This document discusses speed control methods for AC induction motors. It describes several methods including pole changing, stator frequency variation, stator voltage variation using a slip ring induction motor, and rotor resistance variation. It also mentions slip power recovery schemes and basic inverter circuits for variable voltage frequency control. The document provides introductions and explanations of these various speed control techniques for AC induction motors.

1. By
ANAND KUMAR

2.  UNIT-I INTRODUCTION
 UNIT-II SPEED CONTROL OF DC MACHINES
 UNIT-III SPEED CONTROL OF AC MACHINES
 UNIT-IV MOTOR STARTERS AND CONTROLLERS
 UNIT-V HEATING AND POWER RATING OF DRIVE
MOTORS

4.  Fundamentals of electric drives
 Advances of electric drive
 Characteristics of loads
 Different types of mechanical loads
 Choice of an electric drive
 Control circuit components: Fuses,
switches
 Circuit breakers
 Contactors, Relay
 Control transformers

5. INTRODUCTION TO ELECTRIC DRIVES
Electrical Drives
Drives are systems employed for motion control
Require prime movers
Drives that employ electric motors as
prime movers are known as Electrical Drives
Electric Drives and Control 5

6. INTRODUCTION TO ELECTRIC DRIVES
Electrical Drives
• About 50% of electrical energy used for drives
• Can be either used for fixed speed or variable speed
• 75% - constant speed, 25% variable speed (expanding)
Electric Drives and Control 6

7. Example on VSD application
motor pump
valve
Supply
Constant speed Variable Speed Drives
Power
In
Power loss
Mainly in valve
Power out
INTRODUCTION TO ELECTRIC DRIVES
Electric Drives and Control 7

8. Example on VSD application
motor pump
valve
Supply
motor
PEC pump
Supply
Constant speed Variable Speed Drives
Power
In
Power loss
Power out
INTRODUCTION TO ELECTRIC DRIVES
Power loss
Mainly in valve
Power out
Power
In
Electric Drives and Control 8

9. INTRODUCTION TO ELECTRIC DRIVES
Conventional electric drives (variable speed)
• Bulky
• Inefficient
• inflexible
Electric Drives and Control 9

11.  Electrical Motors
 AC motor (Synchronous or Asynchronous motor)
 DC Motor (DC shunt, DC series, DC compound motor)
 Power Modulators
 Rectifier (AC-DC)
 Inverters (DC-AC)
 Choppers or DC-DC Converters
 Cycloconverters
 Sources
 DC Source
 AC Source
 Control Unit

12. INTRODUCTION TO ELECTRIC DRIVES
Advantages of electric drives
• Flexible control characteristics
• Compact in size
• Automatic fault detection system
• Available in wide range of speed, torque and power
• It can operate in all the four quadrants of speed – torque plane
• Control gear required for speed control, starting and braking is
usually simple and easy to operate
Electric Drives and Control 12

13. INTRODUCTION TO ELECTRIC DRIVES
Classification of electric drives
Group Drive
• Single motor which drives one or more line shafts
supported on bearings.
• The line shafts may be fitted with either pulleys and belts
or gears Also called as shaft drive
Advantages
• Single large motor can be used instead of a no.of motors
• Normally induction type motor used, can thus work at
about full load, increasing the efficiency
Disadvantages
• No flexibility .If fault occurs, whole process will come to
stop.
• Addition of extra machine to the main shaft is difficult
Electric Drives and Control 13

14. INTRODUCTION TO ELECTRIC DRIVES
Classification of electric drives
Individual Drive
• Each individual machine is driven by a separate motor.
• Example: Lathes (Three phase squirrel cage type im is used)
• Also called as shaft drive
Advantages
• Easy to control each machine
Disadvantages
• Energy transmitted to different parts of the same
mechanism by means of parts like gears, pulleys, etc.
Hence, these occurs some power loss
Electric Drives and Control 14

15. INTRODUCTION TO ELECTRIC DRIVES
Classification of electric drives
Multimotor Drive
• Several drives, each of which serves to actuate one of the
working parts of the driven mechanism
• Example: Complicated metal cutting machine tools, Crane
Advantages
• Suitable for heavy loads
Disadvantages
• Difficult to control the speed.
Electric Drives and Control 15

