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EE 6361- ELECTRICAL DRIVES & CONTROL II/III MECHANICAL
EE
A Course Material on
EE – 6361 ELECTRICAL DRIVES & CONTROL
By
Mr. S.SATHYAMOORTHI /R.RAJAGOPAL
ASSISTANT PROFESSOR
DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING
SASURIE COLLEGE OF ENGINEERING
VIJAYAMANGALAM – 638 056
1 R.RAJAGOPAL, S.SATHYAMOORTHI,AP/EEE 2015-16
EE 6361- ELECTRICAL DRIVES & CONTROL II/III MECHANICAL
QUALITY CERTIFICATE
This is to certify that the e-course material
Subject Code : EE- 6361
Subject : Electrical Drives & Control
Class : II Year MECH
Being prepared by me and it meets the knowledge requirement of the university curriculum.
Signature of the Author
Name: R.RAJAGOPAL,S.SATHYAMOORTHI
Designation: AP/EEE
This is to certify that the course material being prepared by Mr.S.Sathyamoorthi / R.Rajagopal is of adequate
quality. He has referred more than five books among them minimum one is from aboard author.
Signature of HD
Name: Mr. E.R.Sivakumar
SEAL
2 R.RAJAGOPAL, S.SATHYAMOORTHI,AP/EEE 2015-16
EE 6361- ELECTRICAL DRIVES & CONTROL II/III MECHANICAL
EE6361 ELECTRICAL DRIVES AND CONTROL
Unit-I Introduction
Basic elements-types of electric drives-factors influencing electric drives-heating and cooling curves-
loading conditions and classes of duty-Selection of power rating for drive motors with regard to
thermal overloading and load variation factors
Unit-II Drive motor characteristics
Mechanical characteristics- speed- torque characteristics of various types of load and drive motors -
braking of electrical motors-dc motors: shunt, series, compound motors-single phase and three phase
induction motors
Unit-III Starting methods
Types of d.c motor starters-typical control circuits for shunt and series motors-three phase squirrel and
slip ring induction motors
Unit-IV Conventional and solid state speed control of D.C Drives
Speed control of DC series and shunt motors-Armature and field control, ward-leonard control system-
using controlled rectifiers and DC choppers –applications
Unit-V Conventional and solid state speed control of AC drives
Speed control of three phase induction motor-Voltage control, voltage/frequency control, slip power
recovery scheme-using inverters and AC voltage regulators-applications
TEXT BOOKS
1. VEDAM SUBRAMANIAM “Electric drives (concepts and applications)”, Tata McGraw-Hill.2001
2. NAGARATH.I.J & KOTHARI .D.P,”Electrical machines”, Tata McGraw-Hill.1998
REFERENCES
1. PILLAI.S.K “A first course on Electric drives”, Wiley Eastern Limited, 1998
2. M.D. SINGH, K.B.KHANCHANDANI,”Power electronics,” Tata McGraw-Hill.1998
3. H.Partab,”Art and science and utilization of electrical energy,”Dhanpat Rai and sons, 1994
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PAGE
CHAPTER
CONTENT NO
1
INTRODUCTION TO ELECTRICAL DRIVES
7
INTRODUCTION
1.1 7
BLOCK DIAGRAM OF AN ELECTRICAL DRIVES
1.1.1 8
BASIC COMPONENT (or) ELEMENTS OF ELETCRIC DRIVES
1.2 FACTORS INFLUENCING THE CHOICE OF ELECTRICAL DRIVES 9
1.3 10
CLASSIFICATION OF ELECTRIC DRIVES WITH FACTOR
1.3.1 10
Group drive
1.3.2 10
Individual drive
1.3.3 Multimotor drive 11
1.4 11
LOAD CONDITIONS IN MOTOR
1.4.1 11
Classification of loads
1.4.2 11
Different type of industrial loads
1.5 12
HEATING AND COOLING CURVES
1.6 15
CLASSES OF MOTOR DUTY
1.7 18
SELECTION OF POWER RATING OF MOTORS
1.7.1 18
Continuous duty and constant load
1.7.2 18
Continuous duty and variable load
1.7.3 22
Short time rating of motor
2
DRIVE MOTOR CHARACTERISTICS
2.1 25
TYPES OF ELECTRICAL MACHINES
2.1.1 25
Applications of Dc Motor
2.1.2 25
Characteristics of Dc Motors
2.1.3 26
Types of Electric Braking
2.2 26
DC SHUNT MOTORS
2.2.1 26
Characteristics of Dc Shunt Motor
2.2.2 28
Electric Braking in Dc Shunt Motor
2.3 31
DC SERIES MOTOR
2.3.1 31
Characteristics of Dc Series Motor
2.3.2 33
Electric Braking in Dc Series Motor
2.4 34
COMPOUND DC MOTOR
