Electric drives and controls

1. ELECTRIC DRIVES & CONTROLS

2.  UNIT-I INTRODUCTION
 UNIT-II SPEED CONTROL OF DC MACHINES
 UNIT-III SPEED CONTROL OF AC MACHINES
 UNIT-IV SPECIAL ELECTRIC MACHINES
 UNIT-V

3. UNIT-I INTRODUCTION

4. • Fundamentals of electric drives
• Characteristics of loads
• Different types of mechanical loads
• Control circuit components: Fuses, switches
• Circuit breakers
• Contactors, Relay
Unit-I INTRODUCTION

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. 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 6

7. 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 7

8. INTRODUCTION TO ELECTRIC DRIVES
Conventional electric drives (variable speed)
• Bulky
• Inefficient
• inflexible
Electric Drives and Control 8

9. Modern Electric Drive

10. Main Parts of Electric Drives
 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

11. 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 11

12. INTRODUCTION TO ELECTRIC DRIVES
Classification of electric drives
1. Group Drive
2. Individual Drive
3. Multimotor Drive
1. 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
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 12

13. INTRODUCTION TO ELECTRIC DRIVES
Classification of electric drives
2. 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 13

14. INTRODUCTION TO ELECTRIC DRIVES
Classification of electric drives
3. 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 14

15. Selection of DC or AC drives
Electric Drives and Control 15
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

16. 1.
Eg: Shaping, Grinding or Shearing, require a constant torque
irrespective of speed.
CHARACTERISTICS OF LOAD

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

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

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

20. 4 Quadrant electronic speed control
in modern STEEL MILL

21. FUSE
• A fuse is a special electrical conductor that is
placed in series with a load and melts when
excessive current flows through it, opening
the circuit.

22. FUSE WIRE RATING

23. TYPES
The major two categories of fuses include:
Low Voltage Fuses
Semi Enclosed or Rewire able Type
Totally enclosed or Cartridge Type
High Voltage Fuses

24. Low Voltage Fuses
• Rewire able Fuses
This kind of fuse is most commonly used in the case of
domestic wiring and small scale usage. Another name for this
type is the KIT-KAT type fuse.
The main metals or alloys used in making fuse wire include
lead, tinned copper, aluminum or tin lead alloy.
Advantage : This type of fuse is that it is easy to install and also
replace

25. Totally Enclosed or Cartridge Type
• In this type of fuse, we have a completely closed container
and there are contacts (metal) on either side.
– D type
– Link Type

26. D Type Cartridge Fuses
• D Type Cartridge Fuses: This cannot be interchanged
and comes with the following main components:
fuse base and cap, adapter ring and the cartridge.
• The fuse base has the cap screwed to it and the
cartridge is pushed into it. The circuit becomes
complete when the tip of the cartridge is in contact
with the conductor. In this case, the main advantage
that we get is that of reliability.

27. High Rapturing Capacity Fuses
• HRC fuse or high rupturing capacity fuse- In that type of fuse, the fuse wire or
element can carry short circuit heavy current for a known time period. During this
time if the fault is removed, then it does not blow off otherwise it blows off or
melts.
• The enclosure of HRC fuse is either of glass or some other chemical compound.
This enclosure is fully air tight to avoid the effect of atmosphere on the fuse
materials. The ceramic enclosure having metal end cap at both heads, to which
fusible silver wire is welded. The space within the enclosure, surrounding the fuse
wire or fuse element is completely packed with a filling powder. This type of fuse is
reliable and has inverse time characteristic, that means if the fault current is high
then rupture time is less and if fault current is not so high then rupture time is long

28. SIZE OF FUSE
• Fuses are grouped by physical size. Up to 30
ampsis one physical size; over 30 amps and up
to 60 amps is a larger size

29. MOTOR PROTECTING FUSES
• Because motors draw four to five times their
normal operating current when they start,
standard fuses often blow during normal
motor operation.
• Where fuses are used to protect motors in the
circuit, a special type of fuse is used called a
dual-element time-delay fuse.

30. SWITCHES

31. SWITCHES
• Switches often they are normally meant for
opening(ON) and closing(OFF).

32. TYPES Abbreviation Description Symbol
SPST
Single Pole
Single Throw
A simple on-off switch: The two
terminals are either connected
together or disconnected from
each other.
SPDT Single Pole,
Double Throw
A simple changeover switch: C
(COM, Common) is connected
to L1 or to L2.
DPST
Double pole,
single throw
Equivalent to two SPST switches
controlled by a single
mechanism
DPDT
Double pole, double throw
TPST
Tri Pole Single Throw

33. TYPES
• Proximity switches often
limit motion and are
often operated by the
movement of a
mechanical device
• General-duty switches are
designed for use in
residential and
commercial applications.
air-conditioning and
appliance loads.