16. Selection of DC or AC drives
Electric Drives and Control 16
DC Drives
AC Drives
(particularly Induction Motor)
Motor • requires maintenance
• heavy, expensive
• limited speed (due to mechanical
construction)
• less maintenance
• light, cheaper
• high speeds achievable (squirrel-
cage IM)
• robust
Control Unit Simple & cheap control even for high
performance drives
• decoupled torque and flux control
• Possible implementation using
single analog circuit
Depends on required drive performance
• complexity & costs increase with
performance
• DSPs or fast processors required in
high performance drives
Performance Fast torque and flux control Scalar control – satisfactory in some
applications
Vector control – similar to DC drives

17. 1.
Eg: Shaping, Grinding or Shearing, require a
constant torque irrespective of speed.

18. 2. TORQUE PROPORTIONAL TO SPEED
Eg: Calendaring machines, DC generator connected
with resistive load, eddy current brakes.

19. 3. TORQUE PROPORTIONAL TO SQUARE OF THE
SPEED
Eg: Fans, rotary pumps, compressors and ship
propellers

20. 4. TORQUE INVERSELY PROPORTIONAL TO SPEED
Eg: Lathes, boring machines, milling machines,
steel mill coiler and electric traction load.

21. Choice (or) Selection of electric drives
1. Steady state operation requirements
• Nature of speed-torque characteristics, speed regulation,
speed range, efficiency, duty cycle, quadrants of operation,
speed fluctuations
2. Transient operation requirements
• Values of acceleration & deceleration, starting, braking and
reversing performance.
Electric Drives and Control 21

22. 3. Requirements related to the source
• Type of source, magnitude of voltage, voltage fluctuations, power factor, harmonics
4. Capital and running cost, maintenance needs, life
5. Space and weight restrictions
6. Environment and location
7. Reliability

24.  Introduction of speed control of DC motors
 DC shunt motors- Speed Torque
characterisitics
 Ward Leonard method
 DC series motor, series parallel control
 Solid state DC drives
 Thyristor based bridge rectifier circuits
 Chopper circuits

25. Types of DC Motors
Based on the connections of armature and field
windings DC motors classified in to three types they
are
 Separately excited DC motor [Field and armature
windings are excited separately by independent
sources]
 Shunt excited DC motor [field winding and armature
winding are connected in parallel and are excited by a
common source]
 Series excited DC motor [field winding and armature
winding are connected in series and are excited by a
common source]

26.  Three types of speed control available for a
DC motor
1. Armature Voltage control
Reducing the armature voltage V of the motor reduces the
motor speed.
2. Field flux control
Reducing Field voltage reduces the flux, and the motor speed
increases.
3. Armature resistance control
When the resistance is inserted in the armature circuit, the
speed drop  increases and the motor speed decreases.

27. (i) Torque and Armature current characteristic (Ta/Ia)
It is the curve between armature torque Ta and armature
current Ia of a d.c. motor. It is also known as electrical
characteristic of the motor.
(ii) Speed and armature current characteristic (N/ia)
It is the curve between speed N and armature current Ia of
a d.c. motor. It is very important characteristic as it is
often the deciding factor in the selection of the motor for a
particular application.
(iii) Speed and torque characteristic (N/Ta)
It is the curve between speed N and armature torque Ta of
a d.c. motor. It is also known as mechanical characteristic.

28. ( )
( )
tan ( )
a
a
a a
N Speed rpm
I Armature current Amps
R Armture resis ce ohm
flux
V I R
N








( )
a
a
T Torque
I Armature current Amps
flux
T I







29.  Ta/Ia Characteristic:
◦ Since the motor is operating from a constant
supply voltage, flux f is constant (neglecting
armature reaction).
◦ Hence Ta/Ia characteristic is a straight line passing
through the origin as shown in Fig.

