280
A
Textb
ot M
Mecher
(ii) Shunt wound.
iv) Compound wound.
ACaiOr3
2. A.C. Motors
)
Single phase:
(a) Induction:
Squirrel cage:
-
Split phase
-Capacitor start
Permanent split capacitor
Shaded pole
Two-valve capacitor.
Wound
-
rotor:
Repulsion
Repulsion start
Repulsion induction.
(b) Synchronous:
Shaded pole
Hysteresis
Reluctance
Permanent magnet.
(i) Polyphase:
(a) Induction:
Wound
rotor
ApF
Squirrel cage.
(6) Synchronous.
(iii) Universal motors.
In modern control systems D.C. motors are mostly used.
4.3.4. D.C. Motors
D.C.
designs
(Direct current)
because of the
configurations.
motors find
wide applications in a large number of mechatoni
torque-speed characteristics achievable with different electrial
The speeds of the D.C. motors
reversible.
These motors
can
be
smoothly controlled and
in most
cases a
As
)
(i)
respond quickly since they have a high ratio of torque to roror-i
ateyand
Dynamic braking (where motor generated energy is fed to a resistor
dissude
regenerative braking (where motor-generated energy is fed back to the D.C. po0
supply) can be implemented in applications where quick stops and hig
are desired.
can
Ad
T-inerti
(o)
Tciengy
4.3.4.1. Permanent magnet (PM) D.C. motors
In these motors (See Fig. 4.52) field excitation is
obtained by suitably moun
magnets (which require no power source and therefore produce no heatin
Magnets made from ferrites or rare earth (cobalt samarium) are used.
Lim
Speed.
permanen
he stat
the
4.3
Ref
In
the :
-Mechanical, Eled
lectrical,
Actual
Hydraulic
and
281
Pneumatic
Armature
Magnet
Speed
Torque
Fig. 4.52. Permanent magnet D.C. motor schematic and torque-speed and
A
current-torque curves.
PM motor is lighter and smaller than others, equivalent D.C. motors because the
field strength of permanent magnets
is high.
The radial width of a typical permanent magnet is roughly one-fourth that of an
equivalent field winding.
PM motors are ensily reversed by switching the direction of the applied voltage, because
the current and field change direction only in the rotor.
PM motors can be brushed, brushless, or stepper motors.
Applications:
.The PM motor is ideal in computer control applications because of the linearity of its
torque-speed relation.
When a motor is used in a position or speed control application with sensor
feedback to a controller, it is referred to as "servomotor".
PM motors are used only in low-power applications since their rated power is limited
to 5 H.P. or less, with fractional horsepower ratings being more common.
Advantages:
As compared to field wound motors, these motors possess the following advantages:
() More efficient.
i) More reliable.
(jin) More study and compact.
() The field flux remains constant for all loads giving a more linear speed-torque
characteristic.
(0) In a separately excited motor, failure of field can lead to runaway condition. This
does not happen in PM motors.
Limitation. As the flux is constant in these motors, speed cannot be controlled above base
Speed
4.3.4.2. D.C. Shunt motors:
Refer to Fig. 4.53.
these motors armature and field windings are connected in parallel and powered
by the same
supply.
202
Arm
Variatle
resis1or
Speed
1, Lime current( 1,1,,)
Shunt current
Armnturecurrent
Fig. 4.53. D.C. shunt motor schematic and torquespeed and current-torgue
e d
These motors exhibit nearly constant speed over a long range of lce
ding
They have starting torques about 1.5 tmes the rated operating torque
They
They
starting torgue of any of D.C. motors.
be economically converted to allow adjustable speed by
have lowest
can
potentiometer in series wíth the field windings.
place.
4.3.4.3. D.C. Series motors
In this type of motor (See Fig. 4.54) armature and field windings are connected .
so the armature and field currents are egual.
dine
Field
Arma
ture
Speed
run-away
Speed
Fig. 4.54. D.C. series
motor
Torque
schematic and
Current-torque curves.
torque-speed and
These motors exhibit very high
starting torques, highly variable speed depenaing
and very high speed when the load
is small.
In fact, large series motors
can fail
whenthey
unloaded (eg., in a belt drive
dyT
tn..to
due to dy
fails)
when
the
belt
otorremai
application
forces at high speeds; this is called
re
as the
sudden
catastrophically
loaded,
this poses
no
problem.
"run-away". As long
are
ActuatorsMech
Electrical, Hydraulic
-Mechanical, Ele
and
283
Pneumatic
n
torque-speed
torque-speed curve ford a series motor is hyperbolic in shape, implying
relationship
prse
i n v e r
between the
torque
a wide range.
mstant power over
and
speed
and
nearly constan p
Compound motors
D.C.
3.4.4.