2.4.1 34
Characteristics of DC Compound Motor
2.4.2 35
Electric Braking in DC Compound Motor
2.5 36
APPLICATIONS OF DC MOTORS
2.6 36
SINGLE PHASE INDUCTION MOTORS
2.6.1 36
CONSTRUCTION AND WORKING PRINCIPLE
2.6.2 38
TORQUE-SLIP CURVE FOR INDUCTION MOTOR
2.6.3 38
ELECTRIC BRAKING IN AC INDUCTION MOTOR
2.7 41
THREE PHASE INDUCTION MOTOR
2.7.1 42
CONSTRUCTIONAL DETAILS
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2.7.2 45
WORKING PRINCIPLE OF THREE PHASE INDUCTION MOTOR
3 47
STARTING METHODS
3.1 47
INTRODUCTION
3.1.1 47
Prime Purpose (or) Necessity of a Starter For Motors
3.1.2 47
Protective Devices in a DC/AC Motor Starter
3.1.3 47
Starters for DC Motor
3.2 48
THREE POINT STARTER
3.3 49
FOUR POINT STARTER
3.4 50
TWO POINT STARTER
3.5 52
STARTERS FOR AC STARTERS
3.5.1 52
Necessity for Starter
3.5.2 53
Prime Purpose of a Starter For Motors
3.5.3 53
Need For Starter in an Induction Motor
3.6 53
D.O.L STARTER
3.7 54
STATOR RESISTANCE (OR) PRIMARY RESISTANCE STARTER
3.8 55
PRIMARY REACTANCE STARTER (or) AUTO TRANSFORMER STARTERS
3.9 58
STAR –DELTA STARTER
3.10 59
ROTOR RESISTANCE STARTERS
3.11 60
COMPARE THE INDUCTION MOTOR STARTERS
4
CONVENTIONAL & SOLID STATE SPEED CONTROL OF D.C DRIVES
4.1 62
INTRODUCTION
4.2 62
EXPRESSION FOR SPEED FOR A DC MOTOR
4.3 62
Applications of DC Drives
4.4 62
Advantages of DC Drives
4.5 63
Conventional Methods of Speed Control
4.5.1 63
Speed control of DC Shunt Motors
4.5.2 64
Speed control of DC Series Motors
4.5.3 65
Ward Leonard Control System
4.6 66
Solid state Speed Control of DC Motor
4.6.1 67
Single phase Controlled rectifier fed DC drives
5
CONVENTIONAL & SOLID STATE SPEED CONTROL AC DRIVES
5.1 72
INTRODUCTION
5.2 72
SPEED CONTROL OF DRIVES
5.3 72
Advantages of Induction motor
5.4 72
Applications of Induction motors
5.5 73
Speed control of three phase induction motor
5.6 75
Slip Power Recovery Scheme
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UNIT-I
INTRODUCTION
Basic elements
Types of electric drives
Factors influencing electric drives
Heating and cooling curves
Loading conditions and classes of duty
Selection of power rating for drive motors with regard to thermal
overloading and load variation factors
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INTRODUCTION
UNIT
1
1. INTRODUCTION
Drive:
A combination of prime mover, transmission equipment and mechanical Working load is
called a drive
Electric drive:
An Electric Drive can be defined as an electromechanical device for converting electrical
energy to mechanical energy to impart motion to different machines and mechanisms for various kinds
of process control.
1.1 BLOCK DIAGRAM OF AN ELECTRICAL DRIVES
The basic block diagram for electrical drives used for the motion control is shown in the
following figure1.1
SOURCE POWER MOTOR LOAD
AC (or) DC MODULATOR
INPUT CONTROL SENSING
UNIT UNIT
Fig 1.1 Block Diagram for Electrical Drives
The aggregate of the electric motor, the energy transmitting shaft and the control equipment by which
the motor characteristics are adjusted and their operating conditions with respect to mechanical load
varied to suit practical requirements is called as electric drive.
Drive system=Drive + load
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1.1.1 BASIC COMPONENT (or) ELEMENTS OF ELETCRIC DRIVES
Block diagram of electric drive:
1. Load: usually a machinery to accomplish a given task. Eg-fans, pumps, washing machine etc.
2. Power modulator: modulators (adjust or converter) power flow from the source to the motion
3. Motor: actual energy converting machine (electrical to mechanical)
4. Source: energy requirement for the operation the system.
5. Control: adjust motor and load characteristics for the optimal mode.
Power modulators:
Power modulators regulate the power flow from source to the motor to enable the motor to
develop the torque speed characteristics required by the load.