30.  N/Ia Characteristic :
◦ The flux f and back e.m.f. Eb in a shunt motor are almost
constant under normal conditions. Therefore, speed of a
shunt motor will remain constant
 N/Ta Characteristic
◦ It may be seen that speed falls somewhat as the load
torque increases.

31.  Conclusions
Following two important conclusions are drawn
from the above characteristics:
(i) There is slight change in the speed of a
shunt motor from no-load to fullload. Hence,
it is essentially a constant-speed motor.
(ii) The starting torque is not high because Ta µ
Ia.

32.  Ta/Ia Characteristic :
 This means that starting torque of a d.c. series motor will be
very high as compared to a shunt motor

33.  N/Ia Characteristic :
◦ The speed of the motor reduces as Torque increases.
 N/Ta Characteristic:
◦ A parabolic curve as shown in figure. It is clear that series motor
develops high torque at low speed and vice-versa.

34.  Series Connection (Speed Decreases)
◦ In series connection voltage varies(reduces) but
current remains constant
◦ When two are more dc series motors are connected
in series then the voltage across each motor
decreases, hence motor speed reduces from rated
value.
 Parallel Connection (Speed Increases)
• In parallel connection voltage remains(reduces) but
current remains constant
• When two are more dc series motors are connected in
parallel, hence motor speed increases or remains
constant.

35. 1. Single phase half controlled rectifier
2. Single phase full converter

36.  The converter operates only in Single
quadrant(Forward motoring or Reverse
motoring)
 The converter operates only in the
rectification
36

38.  The average output voltage is:
 The dc voltage can be varied from Vm/π to
0 by varying α from 0 to π.
(1 cos )
m
dc
V
V 

 
Triggering angle
 

39.  The circuit for a single-phase full converter is
shown below.
 The load is assumed to be highly inductive,
so the load current is continuous and ripple
free.
 The converter operates in the rectification
and inversion modes.
39

40. 40
Rectification
Mode
Inversion
Mode

41. Working
 Mode 1 (0< <180)
◦ T1 and T2 conduct
◦ Input voltage Vs=+ve
◦ Output voltage Vo=+ve, Current Io= +ve
◦ Power flows from source to load (rectification process)
 Mode 2 (180< <180+ )
◦ Output voltage Vo=-ve, Current Io= +ve
◦ Power flows from load to source (Inversion process)
 Mode 3 (180< <360)
◦ T3 and T4 conduct.
◦ Input voltage Vs=-ve
◦ Output voltage Vo=+ve, Current Io= +ve
◦ Power flows from source to load (rectification process)
41

 


42.  The average output voltage is:
 The dc voltage can be varied from 2Vm/π to
-2Vm/π by varying α from 0 to π.








cos
2
)
(
sin
2
2 m
m
dc
V
t
d
t
V
V 
 

42

43. 43

44. 44
Ripple-free Armature
Current

45. 45
 The average armature voltage is
 The power supplied to the motor is
s
a kV
V 
a
s
a
a
o I
kV
I
V
P 


46. 46

47. 47
Regenerative Braking Control Power
Control

48. ECE 442 Power Electronics 48
 Q1 and D2 operate
 Q1 ON, Vs applied
to the motor
 Q1 turned OFF, D2
“free-wheels”
 Armature current
decays

49. ECE 442 Power Electronics 49
 Q2 and D1 operate
 Q2 turned ON,
motor acts as a
generator, and the
armature current
rises
 Q2 turned OFF,
motor returns
energy to the
supply via D1 “free-
wheeling”

50. 50

51. 51
Forward Regeneration Forward Power Control
Reverse Power Control Reverse Regeneration

52. ECE 442 Power Electronics 52
 Q1 and Q2 turned ON
 Supply voltage
appears across the
motor
 Armature current
rises
 Q1 and Q2 turned
OFF
 Armature current
decays via D3 and D4

53. ECE 442 Power Electronics 53
 Q1, Q2, and Q3 turned
OFF
 Turn Q4 ON
 Armature current rises
and flows through Q4,
D2
 Q4 turned OFF, motor
acts as a generator,
returns energy back to
the supply via D1, D2
ia reverses