Series
field
Arma-
ture
Shunt
field
Speed
Torque
Fig. 4.55. D.C. compound motor schematic and torque-speed and
Refer
current-torque curves.
1he compound motor has a shunt field winding
t
in addition too
Fig. 4.55.
ding so that the number of magnetic lines of force produced by each of its poles
to
series w i n d i r
resultant of the flux produced by the shunt coil and that due to the series coil. Ihe
is the resultant
TOduced depends not only on the current and number of turns of each coil, but also on
the
ding direction of the shunt coil in relation to that of the series coil. When the two tluxes assis
other, it
nch other the machine is a cumulative compound motor, while if they oppose each
s said to be differential compound motor.
Fig. 4.56 shows the field windings and interpole connections of a differential compound
whilst the series col
mMLnd motor, The shunt coil is made up of many turns of fine wire,
turns of thickwire.
Comprises relatively feuw
Shunt
coil
Series
coil
s
(
Fig. 4.56. Field windings of a differential
Compound motor.
Fig. 4.57. ield windings and interpole
connections of a cumulative compound
wound motor.
A
284
Fig
of
a
cumulative
shows the field windings
worth notingg
457,
in the shunt
n t and
series
he maximmm
ulation
is
in
coil is
spend of
mot as
commound
gand
torgue produoi
of siilar size.
e
as
with
a
by comypound
motor
shunt
Fig:
compoun.
4.6
iniited,
Is
and
Textbook of
ow of curten Actator
57.
unlike aa ser
series motor,
or, but it
moto0
motors Is sonnewhat
lower
thas
hat of serie Spee
fo
voltage polarity
Unlike the permanent magnet
rotatiOn does not cha
ot
diretion
the
is changd.
DC. motor
pound
at the poiarity
of both
the stator and otor chainges, because the field an
motor,
N
Nofe:
when
excited by the
sourv.
same
4.3.4.5. Stepper motors
Introduction: A stepper motor, a special typpe of D.C. motor, is an i
caaracteristios:
(7)
(77)
can
motors
1
the
r o f
Fig
motor
remental motion
1otor and has
the foll
c . It is a rmaent magnet or variable reluctance D.C. nnot
It
ies
a
change. Theheshunt
re Se
reasonserfories, th
ture windinth,
that
ae
NedhaO
motor.4 eThe flo
flow
Fig,
rotate in
lowing
both directions.
It can move in precise angular increments.
It
can
sustain
a
holding torque
at zero
speed.
(F) It can be controlled with digital circuits.
A
stepper
motor moves in accurate
equal angular increments, known
as
circuit. The
T
of the motor shaft. umber
speed of
Generally, stepper motors are manufactured with steps per revolutionshaft.
of 12
144, 180, and 200, resulting shaft increments of
30°, 15°, 5°, 2.5
2.5°,
2°, and 18A,T
step. Special micro-stepping circuitry can be designed to
allow many more
per
s
per revolution, often 10,000
or
steps
steps/revolution more.
The stepper motor is used in
digitally controlled
control system in open
loop mode. The input command is in the form of position
a train of
pulses to turn a shat
through a specified angle.
the application of digital pulses to an electric drive
Irive
and rate of the
pulses control the position and
the
response
steps, n
to
-
Stepper motors are either bipolar,
two power sources or a
switchable
polarity power source, or unipolar,requiring
requiring only one power source. They are
powered by D.C. sources and require digital
for rotation of the motor. Feedback is not circuitry to produce coil energising sequens
an encoder or
always required for control, but the use n
other position sensor can
ensure
control is critical.
accuracy when exact posiuo
Generaly, stepper motors produce less than 1
H.P. and are therefore used
p0sition control applications.
oniy
Construction and working:
A
stepper motor consists ofa slotted
a
stator having
rotor structure
no
multi-pole, mult:-p fo phast
carrying
winding. They typically
phas
windings, the number of poles
use three
an
change per
depends upon the
input pulse.
power
The
rotors may be of the
required
angua
permanent magnet or variable
reluctance type
with
operate
an external
of
is applied to the
drive
When
logic
of
circuit.
the drive
When aa. train O ator
input
logic
currents to the
circuit, the circuit
windings of the motor to rotorthefollows
with the input pulses. Themake
axis of the
supplies
air-gap
b in coind
axis ofthe
the air-gap field cuin
coinc
b
field
arou
Stepper
motors
pulse
It
rever
chuators-Mechar
chanical, Ele
Electrical,
285
Hydraulic and Pneumatic
of the permanentmagnet torque and/or the reluctance
Aca
v
i
u
r
p
t
u
o
torque, depending
the pulse rate and load torque (including inertia effects).
n
net stepper motor:
1 .P e r m a n e n t m a g n e t
c
a
s
o
e
the
n
p o l e s
a
a
permanent magnet stepper motor, the stator
onsists
of
wound
poles,
ermanent magnets.
roor
4.58, s h o w s
the
he
phases
or
stacks of
a
2-phase, 4-pole
manent
magnet stePper
Fig
s
(i) Phase II
() Phase I
Fig. 4.58. Permanent magnet stepper motor.
or rare-earth material which is permanently magnetised.