34. CIRCUIT BREAKERS

35. CIRCUIT BREAKERS
• A circuit breaker is an automatically
operated electrical switch designed to protect an
electrical circuit from damage caused
by overload or short circuit.
• Its basic function is to detect a fault condition and
interrupt current flow.
Unlike a fuse, which operates once and then
must be replaced, a circuit breaker can be reset
(either manually or automatically) to resume
normal operation.

36. SYMBOL OF CB
• The advantage of a
circuit breaker over a
fuse is that it can be
manually reset at the
electrical service panel
after an overload,
rather than replaced.

37. TYPES
• Low-voltage circuit breakers (<1000V, <100A)
– MCB (Miniature Circuit Breaker)—rated current
not more than 100 A
– GFCI (Ground Fault Circuit Interrupter) – Protect
against current that flows somewhere outside of
the normal current path.
– AFCI (Arc Fault Circuit Interrupter)- Protects
against electrical arcs

38. • Medium-voltage circuit breakers (1 to 72 KV,
<6300A)
– Vacuum circuit breakers
• These breakers interrupt the current by creating and
extinguishing the arc in a vacuum container
– Air circuit breakers
– SF6 circuit breakers
• Extinguish the arc in a chamber filled with sulfur
hexafluoride gas.
• High-voltage circuit breakers (>72KV)
– Operates with current sensing
protective relays & current transformers

39. CONTACTORS

40. CONTACTOR
• A contactor is an electrically controlled switch
used for switching a power circuit, similar to
a relay except with higher current ratings.
• It has three components.
– Power contacts, Auxiliary contacts, and Contact
springs.
– The electromagnet (or "coil") provides the driving
force to close the contacts.

41. WORKING
• When current passes through
the electromagnet, a magnetic field is
produced, which attracts the moving core of
the contactor.

42. RELAY

43. RELAY
• A relay is an electrically operated switch. Many relays
use an electromagnet to mechanically operate a
switch.
• Relays are used where it is necessary to control a
circuit by a low-power signal.
• Solid-state relays control power circuits with
no moving parts, instead using a semiconductor
device to perform switching.

44. WORKING
• When an electric current is passed through the coil it
generates a magnetic field that activates the
armature, and the consequent movement of the
movable contact(s) either makes or breaks
(depending upon construction) a connection with a
fixed contact.

45. PROTECTIVE RELAY

46. TYPES
• Latching relay
• Reed relay
– A reed relay is a reed switch enclosed in a
solenoid. The switch has a set of contacts inside
an evacuated or inert gas-filled glass tube which
protects the contacts against
atmospheric corrosion;
• Machine tool relay
• Solid-state relay

47. CONTROL TRANSFORMERS

48. CONTROL TRANSFORMER
• The transformer used in control panels are used for
stepping down to a lower and safer voltage for the
coils of contactors, relays, timers, protective devices,
pilot lamps, meterings.
• Provides greater Isolation & safety to the Control
circuits.
• Control transformer maximizes inrush capability and
output voltage regulation

49. UNIT 3
SPEED CONTROL OF THREE
PHASE INDUCTION MOTOR

50. Introduction to speed control
Stator side control
Stator voltage control
Stator frequency control
V/F control
Pole changing method
Rotor side control
Cascaded control method
Rotor resistance control
Slip power recovery scheme
 Static krammer Drive
 Static Scherbius Drive
TOPICS

51. Two major methods:
1. Stator side control
Applicable-Squirrel cage & Wound rotor motors.
2. Rotor side control
Applicable only for Wound rotor(slip ring induction motor).
Speed control of induction motor

52. 1.Stator voltage control :
Constant supply frequency –stator voltage is varied
Methods
• Using Auto Transformer
• Primary resistor connected in series with stator winding
Stator Side Control

53. Using Auto Transformer
• Input – fixed ac voltage
By varying autotransformer
• Output –variable ac voltage

54. • Simple method of speed control
• By varying the primary resistance voltage drop
across the motor terminal is reduced.
• Reduced voltage is fed to the motor.
• Disadvantage-more power loss
Primary resistor connected in series
with stator winding

