exerts the lowest starting tor'ue and much lower no-load speed and has good speed regulations• Because of the almost constant speed regardless of load, shunt

ound motors are ver$ idel$ used• To reverse the direction of rotation, either the armature or field

supplied oltage must be re ersed


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Applications of shunt ound 4C motor • 0t is a constant speed motor.• 9sed here the speed is re%uired to remain

almost constant from no load to full load.• 'here the load has to be driven at a number of

speeds and an$ one of hich is nearl$ constant.0ndustrial use:

• *athes• 4rills

• Boring mills• 2hapers• 2pinning and 'eaving machines.


El t i l A t ti S t


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• 0n compound motor there are two field indings, one in series iththe armature and the another one in parallel ith the armature

• Compound ound motors aim to get the best features of the seriesand shunt ound motors, namel$ a high starting tor'ue and goodspeed regulations

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• 0n separatel$ excited motor the armature and field circuits areseparated from each other and the motor has separate control ofthe each circuit currents

• 0t is considered to be a special case of the shunt motor

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Electrical Actuation Systems(DC motors with fiel# coils characteristics)

• The choice of motor depends on its application• #or example, the robot wrist might use a series wound motor

because the speed decreases as the load increases• #or example, for the mobile robot application a shunt wound

motor is preferable, because the speed of the heels of the robot

should sta$ constant regardless of the load on the heels

Pr. Prashant

Electrical Actuation Systems


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Electrical Actuation Systems(8rushless %ermanent magnet DC motors)

• A problem ith d.c. motors is that the$ re%uire a commutator andbrushes in order to periodicall$ reverse the current through each

armature coil• The brushes ma e sliding contacts ith the commutator and as aconse%uence sparks )ump bet een the t o and the$ suffer ear

• Brushes thus have to periodicall$ changes and the commutatorresurfaced

• To avoid such problems brushless motors have been designed

Pr. Prashant

l l


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Electrical Actuation Systems(8rushless %ermanent magnet DC motors)

• Brushless d.c. motors consist of a se%uence of stator coils and a permanent magnet rotor

• 'ith the con entional d.c. motor the magnet is fixed !stator" andthe current-carr&ing conductor made to mo e !rotor"

• 'ith the brushless permanent magnet d.c. motor the reverse isthe case, the current-carr&ing conductors are fixed !stator" and theferrite or ceramic magnet mo es !rotor"

Pr. Prashant


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Di0 1 t! DC 2 $C


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Di0erence 1et!een DC 2 $Cotors

AC motors preferred for systems req uiring lot of upfrontpower. On the other hand, DC motors do not perform well

at producing power over extended periods of time. Thereon DC motors are s elf starting and require n o external

help whereas AC motors require effective s tartingequipment to start operation.DC motors are single phase m otors whereas AC motors are

both single and three phase.DC electric m otors work for situations where speed needsto be co ntrolled. Direct current motors a re often found inappliances around the home.

AC motors work great for systems that are hard to start because they need a lot of power up front Three phase, also called poly-phase, AC motors are u sually

found in industrial settings.


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AC motors

• Alternating current motors can be classified into t o groups,

single phase and pol&phase• Each group is further subdivided intoinduction and s&nchronous motors• *ingle-phase motors tend to be used for lo po er

re%uirement hile pol$phase motors are used for higher

po ers• Induction motors tend to be cheaper than s$nchronousmotors and are thus ver$ idel$ used

Pr. Prashant

AC


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AC motors(single9%hase in#uction AC motors)

• +he single-phase s'uirrel-cage induction motor consists of as'uirrel-cage rotor , this being of copper or aluminum bars that fitinto slots in end rings to form compete electrical circuit

• There are no external electrical connections to the rotor • The stator has a set of indings

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• 'hen an alternating current passes through the stator indingsan alternating magnetic field is produced

• As a result of electromagnetic induction, e.m.f.(s are induced inthe conductors of the rotor and currents flow in the rotor

• Currents in the rotor produce the magnetic fields around theconductor and due to the interactions bet een the t o magneticfields the tor%ue is applied to the rotor

Pr. Prashant

AC t


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• Initiall& , hen the rotor is stationar& , the forces on the currentcarr$ing conductors of the rotor in the magnetic field of the statorare such as to result in no net tor'ue