The rotor is made of ferrite
of 90°.
II is staggered from that of phase I by an angle
The stator stack of phase
If now
rotor is aligned as shown in Fig. 4.58(i).
When the phase T is excited, the
effective stator poles shift anti-clockwise by 22.5
the phase 'T is also excited, the
T
the rotor to move accordingly. Now, keeping the phase
causing
4.58(ii)]
Fig.
the rotor will move another step
still energised, if the phase T is now de-energised,
current will produce a further forward
of 22.5°. The reversal of phase T winding
movement of 22.5°, and so on.
It
can
be
easily
observed/visualised
as
to how the direction of movement
can
be
reversed.
-Each phase is provided with double coils to simplify the switching arrangement
(which is electronically accomplished).
-This type of motor has the advantage of small residual holding torque,
called detent
torque, even when stator is not energized.
2.Variable reluctance stepper motor:
A
variable-reluctance
rotor
stepper motor has no permanent magnet
on
the rotor and the
one.
employed is a ferro-magnetic multi-toothed
h e large differences in magnetic reluctances that exist between the direct and quadrature
0Xes develop the torque. The stationary field developed by the direct current in some
Stator coils tends to develop a torque which causes the rotor to move to the position
Where the reluctance of the flux
path is minimum.
Fig.4.59 shows the basic form of the variable
reluctance
stepper
motor.
With this form the rotor is made of soft-steel and is cylindrical with four poles, i.e.
Tewer poles than on the stator.
A
286
Textbook
of
Me
Stator
Staty
(i) This pair of poles
current being switchenergised ty
to
to give next
step. them
T h i s pair of poles energised by
Current being switched to them and
rotor
rotates to
A
position shown ()
Fig. 4.59. Variable reluctance stepper motor.
When an opposite pair of windings has current switched to them
a
is produced with lines of force which pass from the stator poles
nearest set of
poles on the rotor. Since lines of force
can
be consida
like elastic thread and always trying to shorten
until the rotor and stator poles line up. This is termed the
themselves, the
reluctance.
magnetOughic fe
red to
rotor
position
sition
This form of
stepper generally gives step angles of 4.5° or 15.
Stepping angle, irespective of the type of stepper motor is given as,
360
Number of phases
3.
x
number of
be rathe
of wimil nimm
un
imum
360
poles
Hybrid stepper motor:
This is infact a
permanent magnet stepper motor
with constructional
features of toothed and stacked
rotor adopted
from the variable-reluctance motor..
The stator has
only one set of winding-excited
which
poles
interact with the two rotor stacks.
The
permanent magnet is placed axially along
the rotor in the
form of an annular cylinder over
the motor shaft (See
Fig. 4.60).
np
Teeth
on
end capss
Permanent
magnet
Fig. 4.60. Hybrid
motor roto
The stacks at each end of
the rotor are toothed. So all
the teeth on the
end of the rotor
the same polarity while
acquire
the teeth of the stack
end of the rotor
the opposite
acquire
polarity. The two sets of the
from each other
by
stackat
a one
other
at
tnisplaced
teeth arc
half of the tooth pitch (also called pole
The primary advantage of the hybrid motor
is that if stator excitation
ed, the
pitch)1 ciatit
one
rotor continues to remain
locked into the same
This is due to the reason
position, as before removal irectionby
that the rotor is
prevented to move in eitn
torque because of the
irection
permanent excitation.
Which
MChaOrsMoch
echanical, Electrica Hydraulic and
Pneumatic
287
eed characteristics
of a stepper motor
shows the torque-speed
Torque-speed
l i g .4 . 6 1
teristics
alor
In the
otor
-
stepper motor.
of a
"locked step mode', the
ecelerates and may
to rest
even come
between
oach step. Within this range,
the
motor
be
can
Slewing
iiillIIIT
mode
Locked step mode
instantaneously started, stopped
or reversed without losing step
integrity.
In the "slewing mode", the
speed is too fast to allow
instantaneous
starting,
stopping, or reversing. The
netic field
rough te
be rathe
rotor must be gradually
accelerated to enter this
Speed-
Fig. 4.61. Torque-speed curves
of stepper motor.
mode and
gradually decelerated to leave the mode. While in slewing mode, thee
synch with the stator field rotation and does not settle between stepps.
The curve between the regions in the figure indicates the maximum
torques that the stepper can
rotor is in
will moe
imroride at ditferent speeds without slewing. The curve bordering the outside of the slewing mode
minimun
region represents the absolute maximum torgues the stepper motor can provide at diferent speeds.