54. ECE 442 Power Electronics 54
 Q3 and Q4 turned ON
 Supply voltage
appears in the
reverse direction
across the motor
 Armature current
rises and flows in the
reverse direction
 Q3 and Q4 turned
OFF
 Armature current
decays via D1 and D2
ia

55. ECE 442 Power Electronics 55
 Q1, Q3, Q4 turned
OFF
 Q2 turned ON
 Armature current
rises through Q2 and
D4
 Q2 turned OFF
 Armature current
falls and returns
energy via D3 and D4
i a

57.  Introduction of speed control of Induction motors
 Pole changing
 Stator Frequency variation
 Slip ring induction motor – Stator voltage
variation
 Rotor resistance variation
 Slip Power recovery scheme
 Basic Inverter circuits
 Variable voltage frequency control

58.  AC motor Drives are used in many industrial and
domestic application, such as in conveyer, lift, mixer,
escalator etc.
 There are two type of AC motor Drives :
1. Induction Motor Drives
2. Synchronous Motor Drives

59. Two methods of speed control
1. Stator side control
a. Pole changing control
b. Stator voltage control
c. Stator frequency control
2. Rotor side control
a. Rotor resistance control

60. a. Pole changing control
Series Connection
• Reduces the number of poles in the stator
• Increases the speed
• All the winding groups are connected in series
Parallel Connection
• Increases the number of poles in the stator
• Decreases the speed
• Winding groups are connected in parallel
Ns is syncronous speed [rpm]
p is numbers of poles
f is the supply frequency [Hz]
p
f
Ns
120


61. b. Stator voltage control
The speed of the induction motor is varied by
varying the stator voltage. Here, supply frequency is
constant.
The stator voltage can be controlled by two methods
i. Using auto transformer
ii. Using resistance connected in series with stator
winding

62. i) Using auto transformer
Auto transformer provides variable AC voltage
without change in frequency. This voltage fed to
induction motor and hence speed varies
accordingly.
3 phase
auto
transformer
3
phase
IM
R
Y
B

63. ii) Using resistance connected in series with stator
winding
By varying the primary resistance, the voltage drop across
the motor terminal is reduced.
Reduced voltage is applied to the
motor, hence the speed of the motor
is reduced.
Torque is proportional to square of
its stator voltage 2
T α V

64. a. Rotor resistance control

65. Instead of wasting the slip power in the rotor
circuit resistance, a better approach is to convert it
to ac line power and return it back to the line. Two
types of converter provide this approach:
1) Static Kramer Drive - only allows operation at
sub-synchronous speed.
2) Static Scherbius Drive – allows operation above
and below synchronous speed.

66. b) Static Kramer drive

67. c) Static Scherbius drive

68. MOTOR STARTERS AND
CONTROLLERS
Book: Electric drives
Author : n.k.de & p.k sen

69. Introduction
 DC motor starter
◦ Voltage sensing relay
◦ Current sensing relay
◦ Time delay relay
 AC motor starter
◦ Frequency sensing relay
◦ DOL (Direct Online Starter)
◦ Autotransformer starter

70. Why we use starter ?

71.  To limit the starting current the starters are
used.
 At the time of starting the back emf of DC
motor is zero.
Eb=V-IaRa
 For 220V Machine, Ra=1ohm then the
starting current will be 220A.

72. What happens if starters
where not employed?

73.  Strating current goes high hence insulation of
the wires gets failure
 Damages the motor windings.

75. Starter using
 Voltage sensing relay
 Current sensing relay
 Time delay relay

76. Main
coil
energi
zes
Main contactor
(NO) closes
hence motor
starts
When Start
button is
pressed
When motor gains its speed,
corresponding voltage sensing relay
(1AR, 2AR, 3AR) works accordingly
1AR
energizes
when speed
gains
1A (NO)
closes and
cuts the
resistance
Clo
ses
Energiz
es