The common function of the power modulator is,
They contain and control the source and motor currents with in permissible limits during
the transient operations such as starting, braking, speed reversal etc.
They converts the input electrical energy into the form as required by the motors.
Adjusts the mode of operation of the motor that is motoring, braking are regenerative.
Power modulators may be classified as,
Converters uses power devices to convert uncontrolled valued to controllable output.
Switching circuits switch mode of operation
Variable impedance
Converters
They provide adjustable voltage/current/frequency to control speed, torque output power of the
motor.
The various type of converters are,
AC to DC rectifiers
DC to DC choppers
AC to AC choppers
AC to AC –AC voltage controllers (voltage level is controlled)
Cyclo converter (Frequency is controlled)
DC to AC inverters
Switching circuits
Switching circuits are needed to achieve any one of the following.
Changing motor connection to change its quadrant of operation.
Changing motor circuits parameters in discrete steps for automatic starting and braking
control.
For operating motors and drives according to a predetermine sequence
To provide inter locking their by preventing maloperation
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Disconnect under up normal condition
Eg: electromagnetic contacters,
PLC in sequencing and inter locking operation,
solid state relays etc.
Variable impedance
Variable resisters are commonly used for AC and DC drives and also needed for dynamic
braking of drives
Semiconductors switch in parallel with a fixed resistance is used where stepless variation
is needed. inductors employed to limit starting current of ac motors.
1.2 FACTORS INFLUENCING THE CHOICE OF ELECTRICAL DRIVES
(i) Nature of electric supply
Whether AC or DC supply is to be used for supply
(ii) Nature of the drive
Whether the particular motor is going to drive individual machine or a group of
machines
(iii)Capital and running cost
(iv) Maintenance requirement
(v) Space ad weight restrictions
(vi) Environment and location
(vii) Nature of load
Whether the load requires light or heavy starting torque
Whether load torque increases with speed remain constant
Whether the load has heavy inertia which may require longer straight time
(viii) Electrical characteristics of motor
Starting characteristics,
running characteristics,
speed control and
Braking characteristics
(ix) Size, rating and duty cycle of motors
Whether the motor is going to the operator for a short time or whether it has to
run continuously intermittently or on a variable load cycle
(x) Mechanical considerations
Type of enclosures, type of bearings, transmission of drive and Noise level.
Due to practical difficulties, it may not possible to satisfy all the above
considerations.
In such circumstances, it is the experience and knowledge background which
plays a vital role in the selection of the suitable drive.
The following points must be given utmost important for the selection of motor. The factors are:
Nature of the mechanical load driven
Matching of the speed torque characteristics of the motor with that of the load
Starting conditions of the load.
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1.3 CLASSIFICATION OF ELECTRIC DRIVES WITH FACTOR
The choice of the electric drives
There are three classification namely
grope drive
individual drive
multimotor drive
1.3.1 Group drive
One motor is used as a drive for two or more than machines. The motor is connected to a long
shaft. All the other machines are connected to this shaft through belt and pulleys.
Advantages:
Grope drive is most economical because, the rating of the motor used may be comparatively
less than the aggregate of the individual motors required to drive each equipment, because all of
they may not be working simultaneously.
Grope drive reduces the initial cost of installing a particular industry.
Cost is less because of investment in one motor which is lesser in HP rating.
Disadvantages:
The use of this kind of drive is restricted due to the following reasons:
It is not possible to install any machine as per our wish. so, flexibility of lay out is lost.
The possibility of installation of additional machines in an existing industry is limited.
In case of any fault to the main driving motor, all the other motors will be stopped
immediately.
so, all systems will remain idle and is not advisable for any industry.
Level of noise produced at the site is high.
Because of the restrictions in placing other motors, this kind of drive will result in untidy
appearance, and it is also less safe to operate.
Since all the motors has to be connected through belts and pulleys, large amount of energy is
wasted in transmitting mechanisms. Therefore, power loss is high.
1.3.2 Individual drive
In this drive, there will be a separate driving motor for each process equipment.
One motor is used for transmitting motion to various parts or mechanisms belonging to signal
equipment.
Ex: Lathe
One motor used in lathe which rotates the spindle, moves feed with the help of gears and
imparts motion to the lubricating and cooling pumps).