Advantages and applications of stepper motor:
Advantages: The stepper motor (a position control device) entails the following advantages:
1. Compatibility with digital
systems.
2. The angular
displacement can be precisely controlled without any feedback
arrangement.
3. No sensors are needed for
position and speed sensing.
4 It can be
readily interfaced with microprocessor (or computer based controller).
Applications: Stepper motors have a wide range of applications, mentioned below:
Paper feed motors in typewriters and printers.
2.
7anent
gnet
or roto.
a c ka to t
t h eoth
3.
Positioning of print
Pens in
heads.
XY-plotters.
Recording heads in computer disc drives.
.
Positioning of worktables and tools in numerically controlled machining equipment.
AISO employed to perform many other functions such as metering, mixing, cutting,
ending, stirring etc. in several commercial, military and medical applications.
4.3.4.6. Servomotors
Introdction: The term servo or servo mechanism refers to a feedback control system in
which the controlled
variable is:
exciahis
Mechanical position, or
d i s p l a c
whic
m o r e n ,
~rection
Tin
Foll
aerivatives eg.,
velocity
and acceleration.
g characteristics
are usually required for a feedback control system:
lowing
characte
) High accuracy.
ATextbook ot
288
o Mechtt
() Remote operation.
(ii) Fast-response.
(7) Unattended control.
control system
the essentinls of a feedback
1. An error detecting device. It determines when the regulated
Following
are
guartity is dif
changes the regulatedSupplies pH
guartity st
AServo-motor should entail the following characteristics
from the reference quantity and sends out the error signal totity
An amplifier. The amplifier receives the error signal and then
t
in tun
the error-correcting devices, which
it matches the reference input.
The output torque of the motor should be proportional to the voltage
control voltage whichisdeveloped by the amplifier in responset o d i ,
to an
error s
2. The direction of the torque developed by the servo-motor should a 0
7stantaneous polarity of the control voltage.
Types of servo-motors:
The servo-motors are of the following two types
1. D.C. seroo-motors.
2. A.C. servo-motors.
1. D.C. servo-motors:
These motors are preferred for very high power systems since they operate
compared to A.C. servo-motors).
These motors may be of the following types
more
eficimt,
Series motors;
Split series motors;
Shunt control motors
Permanent magnet (fixed excitation) shunt motor.
( Series motors:
This motor has
torque.
a
high starting
It draws large current.
The
Source
speed regulation is poor.
Reversal can be obtained by
reversing field voltage polarity
with split series field
winding
(in Split series motor:
The D.C. series motor with
split
field (small fraction kW)
may
be
Constant current -
operated
as a
separately
excited field-controlled motor
(Fig. 4.62).
The armature may be
supplied
from a constant current source.
Armature
M0
Auxiliary
winding
Main
winding
o
D.C. amp
Fig. 4.62. From
l
i
i
e
r
ACtalO
ActuatorsM
ydraulic and Pneumatic
ical torque curve shows the following:
289
-Mechanical, E
le
Electrical,
High stall torque;
apid reduction in torque with increase in speed.
motor
S
h
u
n
t
c
o
n
t
r
o
l
mot has too separate windings: Field winding placed on the stato
ofmotor
Ssis type
type of
This
and the armature
winding placed on the rotor of the machin Both the windings
connected to a D.C. supply source.
are
reas in a conventional D.C. shunt motor, the two windings are connectea
Where
parallel across the D.C. Supply mains, but in a servo-application the windings
inp
D.C. supplies.
driven by separate
shunt motor
( i ) P e r m a n e n t magnet
Permanent
shunt motor where the field is actually
by a
a fixed excitation
are
supplied
is
It i
permanent magnet.
Its
2.A.C.
performance
servo
is similar
to that
of armature controlled
fixed field motor.
motors:
Applications:
These motors
Precision
-
are best suited for
servo-motors
are
low power
applications.
used in
Instrument servos;
-Computers;
-
Inertial guidance systems
The mechanical output
etc.
power of A.C.
servo-motor
varies from 2 watts to
a
few
hundred watts.
certain
induction motor except for
An A.C. servo-motor is basically a two-phase
between a standard splitspecial design features. The main important difference
has thinner conducting bars
motor and an A.C. servo motor is that the latter
phase
in the squirrel cage motor, so that the motor resistance is higher. The torque-speed
characteristics should be linear
as
shown
by
the curve I in
Fig.
I: For normal-split phase motor
or with larger X/R ratio
I : For servo motor or
with small X/R ratioo
-Synchronous speed
Speed, N
Fig. 4.63. Torque-speed
characteristics.
4.63.
290
A
Description of A.C. servo-motors
.Drag-cup rotor servo-motor. Refer
Drag-cup
construction is
used for very low inertia
ayplications.
to
Fig.
extbook
of M
4.64.
