DC Machines Week 5,6

7/24/2019 DC Machines Week 5,6

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KL3073

DC Motors


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Direct Current (DC) MachinesFundamentalsDc motors are dc machines used as

motors

the same physical machine can

operate as either a motor or agenerator-it is simply a question othe direction o the po!er "o!

through it#Dc motors are compared $y their

speed regulations#


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%ypes o dc motors

%he separatelye&cited dc motor

%he shunt dc motor#

%he permanent-magnet dc motor#

%he series dc motor#

%he compounded dcmotor


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%he 'quialent Circuit o a DC Motor

its equialent circuit is e&actly thesame as that o a generator e&ceptor the direction o current "o!#

Figure (a) %he equialent circuit o a dc motor# ($) * simpli+edequialent circuit


5/26

%he 'quialent Circuit o a DC Motor

%he internal generated oltage in thismachine is gien $y the equation#

induced torque deeloped $y themachine is gien $y

These two equations, the KVL equation of the

armature circuit and magnetization curve, arenecessary to analyze the behavior and

performance of a dc motor


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,eparately e&cited and shunt dcmotors* separately e&cited dc motor is a motor

!hose +eld circuit is supplied rom aseparate constant-oltage po!er supplya shunt dc motor is a motor !hose +eld

circuit gets its po!er directly across thearmature terminals o the motorhen the supply oltage to a motor is

assumed constant. there is no practicaldi/erence in $ehaiour $et!een theset!o machines


7/26

%he ,eparately e&cited and shunt dcmotors

%he KL equation or the armaturecircuit is

(a) The equivalent circuit of a separately

excited de motor.

(b) The equivalent circuit of a shunt dc

motor.


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%he %erminal Characteristic o a,hunt DC Motor

%he terminal characteristic o amotor is a plot o its output torqueersus speed#

1o! does a shunt dc motor respondto a load2When load load> ind so E ! "#

$

! (%T

& E

)'

ind ! "#$*+,)

load= ind


9/26

nduced %orque in the 4otating Loop

5utput characteristic o a shunt dcmotor

(a) Torque-speed characteristic of a shunt

or separately excited dc motor ith

compensatin/ indin/s to eliminate

armature reaction.

(b) Torque-speed

characteristic of the motor ith

armature reaction present.

ind

AT

K

R

K

V

2)=


10/26

,peed Control o ,hunt DC Motors

n general. there are 3 methods to controlthe speed o dc shunt motor

*d6usting the +eld resistance 4F(and thusthe +eld "u&)

*d6usting the terminal oltage applied tothe armature#

Less common methodnserting a resistor in series !ith the

armature circuit


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Changing the FieldResistance


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Changing the FieldResistance

The effect of field resistance speed control

on a shunt motor0s torque speed

characteristic


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Changing the Armature Voltage

The effect of armature volta/e speed

control on a shunt motor0s torque-speed

characteristic.


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DC Machine Construction

Inserting a Resistor in Series with theArmature Circuit.

ind

AT

K

R

K

V

2)=

The effect of armature resistancespeed control on a shunt motor0s

torque-speed characteristic.


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,ae 4anges o 5peration or the common methods

If a motor is operating at its rated terminalvoltage, power and eld current, then it will

be running at rated speed, also known asbase speed.

ield resistance control can control thespeed of the motor for speeds above basespeed but not for speeds below base speed.

!o achieve a speed slower than base speedb" eld circuit control would re#uiree$cessive eld current, possibl" burning upthe eld windings.

Field ResistanceControl


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,ae 4anges o 5peration or the common methods

If a motor is operating at its rated terminalvoltage, power and eld current, then it will

be running at rated speed, also known asbase speed.

Armature voltage control can control thespeed of the motor for speeds below basespeed but not for speeds above base speed.

!o achieve a speed faster than base speed b"armature voltage control would re#uiree$cessive armature voltage, possibl"damaging the armature circuit.

Armature VoltageControl


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%he '/ect o an 5pen Field Circuit

%hat would happen if the eld circuit wereactuall" opened while the motor isrunning&

!he 'u$ in the machine will drop, and (Awill drop as well. !his would cause a reall"large increase in the armature current,and the resulting induced tor#ue would be#uite a bit higher than the load tor#ue of

the motor. !herefore, the motor)s speedstarts to rise and *ust keeps going up.

!his condition is called runawa".


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%1' 8'4M*9'9%-M*:9'% DCM5%54A permanent magnet dc motor

+-C/ is a dc motor whose polesare made of permanent magnets.Advantage1

2ince the motors do not require an external field circuit.3o field circuit copper losses.2maller than correspondin/ shunt dc motors.

Disadvantages1cannot produce as hi/h flux density 4loer induced torque5678 motors run the ris9 of dema/neti:ation.

not possible to control the speed of the 5678 motor by varyin/

the field current or flux.


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%1' ,'4', DC M5%54

A series dc motor is a dc motorwhose eld windings consist of arelativel" few turns connected inseries with the armature circuit.

!he 0irchho12s voltage law e#uationfor this motor is

The equivalent circuit of a series dc motor.


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nduced %orque in a ,eries DC Motor

the 'u$ is directl" proportional tothe armature current, at least untilsaturation is reached.

the load on the motor increases, its'u$ increases too.so the speed deceases

%he induced torque is ;ind


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%he %erminal Characteristic o a,eries DC Motor

>sing equation =< c* the torque-speed

characteristic cure or the series motorcan $e deried as

ideal torque-speed characteristic is

Kc

RR

Kc

VSA

ind

T +

=

!


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,peed Control o ,eries DC Motors

3nlike with the shunt dc motor, thereis onl" one e4cient wa" to change thespeed of a series dc motor.

!hat method is to change the terminalvoltage of the motor. If terminalvoltage is increased, the speed willincrease for an" given tor#ue.


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%1' C5M85>9D'D DC M5%54

A compounded dc motor is a motor withboth a shunt and a series eld. Such amotor is shown in igure below.

The equivalent circuit of compounded dc motors1 (a) lon/-shunt connection; (b)

short-shunt connection.


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%1' C5M85>9D'D DC M5%54

%he KL or a compounded dc motor is%< '*? *(4*? 4,)

and the currents are

*< L- FF< %@4F

%he net mm and the e/ectie shunt +eld current are

Fnet< FFB F,'- F*4F< F B (9,'@9F) * F*4@9F


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Cumulatiely Compounded DC Motor

(CC)

%here is a component o "u& !hich is constantand another component !hich is proportional toits armature current (and thus to its load)#

CC motor has a higher starting torque than a

shunt motor (!hose "u& is constant) $ut a lo!erstarting torque than a series motor (!hoseentire "u& is proportional to armature current)#

%he CC motor com$ines the $est eatures o

$oth the shunt and series motors# LiEe a seriesmotor. it has e&tra torque or starting liEe ashunt. it does not oerspeed at no load#


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Cumulatiely Compounded DC Motor

(CC)

* comparison o the torque-speedcharacteristics o each o these types omachines is sho!n $elo!

(a) The torque-speed characteristic of a

cumulatively compounded dc motor

compared to series and shunt motors ith

the same full-load ratin/.

(b) The torque-speed characteristic of a

cumulatively compounded dc motor

compared to a shunt motor ith the same

no-load speed.

DC Machines Week 5,6

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