55. • T α v2
• By varying stator voltage torque can be
changed

56. • 2.Stator frequency control
Under steady state condition we can vary the
input frequency of the motor
Speed of the IM is always close to synchronous
speed of rotating flux.
Synchronous speed of IM is:Ns=120f/P
Where
f=frequency of the supply voltage
P=Number of poles
When supply frequency changes the motor speed
also changes,.it is possible by controlling the
speed of prime mover of the generator

57. • The emf induced in the stator winding of IM is V=2 πf
T1Økw
• where Ø=flux/pole
kw =winding factor
f=frequency of stator voltage
T1=no,of turns in stator winding
Here we consider two cases
I. Low frequency operation at constant voltage
II. High frequency operation at constant voltage

58. Decreasing supply frequency at constant voltage the air gap
flux increased and the magnetic circuits gets saturated
Emf equation V=constant
f=decreases
Ø=increases
Due to this low frequency operation:
1. The resistance will be low leading to high motor currents.
2. More losses
3. Very low efficiency
Low frequency operation at
constant voltage

59. • Constant input voltage stator frequency is increased so the
motor speed also increased
• Due to increase in frequency flux and torque are reduced.
V=constant
f=increases
Ø=decreases
By increasing supply voltage:
1. The no-load speed increases
2. The maximum torque decreases
3. Starting torque reduces
High frequency operation at
constant voltage

60. • Ø=(1/(2 πf T1kw))*(V/f)
• From this by varying the supply voltage the
airgapflux changes.
• This will lead to saturation of the motor.
• To avoid this airgapflux should be maintained
constant
• So, the V/f parameter ratio is maintain
constant
• This is known as V/f control method
3.Voltage/frequency control

61. • It is obtained by using power electric converter
• It is one of the most powerful method and it is
applicable for only below base speed

62. • Input fixed AC voltage
• Rectifier-ACtoDC
• Inverter –Dcto variable AC voltageand variable
frequency
• Output is fed to IM
• By varying(V/f)ratio speed of IM can be
changed

63. • Curve (1)-rated voltage and rated frequency
• Curve (2)-reduced voltade and reduced
frequency
• Here the motor speed is decreased but the
maximum torque is costant
• Speed toque characteristics of V/f method.

64. • For squirrel cage IM
• Slip ring –arrangement is complicated.
• For a constant frequency,the synchronous speed
of the motor is inversely proportional to the
number of poles.
• Nsα(1/P)
• By changing the poles the motor synchornous
speed can be varied.
• Machines-Pole changing motors or multispeed
motor
4.Pole changing method

65. • A very simple method
• Expensive arrangement for changing no. Of
stator poles
• 2 separate stator windings are used for 2
different poles but for an economical single
stator winding-divided into few coil group.
• By changing these coil no. of poles can be
changed.
• For simplicity 2motor winding group is divided
• This allow change in pole by a factor 2

66. • Here a phase winding consists of 2 groups(AB&CD)
• Winding consists of 6 coils(1to6)
• AB-(1,3,5)connected in series
• CD-(2,4,6)connected in series
• Currents are made to carry either in series or parallel.
• Current flow AtoB and DtoC-has 6 poles

67. • If AB is reversed then the coil will produce north
poles
• Consequently it will produce south pole in the
inter-pole spaces
• Now the machines has 12 poles
• Connected in series or parallel for both pole no. 6
and 12

68. 1.cascade control method:
• It is also known as tandem control
• Used for speed control of slip ring IM
• It consists of 2 motors:1st–mainmotor(M1),2nd-auxilary
motor(M2)
• Input 3 phase supply to(M1),
• The slip ring voltage of (M1)is fed to(M2)
• This connection is called cascade connection
Rotor side control

69. • In this method,if both motors produces the
toque in the same direction it means
cumulative cascading and opposite direction
• It means differential cascading

70. Contn..

71. • Disavantages
1. This method requires two motors .
2. More expensive.
3. Wide range of speed control is not possible.
4. It cannot be operated when P1=P2or P1‹P2.