Stationary
rotor core
Mechatre
Stator
In this type of motor the rotor
construction is usually of
squirrel cage or drag-cup
hpe: here only a light cup
rotates tuile the rotor corc is
stationary (thus inertia is
Shath
Drag
cup motor
quite small).
The servo-motors contains
Stator
wo windings namely, 1main
Fig. 4.64. Orag-cup roto
winding (sometimes called
servo-motor,
fixed or reference winding)
and control winding. The voltage
are at right.
ang t
another. Usually one winding is excited with a fixed voltage while thes
the
other
is excited by the control
rom servo-amplifier
voltage (which is the output from
applied to the windings
servo-amplis
ona
motor roughly proportional to
While
is
the output torque of the
operation,
control voltage, and the direction of torque is determined by the
in
voltage.
polarity
id
of the appl
Control
2. Shaded-pole type servo-motor:
This type of motor employs a phase-sensitive relay to actuate those contactsne
produce a short-circuit of the shaded-pole winding to produce rotation in the desid
direction.
The main
15
shortcoming of this motor is that it responds only when
of adequate magnitude to cause the relay to operate.
the
amplifier error sia
4.3.4.7. Moving coil motors
There are certain applications which require acceleration much higher than
be achieved in a conventional D.C. servo-motor. The armatures of moving coil D.C. motus
have special constructions which allow a substantial reduction in armature inertia and
inductance, permitting very high accelerations.
Moving coil motors are of the following two types:
1. Shell type.
2.
Disc
or
Pancake
type.
1. Shell type moving coil motor. Refer to Fig. 4.65.
thas
In this type of motor, the rotor consists of only armature winding due to windn
very low inertia ; consequently high acceleration is obtained. Armature
consists of conductors assembled to fornm a thin walled cylinder. Thei s c
or
may have a cylindrical construction as in conventional D.C. moro
nutdts
construction.
Low reluctance path for the stator field is provided by a stationa
material cylinder.
In such a motor the current is axial and flux is radial.
nagneti
4cdta1OYSMechar Electrical, Hydraulic
and
Pneumatic
Stationary
Shell lype
magnets
armature
291
Stationary
iron core
Disc type
Conductors
Commutator
Fig. 4.65. Shell type
moving coil motor.
notors (Tiny motors wth
diameters
around 1 cm)
sting of simply varnished
wires
consist
have
armature winding
arranged in cylindrical form
andla disc type
commutator.
otors find wide applications in card
readers, video systems, cameras
size motors the armature
is
winding made by bonding conductorsetc
and fibre
rmer
resins an
to
glass
together
sing
provide adequate mechanical
Pancak
strength.
cake type moving coil
2.Disc
motor. Refer to
Fig, 4.66.
armatu
motor
is
this
made
in
In
disc
pancake form, and armature conductorss
hle spokes on wheel. Ihe armature
Such m o t o r s
poilyrner
resins
or
or
resemble spokes (
:
a
is formed by
stamping conductors from
fcopper, welding them together andwinding
them on a light weight disc. Conductor
placing
then
joine
with
a
nents
commutator at the centre of the
Here the direction ot flux is axial and armature current is radialdisc.
are
(just opposite
type conventional motors).
Printed
circuit
armature
Conductors
Brushes
Magnets
Fig. 4.66. Disc or Pancake type
moving coil
motor
to
shell
A
292
he
rinciple of
operation is
T h e s e motors a r e m o r e
They
same as
that
robust and
find applicntions where
of
a
conventional
a v a i l a b l e in
ax ial space
1s at a
Textbook
D.C. motor.
sizes
upto
pto fezn
few
preniium
such
of
Mer.
Ac
kilowatts
machine tools, di
as
drives etc
brus
4.3.4.8. Torque motors
Torquc
motors"
are
Spred condition. Some
Ihe
()
(77)
(17)
forque
the D.C. motors
torque
motor
motors are
applications
can
designed
perio
designed
be divided
eds intermitten
stal ed
run for long periods inn a
to operate at low speeds e
d or
speeds intermit
almy
follov
into the
the following three
to
stalled condition.
operate in
Motor is required
to develop the required tension
The purpose of the motor is
material, similar to spring
to
Motor is required to
through only a few revolutions or degrees
devices etc.
valves, switches, clamping
move
of
This category involves continuous movement of the motor at low speed
Examples. Opening
t erntly
sSure ona
evolution,
Example. Reel drive.
motors
(or trapezoidal PMAC)
4.67 shows the cross-section of a 3-phase 2-pole trapezoidal
4.3.4.9. Brushless D.C.
Fig.
PMAC ma
otor.
AA, BB, CC'
concentred
phasewindings
B
C
m
A
N
Permanent
magnet rotor
A
Fig. 4.67. Cross-section of a trapezoidal PMAC motor.