• The motor thus is not self-starting • To give an initial impetus to start rotation of the rotor an auxiliar&

starting windings are used

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• The rotor rotates at a speed determined b$ the fre%uenc$ of the

alternating current applied to the stator • #or a constant fre%uenc$ suppl$ to a t o pole single phase motor

the magnetic field ill alternate at this fre%uenc$• This speed of rotation of the magnetic field is termed the

synchronous speed

• The rotor ill never %uite match this fre%uenc$ of rotation, t$picall$differing from it about ; to )<• This difference is termed slip• Thus for a = /+ suppl$ the speed of rotation of the rotor ill be

almost = revolutions per second

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Sin/le "hase vs three"hases

AC motors


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AC motors(three9%hase in#uction AC motors)

• +hree-phase induction motor is similar to the single phaseinduction motor but has a stator ith three indings located ;&= ° apart, each inding being connected to one of the three lines ofthe suppl$

• Because the three phases reach their maximum currents atdifferent times, the magnetic field can be considered to rotateround the stator poles , completing one rotation in one full c$cle of

the current•

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Rotatin/ Ma/netic Field


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• Because the three phases reach their maximum currents atdifferent times, the magnetic field can be considered to rotateround the stator poles , completing one rotation in one full c$cle ofthe current

Pr. Prashant

AC


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• The rotation of the field is much smoother than ith the singlephase motor

• +hree-phase induction motor has great advantage over the singlephase motor of being self starting

• The direction of rotation is reversed b$ interchanging an$ t o ofthe line connections, thereb$ changing the direction of rotation ofthe magnetic field

Pr. Prashant


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AC motors(synchronous AC motors)

• *&nchronous motors have stators similar to the three phaseinduction motors but a rotor which is a permanent magnet

• The magnetic field produced b$ the stator rotates and so themagnet rotates ith it

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• 'ith one pair of rotor poles per phase of the suppl$, the magneticfield rotates through )>= ° in one c$cle of the suppl$ and so thefrequency of the rotation with this arrangement is the same asthe frequency of the supply

• 2$nchronous motors are used hen a precise speed is required• The$ are not self-starting and some s$stem has to be emplo$ed to

start them

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• The more number of pair of poles per phase of the suppl$, theslo er is the speed of the rotor rotation

• 7oltage fre%uenc$ of the suppl$ and the mechanical fre%uenc$ ofthe rotor rotation are related b$

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AC motors(AC motors! s%ee# control)

• A.C. motors have great advantage over d.c. motors of beingcheaper, more rugged, reliable, and maintenance free

• /o ever, speed control is generall$ more complex than ith d.c.motors

• 2peed control of a.c. motors is based around the provision of avariable fre%uenc$ suppl$, since the speed is determined b$ thefre%uenc$ of the suppl$

• The tor5ue developed b$ an a.c. motor is constant when theratio of the applied stator voltage to frequency is constant

• +o maintain a constant tor'ue at the different speeds when the

fre'uenc& is aried the oltage applied to the stator has also to bearied

Pr. Prashant


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AC motors(AC motors! s%ee# control)

• 'ith one method, the a.c. is first rectified to d.c. b$ a con erter and then in erted back again but a fre%uenc$ that can be selected

• Another method that is often used for operating slo speedmotors is the c&clocon erter hich converts a.c. at one fre%uenc$directl$ to a.c. at another fre%uenc$ without intermediate d.c.con ersion

Pr. Prashant


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$ir and vacuu have hi/h reluctance3 !hileeasil4 a/neti5ed aterials such as so#t

iron have lo! reluctance. 7he concentration o# 8u9 in lo!:reluctance

aterials #or s stron/ te "orar4 "oles 7his causes echanical #orces that tend to

ove the aterials to!ards re/ions o# hi/her8u9 so it is al!a4s an attractive #orce%"ull'.

Ste%%er motors


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Ste%%er motors

• 2ince lines of force can be considered to be rather like elasticthreads and al a$s tr$ing to shorten themselves, the rotor ill

move until the rotor and stator poles line up• This is termed the position of minimum reluctance• This form of stepper generall$ gives step angles of @. ? or ; ?

Pr. Prashant Ja !al


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Increase resolution 14 either increasin/


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"oles on the rotor or stator.Or 14 usin/ di0erent ste""in/

techni;ues .

Ste""in/ se;uence in a ,ermanent magnetstepper .