77. 2AR
energizes
when speed
exceeds
Clos
es
Clos
es
Energ
izes
Closed
state

78. Inter Locking
Relay

79. Starting
resistance
can also be
cut off at
specific
intervals of
time by
using time
delay relays
1AR, 2AR, 3AR are
off time delay
relays.
1. Start Button
In
Energized
state
Opens
Before
pressing
start
button

80. Starting
resistance
can also be
cut off at
specific
intervals of
time by
using time
delay relays
1AR, 2AR, 3AR are
off time delay
relays.
1. Start Button
OPENS
CLOSE
S
CLOSE
S
De-
energize
with time
delay
Closes with
time delay
CLOSE
S
Energizes
De-
energize
with time
delay
Closes with
time delay

84. Main contactor
(NO) closes
hence motor
starts
When Start
button is
pressed
Main Coil
Energizes
Resistance
added during
starting
Frequency
sensing relay
-1 energizes
when speed
picks-up
1A contactor
closes &
removes the
resistance
1FR
closes
1A closes Frequency
sensing relay
-2 energizes
when speed
picks-up
2FR
closes
2A closes
2A contactor
closes &
removes the
resistance

86. UNIT-5
HEATING AND POWER RATING
OF DRIVE MOTORS
Book: Electric drives
Author : n.k.de & p.k sen

87.  LOAD DIAGRAM
 OVERLOAD CAPACITY
 INSULATING MATERIAL
 HEATING AND COOLING OF MOTORS
 SERVICE CONDITION OF ELECTRIC DRIVE
 CONTINUOUS, INTERMITTENT & SHORT TIME DUTY
 INDUSTRIAL APPLICATION

88.  If there is no cooling in the motor the machine
cannot dissipate the heat to the external
medium. So the temperature in the motor
increases to the high value.
 Due to increase in the temperature in the motor,
insulation in the windings get damaged.

89.  It should provide a suitable speed-torque
characteristics to drive the load.
 When the motor is loaded its final steady-state
temperature must be with in the permissible value
of class of insulation used.
 The motor selected should be capable of driving
the load satisfactorily both steady-state and
transient conditions.
 If the motor is fully loaded it must not have excess
temperature rise and also capable of with stand
short time overloads.
 It should have enough starting torque to accelerate
the motor to the desired speed with time.

90.  The right choice of
motor for a given
application can be
found from the
load diagram.
 Motor rating must
be > than load
torque
 Load diagram of
two types
◦ Static or Steady
state component
◦ Dynamic component

91.  Torque(T) vs Time(t) in load
diagram for crane
 Time t1-t2  Hoisting of
load (Load is constant ),
hence Torque T remains
constant.
 t2-t3  Pulley is blocked
by clutch(No load)
 t3-t4  Lowering process(
Load raises and becomes
constant)
 Dynamic components of
load (during hoisting &
lowering)
Hoisti
ng
Loweri
ng
No
loa
d

92. max
max
T =Maximum torque of motor
= Instantaneous torque overload capacity of the motor
r
T
T




93. CLASSIFICATION OF
SUBSTANCES
 Conductors
 Insulators
 Semiconductors

94. CONDUCTORS
The substances through which electric
current can flow easily are called
conductors.
e.g. Silver, gold, copper, aluminum
etc. Conductors have a large number
of free electrons. Generally metals
have a large number of free electrons,
So all metals are good conductors.

95. The materials which have very high
resistivity i.e. offers a very high resistance
to the flow of electric current. Insulating
materials plays an important part in various
electrical and electronic circuits.
In domestic wiring insulating material
protect us from shock and also prevent
leakage current.
So insulating material offers a wide range
of uses in engineering applications. e.g.
Glass, Mica, dry Air, Bakelite etc.

96. SEMICONDUCTORS
The substances whose resistivity lies
between the resistivity of conductors
and insulators are called
semiconductors. e.g. Germanium,
Silicon, Carbon etc.

97. RESISTIVITY
 Resistivity is the resistance between the two opposite faces
of a cube having each side equal to one meter.
Resistivity of
 CONDUCTORS 10-8 to 10-3 ohm-m
 INSULATORS 1010-20 ohm-m
 SEMICONDUCTORS 100-0.5 ohm-m

98.  Operating condition : Before selecting an insulating
material for a particular application the selection should be
made on the basis of operating temperature, pressure and
magnitude of voltage and current.
 Easy in shaping : Shape and size is also important affect.
 Availability of material : The material is easily
available.
 Cost : Cost is also a important factor.