The stator has three concentrated phase windings (AA, BB' and CC') which a
ced
displaced by 120 and each phase winding spans 60° on each side. The voltagesn ss
in three phases are shown in Fig. 4.68. The reason for getting trapezoidal wavero
explain below:
position
When revolving in the counter-clockwise direction, upto 120° rotation from teatom
shown in Fig. 4.67, all top conductors of phase A will be linking the S-polean
conductors of
phase
A will be
linking
the
N-pole.
Hence the
voltage
nduced
phase
inphase
top
in the
o m e conductors.
will be the same during 120° rotation (Fig. 4.67). Beyond 120°, some
conductoence, the
te
eformof
link N-pole and others the S-pole. Same happens with bottom
veform
Hence,
conductors.
voltage induced in phase A linearly reverses in next 60° rotation. Rest
phase A and waveforms of B and C can be explained on the same lines.
becr
ators-Mechanical
ical, Electrical, Hydraulic and
Pneumatic
shows
the indu
ACUG
s the
induced voltage, phase current and
F
i
g
.4
.
6
8
s
D Cmotor
293
torque waveforms of a
brushless
trapezodal
PMAC motor
called a
is calle
mode is
This motor is also drive operating in self-controlled
controlled mode
motor.
ele
conceived
as
as
brushless.
ed D.C. notor,
ceived
electronically commutated
borrhere
same function
here pertorms
pertorms the samo
as the brushes
and commutator in a D.C
se invift currents between armature
conductors to keep the stator and rotor fields
in
in quadrature with
respect to each other.
inoe
fed
An
r
i , tosh
motor
ie,
a1
and
statiornary
E
60
120 180
240
300
360
(a)
6)
0
0
60
1209
180
240
300
360
(c)
Fig. 4.68. Induced voltage, phase current and torque waveforms of a
brushless D.C. motor.
Advantages:
tollowing advantages the conventional D.C. motors
() Long life.
(i Require
maintenance.
practically
(i) High reliability.
wng to the absence of brushes and commutator, brushless D.C. motors claim the
over
no
A
(i7) Low
0Low
(7)
inertia and friction.
interference
radio frequency
m a t u r e windings
Because
specific outputs
(7i1) They
much
(n)
can
be
Textbook of Man
and
4Mus
no1Se.
cooling is mud
the stator, co
are on
obtained.
acceleration
(due
ertia and friction).
to low inerti
better, 1R., hi
5
and can
higher aare common
100000 r-p.m.
speeds-upto
higher
(whereas wound field mo.
otors
exceeding 75 percent
have
a faster
AdaisM
decher
294
and
be Tun
Hig
4
Eco
&
Hig
9
n
High efficiency,
ratings have
efficiency).
much lower
m a c
DCM
Disadoantages:
)
Con
High cost.
Therea
(i) Low stalling torque.
D.C. motor is
The size of a brushless
nearly the
same as
motor.
that of the
co
nventional D
1
Arm
This
Applications
applications
The brushiless D.C. motor finds
recorders
() Tape drive for video
Turn table drives in record
in:
t h i s
s c h e m
Th
Th
players
(i)
disk drives for computers;
(ti) Spindle drives in hard
ents and cOnt
drives in computer peripherals, instruments.
Low cost and low power
(o)
br
th
di
CC
systems.
in
() Gyroscope motors
(i) Cryogenic coolers
(vii) Artificial heart pumps;
(vii) Cooling fans for
sinks.
electronic circuits and heat
4.3.4.10. Electronic control
1-pha
A.C
of D.C. motors
of electrical drives in diferm
the
Normally, it is essential to vary speed desired that the
systembega
industries, it is
fields of application. Usually, in all process
and then gradually increased to meet the maximum
at slow speed in the beginning
production rate, eg, newspaper printing press.
Introduction:
in the field of corntrol isthecon
One of the major achievements of thyristor technology
schemes have totally dominated thehel
of D.C. and A.C. motor drives. Thyristor controlled
of the following advantages :
of control of D.C. as well as A.C. motors because
() Compactrness
(i7) Fast response.
(iin) More efficiency.
(w) More aontrol capabilities.
() More reliability.
Less cost etc.
Advantages of electronic control systems:
The electronic control system claims the following advantages over corivent
(vi)
1. Very compact and small in size.
2. Consumes very less power.
3. Very fast in response.
Fig.
methab
actualorsMechar
ctrical, Hydraulic
ical,
4
M u c h
re
accurate
m o r e .
ranges
ar
Controf
5
reliability
6.
h
Economical
4.
295
Pneumatic
and
efficient than a conventional
much more than any other
mainter
a n c e
system.
systems.
comparatively.
since
cost in
minimum.
protective.
Highly
8
and
electronic
systems more automation, as required for highly sophisticated
In
9. machines, Is possible.