72. • A simple and primitive method for speed control
by mech variation.
• Applicable for slip ring IM
• External resistance can be added in the rotory
circuit.
Rotor resistance control

73. By increasing rotor circuit resistance the starting torque also
increases

74. 1. Smooth and wide range of speed control
2. Absences of in-rush starting current
3. Availability of full-rated torque at starting.
4. High line power factory.
5. Absence of line current harmonics,
6. Starting torque can be improved
Advantages

75. 1. Reduced efficiency because the slip energy is
wasted in the rotor circuit resistance.
2. Speed changes vary with load variation.
3. Unbalance in voltage and current if rotor
circuit resistance are not equal.
Disadvantages

76. 1.cascade control is also known as___________
2.stator side control is applicable for_________
3.in change in stator frequency control the IM operates in
the small region under__________condition.
4.primary resistor connected in_________with stator
winding.
5.in stator voltage control variable AC can obtained
by__________transformer.
6.pole changing method is applicable for (slip
ring/squirrel cage)
7.In rotory side control the type we left is________
One mark

77. • 1.cascade control is also known as_
• 2.stator side control is applicable
• 3.in change in stator frequency control the IM operates in
the small region under_ _condition.
• 4.primary resistor connected in_ _with stator
winding.
• 5.in stator voltage control variable AC can obtained
by_ _transformer.
• 6.pole changing method is applicable for (slip
ring/ )
• 7.In rotory side control the type we left is
Answers

79. UNIT - 4
MOTOR STARTERS AND
CONTROLLERS
Book: Electric drives
Author : n.k.de & p.k sen

80. TOPICS
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

81. Why we use starter ?

82. • 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.

83. What happens if starters
where not employed?

84. • Strating current goes high hence insulation of
the wires gets failure
• Damages the motor windings.

85. RELAY DIAGRAM

86. DC MOTOR STARTER
Starter using
 Voltage sensing relay
 Current sensing relay
 Time delay relay

87. STARTER USING VOLTAGE
SENSING RELAY
Main
coil
energiz
es
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
Clos
es
Energizes

88. Contd….
2AR energizes
when speed
exceeds
Close
s
Close
s
Energiz
es
Closed
state

89. STARTER USING CURRENT
SENSING RELAY
Inter Locking
Relay

90. Time delay relay
(OFF Time Delay)
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 pressed
2. M coil energized, M
contacts of NC
In Energized
state
Opens
Before
pressing start
button

91. Time delay relay
(OFF Time Delay)
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 pressed
2. M coil energized, M
contacts of NC
OPENS
CLOSES
CLOSES De-energize
with time
delay
Closes with
time delay
CLOSES
Energizes
De-energize
with time
delay
Closes with
time delay

92. Wound rotor induction motor starter
using time delay relays

93. DOL (Direct Online Starter)

94. DOL (Circuit Diagram)

95. FREQUENCY SENSING RELAY
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

96. AUTO TRANSFORMER STATER

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

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

99. Need of cooling in motors
• 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.

100. REQUIREMENTS OF A DRIVE MOTOR
• 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.

101. Load diagram, Overload capacity
• 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

102. Load diagram of crane
• 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

103. • Where Tr is the rated full load torque
Overload capacity varies for different motors.
DC motor > 2.5
AC motor  lies between (1.7 to 3.4)
max
max
T =Maximum torque of motor
= Instantaneous torque overload capacity of the motor
r
T
T




104. CLASSIFICATION OF
SUBSTANCES
 Conductors
 Insulators
 Semiconductors

105. 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.

106. 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.
INSULATORS

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

108. 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

109. FACTORS AFFECTING SELECTION OF AN
INSULATING MATERIAL
 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.

110. 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

111. 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.

112. 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.

113. 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.

114. HEATING COOLING OF MOTORS
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.

115. Contd…..
Equation 1

116. Contd…..
From
Equation 1
H H
t t
- -
T T
ss 0
Γ=Γ (1-e )+Γ e

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

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

119. Contd…..
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.

120. Contd…..
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.

121. Selection of motor power capacity
Method of average losses
Equivalent current method
Equivalent torque method
Equivalent power method

122. SELECTION OF MOTOR RATING
 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



123. Power Rating for Continuous Duty
As shown in figure, Load does not remain constant.
To find the equivalent power
Then rated power of motor
is taken as
2 2 2
1 1 2 2
1 2
....
........
n n
eq
n
P t P t P t
P
t t t
  

  
r eq
P =(1.1 to 1.3)*P

124. Power Rating for Continuous Duty
To find the Average losses
To find the equivalent current
To find the equivalent torque
1 1 2 2
1 2
..........
........
L L Ln n
av
n
W t W t W t
W
t t t
  

  
2 2 2
2 1 1 2 2
1 2
..........
........
n n
eq
n
I t I t I t
I
t t t
  

  
2 2 2
2 1 1 2 2
1 2
..........
........
n n
eq
n
T t T t T t
T
t t t
  

  