99. CLASSIFICATION ON THE BASIS OF OPERATING
TEMPERATURE
CLASS ‘Y’ INSULATION - 90 ºC
CLASS ‘A’ INSULATION - 105 ºC
CLASS ‘E’ INSULATION - 120 ºC
CLASS ‘B’ INSULATION - 130 ºC
CLASS ‘F’ INSULATION - 155 ºC
CLASS ‘H’ INSULATION - 180 ºC
CLASS ‘C’ INSULATION - >180 ºC

100. CLASS ‘Y’ INSULATION
Material if un-impregnated fall in this category with operating
temperature up to 90 ºC. e.g. paper, cardboard, cotton, poly vinyl
chloride etc.
CLASS ‘A’ INSULATION
Insulators of class Y when impregnated fall in class A with
operating temperature of about 105 ºC.
CLASS ‘E’ INSULATION
Insulation of this class has operating temperature of 120 ºC.
Insulators used for enameling of wires fall in this category. e.g.
pvc etc.

101. CLASS ‘B’ INSULATION
Impregnated materials fall in class B insulation category with
operating temperatures of about 130 ºC. e.g. impregnated mica,
asbestos, fiber glass etc.
CLASS ‘F’ INSULATION
Impregnated materials, impregnated or glued with better
varnishes e.g. polyurethane, epoxides etc. fall in this category
with operating temperature of about 155 ºC.
CLASS ‘H’ INSULATION
Insulating materials either impregnated or not, operating at 180 ºC
fall in this category. e.g. fiberglass, mica, asbestos, silicon rubber
etc.

102. CLASS ‘C’ INSULATION
Insulators which have operating temperatures more
than 180 ºC fall in class C insulation category. e.g.
glass, ceramics, polytera fluoro ethylene etc.

103. The following assumptions are made in determining the
variation of temperature rise(motor temperature minus ambient
temperature) with time
 The atmosphere possesses an infinity thermal capacity, so
the temperature does not change due to heat received from
motor.
 The internal conductivity is infinite and as a result, all parts
in the motor has same temperature.
 The motor is homogeneous, i.e the condition for the cooling
are identical at all the points on the surface of the motor.

104.  Equation 1

105.  From
Equation 1
H H
t t
- -
T T
ss 0
Γ=Γ (1-e )+Γ e

106. 1
2
Гss
Г
o
0
t
Г
1-Initially load
2-Intially at Г=Гo
Variation of temperature rise vs time for heating

107. t
Гss
Гo
Г
Variation of temperature rise vs time for cooling
2
1
1-Load disconnected
2-Load decreased

108. After the disconnecting the motor from
the circuit, the load of the motor has
been decreased, the steady state
temperature rise is not equal to zero.
Motor reaches its steady state
temperature after three to four times of
TH.
TH for squirrel cage self-ventilated
motor lies between 11 to 22 minutes.
TH for wound rotor induction motor lies
between 25 to 90 minutes.

109. Time constant TH does not vary with
load it is determined by the parameters
C and A.
C=G.H and A=S.λ
G=Weight of the active parts of the
machine, kg.
H=Specific heat, cal per kg peroc.
S=cooling surface, m2.
λ=Specific heat dissipation or
emissivity, cal per sec per m2 peroc.

110.  Method of average losses
 Equivalent current method
 Equivalent torque method
 Equivalent power method

111.  The HP rating of a motor to drive a particular load is selected on the basis of thermal
loading.
Continuous Duty
Selection of motor power rating is simple with load as constant.
kW rating of motor is found using kW rating of load(FAN) is found using
N-Speed (rpm) Q- Volume of air (m3 /sec)
T- Load Torque (kg-m) h- pressure (kg/m2 )
Efficiency Efficiency
975
kW
NT
P 

102
kW
Qh
P