C. Motorspeed control:
p.
everal methods by which the speed of a D.C. shunt motor can be controlled
of the commonly used methods are discussed below.
There
hyristors, some
usig
A
.
r
m
a
t
u
r
e
voltage
control
method
also called the phase control method of speed control. The complete diagramtor
is shown in Fig. 4.69.
Thisi s also
heme
this
The field of motor is excited by a constant D.C. obtained from the full-wave rectifie
The
The armature voltage is varied by varying the firing angle of the SCRs of the thyristor
bridge. Voltage across the armature terminals will be variable D.C. obtained from
and
the full-wave half-controlled thyristor bridge. In the positive half-cycle SCR
and diode D, Will
conduct. Gates of SCRs will be given signal from the triggering circuit (not shown
diode D, will conduct whereas in the negative half cycle SCR2
in the Fig. 4.69).
1-phase
A.C.
D2
SCR,
D3
D
50
Hz
M
D1
SCR2
Shunt
field
D6
D
Full-wave
half controlled
thyristor bridge
Full-wave
rectifier
M Shunt motor
D, D2 Dg, D4, D5,
Dg
Diodes
SCR, SCR, Silicon-controlled
=
rectifiers
g. 4.69. Complete circuit diagram for the armature voltage control method for speed
control of D.C. shunt motor.
The wave
are
shapes for the A.C. input voltage and controlled D.C. armature voltage
shown in
Fig.
4.70.
A
296
Textbook of
A.C. input voltage
hechaton
4ctalo
Time
Full-wave
rectified voltage
across the armature
a
Fig.
4.70. Wave
Time
shapes for A.C. input voltage and
D.C. armature voltage.
controlled
2. D.C. chopper speed control:
A D.C.
purpose of
popularity
This variable
chopper can give variable D.C. at its output.
od of
motors. This metho
speed control of D.C. shunt
devices.
since the introduction of semiconductor
can
be
uilios
ilised
speed control has
gainel
Tachogeneralo
L
3-phase
A.C. input
Chopper
3-phase
circuit
rectifier
D.C.
L.C. filter
Comparator
Error
amplifier
Reference
voltage
Fig.
4.71. Block
shunt
motor
Logic circuit
or firing
control circuit
scheme
diagram representation of a D.C. chopper speed control
shunt motors.
for D.C
cont
Figure 4.71 shows the block diagram representation of a D.C. chopper spe
scheme for D.C. shunt motors.
haserectifin
I n this scheme the 3-phase A.C. is rectified into D.C. by means of a spa
The ripples are minimised with the help of a proper LC fiurer
S
Pper circuit. The
This filtered rectified D.C. serves as the input for the chopper
The ON O
circuit' which decides the firing of the thyristors used in the chopper
durations for the thyristors used are decided by this unit. The input sigra
S I g n a l
circuit is obtained from a 'comparator through an 'error amplifier.
log"
this
to
ACchualorsMec anical, Electrical, Hydraulic and Pneumatic
fechand
297
feedback from the D.C. shunt motor is
equivalent voltage
converted into
into equivalen
Thehy means of "tachogenerator'. The
speed
to the
feedback in the form of voltage
ignal is given
comparator
where
it
with the set reference
is
o If there is a difterence between
compared
the two, it will
an
arn error signal
the error
generate
whichis amplified by
and
amplifier sent to the logic circuit to decide the
OFF durati
tions of the
thyristors connected in the chopper.
a
signal
ON
Choppers
are
Is are
circuitry used.
built by using
This is a very
hecaSe of its fast response.
one or
efficient
two SCRs
method and is
depending upon the tyP and
widely used in industries these days
Fig. 4.72 ws a simple circuit diagram for speed control of a D.C. shunt motor.
An L-C filter Is usea in the input side of the chopper to reduce ripples in the
D.
anut. Diode is the freewheeling diode.
Chopper
circuit
Vp.c
Shunt
field
M
Armature
L-C filter
Fig. 4.72. Circuit diagram of a
D.C.
chopper for speed
control of a D.C. shunt motor.
Fig. 4.73 shows a simple chopper circuit which may be used for controlling the
speed of a D.C. series motor.
L
Series flied
Chopper
circuit
DA
(M
Armature
L-C filter
Fig. 4.73. DC. chopper application for speed control of a D.C, series motor.
variation of ToN and ToF will vary the load voltage at the output of the chopper
Which will change the speed of the motor accordingly.
DIode D has been used as a freerwheeling diode to provide low resistance path for the
urent which will low even at the OFF period of the thyristors. This current flows for
a ittle
time due to the stored energy in the winding which is inductive in nature.
A
298
the
Textbook
chopper to
An 1C filter has been used in the input side of the chopper
in the D.C. input voltage.
converter:
control by using a dual
A dual converter, as the name indicates, uses two converters a ve
3.