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Ste%%er motors

• Current is supplied from a d.c. source to the indings throughs itches

• 'hen a pair of stator poles has a current s itched to it, thepermanent magnet rotor ill move =? to line up ith it

• 0f the current then s itched so that the polarities are reversed, therotor ill move a further =? in order to line up again

• Thus b$ s itching the currents through the coils the rotor rotatesin =? steps. T$pical step angles are ;. ?, @. ?, ; ?, )=?, )-? or =?

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• The rotor sets itself in the minimum reluctance position inresponse to a pair of stator coils being energi+ed

Pr. Prashant

Ste%%er motors


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(Ste%%er motor s%ecifications)

• Phase . This term refers to the number of independent windingson the stator , e.g. a four phase motor

• The current re%uired per phase and its resistance and inductanceill be specified so that the controller s itching output ill be

specified• +wo-phase motors tend to be used in light dut$ applications• +hree-phase tend to be variable reluctance steppers• Four-phase motors tend to be used for higher po er applications

Pr. Prashant

Ste%%er motors

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(Ste%%er motor s%ecifications)

• Step angleThis is the angle through hich the rotor rotates for one switchingchange for the stator coils

• Holding torque This is the maximum tor'ue that can be applied to a po ered motorwithout mo ing it from its rest position and causing spindle rotation

• *ull9in tor5ueThis is the maximum load tor'ue against which a motor will start! for

a gi en pulse rate! and reach s$nchronism ithout losing a step

Pr. Prashant

Ste%%er motors

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• Pull-out torqueThis is the maximum tor'ue that can be applied to a motor, alread$running at a gi en stepping rate , ithout losing s$nchronism

• Pull-in rate This is the maximum switching rate at hich a loaded motor canstart from the rest ithout losing a step

• *ull9out rate This is the minimum switching rate at hich a loaded and runningmotor ill remain in s$nchronism as the switching rate is reduced

Pr. Prashant

Ste%%er motors(Ste%%er motor s%ecifications)

St %% t


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Ste%%er motors(Ste%%er motor s%ecifications)

• Slew rangeThis is the range of switching rates bet een pull in and pull out

ithin hich the motor runs in s$nchronism but cannot start up orreverse

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Ste%%er motors


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• Bipolar motors can be driven b$ t o circuits• Table sho s the full step switching se'uence re%uired for the

transistors to carr$ out the c$clic repeating of four steps toadvance the rotor for ever$ =? in cloc ise direction

• #or reversing the motor the se%uence needs to be reversed

Pr. Prashant

g g

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1$

;0

/,

-&

1458

1467

2367

2358

Ste%%er motors

S!itchin/ se;uence


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S!itchin/ se;uence

A B C D

145

8

S S N N

1467

S N N S

2367

N N S S

235 N S S N

1$

;0

/,

-&

A 8

CD

2 22

2

2

22

2

S <

S S

Ste%%er motors


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• Half-steps , and a finer resolution, are obtainable if instead of thefull stepping se%uencing needed to implement a pole reversal to

get from one step to the next, the coils are s itched so that therotor stops at a position halfwa& to the next full step

Pr. Prashant

1458

1400

1467

0067

2367

2300

2358

0058

1458

1467

2367

2358

=ull ste% #ri e Half ste%%ing 2icro9ste%%ing


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Single9Coil e&citation3wo9Coil e&citation

Ste%%er motors


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• T o phase motors are termed unipolar hen the$ have sixconnecting wires for the generation of the s itching se%uence !t ocoils divided into four"

• Each of the coils has a center-tap hich connects coils togetherand such a form of the stepper motor can be s itched with )ust

four transistors

Pr. Prashant

Ste%%er motors

Ste%%er motors


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• 2ome application re%uire even smaller step angles• 'hile the step angle can be made small b$ increasing the number

of rotor teeth andDor the number of stator phases , generall$ morethan four phases and to 0 teeth are not used

• 0nstead the mini-stepping is used !dividing each step, example;. ?, into a number of e%ual si+e sub steps, example ;= substeps"

• This can be achieved b$ using different currents to the coils sothat the rotor moves to intermediate positions bet een normal steppositions

• Besides of using the stepper motor for the specific angle rotation,it can be used for continuous rotation here the speed of rotationis controlled b$ the rate of input pulses that ere used for stepping

Ste%%er motors(Ste%%er motor control)

Documents

Lecture First and Second