Specd
Both the bridges are built by using SCRs. A dual converter may
may
tollowing
controls
of a D.C.
motor:
of
reduce
oduce hethhechaterpp
Me
rectbeifier and an
sed
to
Reversible speed control.
gene
imos
obtain
Plugging.
Regenerative braking.
The above controls are discussed below.
Fig. 4.74 shows the circuit diagram for speed control a D.C.
C. shunt mMot
shunt motor
conTeTtC
usin
a du
L
C
1-phase
A.C
1phase
A.C.
Field
Bridge-1
Bridge-2
L-C filter
Fig. 4.74. Circuit diagram for speed control of a D.C. shunt motor using a
dual converter.
1. Reversible speed control and plugging:
Four SCRs, 1,2, 3 and 4 form the first bridge (Bridge-1) which serves as a 1-phaa
full-wave fully-controlled bridge and rectifies the 1-phase A.C. into D.C. This DC
is filtered by an L.C. filter to remove the ripples. In the positive half cycle SCR
and 2 conduct simultaneously and in the negative half cycle SCRs, 3 and 4 condut
armature
simultaneously. The direction of flow of
current
I is clockwise as showa
in Fig. 4.74.
the second bridge (Bridge-u
reversing the direction of rotation of the motor,
is constituted by tne
gated after commutating the first bridge. The Bridge-2
in the positive haite
5, 6,7 and 8. SCRs 5 and 6 conduct simultaneously
tne
For
a
half cycle. Thus, rotate
SCRs 7 and 8 conduct simultaneously in the negative
tries
and the motor
of flow of armature current is reversed in this case
the opposite direction i.e. in the anticlockwise direction.
the
be
S
clockwise directiorn,
Because the motor was originally running in the
two0 O etOpi
the
in the anticlockwise direction. When
oppose the torque developed
otor wou
commutated. This pro
motor becomes stationary provided bridge-2 is
the
egual,
conduct,
dire
tuite
the bridge-2 further continues to
the motor is called plugging. If
oppos
the
In
direction resulting in speed reversal.
e
ug
start-running in the opposite
firing
the
be
controlled
can
varying
the
by
motor,
speed
of rotation of the
WOud
of
t h es e c a n
bridge.
2t
Ma
ActvatorsMecha
hanical, Electrical, Hydraulic and Pneumatic
299
erative braking
is commutated and bridge-2 is triggered the counter e.m.f.
.casese. after bridge-1
ure of the motor acts as input for bridge-2 whichi connected in the
the armature
generatedi n
The
jnterier
moae. T h
output of bridge-2 which is 1-phase A.C. may be fed
back to the mains
we see that bridge-1 acts as a rectifier and bridge-2 acts as inverter. Iherero
ore,
p
i
v
e
braking
the
K.E.
of
the
motor
is
converted
into
electrical
energy
and
i
nd fed
back
in
regenerative
s u p p l y1 h u s , w e s
pply system there
saving energy.
4.3.5. Single-Phase Motors
43.5.1. General aspects
Thenumber
of machines operating from single-phase supplies is greater than all
her y
types taken in total. For the most part, however, they are only used in the
ott
emaller sizes, less than 5 kW and mostly in the fractional HP. range. They operate
atlower poueryjactors and are relatively nefficient when compared with polyphase
notors. Though simplicity might be expected in view of the two-line supply, the
analysis is quite complicated.
Single-phuase motorspertorm a great variety of useful services in the home, the office,
the factory, in business establishments, on the farm and many other places where
electricity is available. Since the requirements of the numerous applications differ
so widely, the motor-manufacturing industry has developed several types of such
machines, each type having operating characteristics that meet definite demands
For example, one type operates satisfactorily on direct current or any frequency
upto 60 cycles; another rotates at absolutely constant speed, regardless of load;
another develops considerable starting torque and still another, although not
capable of developing much starting torque, is nevertheless extremely cheap to
make and very rugged.
Applications and Disadvantages
Aplications:
4.3.5.2.
Single-phase induction motors are in very wide use in industry especially in
fractional horse-power field.
They are extensively used for electric drive for low power constant speed apparatus
such as machine tools, domestic apparatus and
agricultural machinery in circumstances
where a three-phase supply is not
readily available.
There is a large demand for single-phase induction motors in sizes
ranging from
a
fraction of horse-power upto about 5 H.P.
Disadoantages:
hough these machines are useful for small outputs, they are not used for
large powers
suter from many
and are never used in cases where
disadvantages
three-phase
machines can
as
be
adopted.
ne main disadvantages of single-phase induction motors
Their output is only 50% of the
temperature rise.
three-phase motor,
are
for
a
givern
They have lower power factor.
3. Lower-efficiency.
5
hese motors do not
have
Ore
inherent starting torque.
expensive than three-phase motors of the same output.
frame size and