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Advantages:
Machines can be located at convenient places.
Continuity in the production of the processing industry is ensured to a high level of
reliability.
If there is a fault in one motor, the effect on the production or output of the industry will
not be appreciable.
Disadvantages:
Initial cost is very high.
1.3.3. Multimotor drive
In this type of drive, separate motors are provided for actuating different parts of the driven
mechanism.
Ex: cranes, drives used in paper mills, rolling mills etc.,
In cranes, separate motors are used for hoisting, long travel motion and cross travel motion.
1.4 LOAD CONDITIONS IN MOTOR
The load requirements are in either of
Speed control
Torque control
Depending upon the load requirements the motor has to be chosen.
For example in traction system the load (traction network) needs high starting torque
(initiali.e.,high current value is needed at t6he start. A series motor provides a high starting
torque as .Hence series motor should be chosen for traction system.
1.4.1 Classification of loads
Torque dependent on speed
(Ex-hoists, pumping of water or gas against constant pressure)
Torque linearly dependent on speed
(Ex- motor driving a DC generator connected to a fixed resistance load generator field
value is kept constant)
Torque proportional to square of speed
(Ex- fans, sentrifugal pumps, propellers)
Torque inversely proportional to speed
(Ex-milling and boring, machines)
1.4.2 Different type of industrial loads
There are three types of industrial loads under which electric motors are required to work.
they are
Continuous load
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Intermittent load
Variable or fluctuating load
Continuous load
Load is continuous in nature
Ex- Pumps or fans require a constant power input to keep them operating.
Intermittent load
This type classified in to two types
Motor loaded for short time and then shunt off for sufficiently longer duration
temperature is brought to the room temperature
Eg: kitchen mixie.
The electrical loss is more due to constant ON/OFF delay period
Moor loaded for short time and shunt off for short time .
Here the motor cannot be cooled down to the room temperature comparison of the two
methods it can be Inferred.
The temperature level of motor is not brought to the room temperature.
1.5 HEATING AND COOLING CURVES
A machine can be considered as a homogeneous body developing heat internally at uniform rate
and dissipating heat proportionately to its temperature rise,
RELATION SHIP BETWEEN TEMPERATURE RISE AND TIME
Let,
P =heat developed, joules/sec or watts
G =weight of active parts of machine, kg
h =specific heat per kg per deg cell
S = cooling surface, m2
2
= specific heat dissipation (or) emissivity, J per sec per m of
Surface per deg cell difference between surface and ambient cooling medium
= temperature rise, deg cell
=final steady temperature rise, deg cell
m
t =time, sec
=heating time constant, seconds
'
=cooling time constant, seconds
Assume that a machine attains a temperature rise after the lapse of time t seconds.
In an element of time “dt” a small temperature rise “d” takes place.
Then,
Heat developed = p.dt
Heat developed = Gh.d
Heat dissipated = S.dt
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Therefore, total heat developed=heat stored + heat dissipated
.
Ghd + S dt= p.dt
d s p
.
dt Gh Gh
This is a differential equation and solution of this equation is,
p
(s /Gh)t
ke
s
Where k is a constant of integration determined by initial conditions.
Let the initial temperature rise to be zero at t=0.
p
Then, 0 k
s
p
k
s
s
()
p
Gh
Hence, (1et) - - - - - - - - - - - - - (1)
s
p
When t= , , the final steady temperature rise.
m
s
p Gh
Represent and - - - - - - - - - - - - - -(2)
m
ss
Equation 1 can be written as
1
(1 e ) - - - - - - - - - - -(3)
m
Where is called as heating time constant and it has the dimensions of time.
Heating time constant
Heating time constant is defined as the time taken by the machine to attain 0.623 of its
final steady temperature rise.
When t= ,
1
(1 e )
m
0.632
m
The heating time constant of the machine is the index of time taken by the machine to attain
its final steady temperature rise.
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Gh
We know that , therefore, the time constant is inversely proportional to has a larger
s
value for ventilated machines and thus the value of their heating time constant is small.
The value of heating time constant is larger for poorly ventilated machines with large or totally
enclosed machines, the heating time constant may reach several hours or even days.
When a hot body is cooling due to reduction of the losses developed in it, the temperature
time curve is again an exponential function
t
() e - - - - - - - - - - - - - (4)
f i f
Where,
=final temperature drop (the temperature at which whatever heat is generated is
f
dissipated)
p
= where, is rate of heat dissipation while cooling
'
s
= the temperature rise above ambient in the body at time t=0
i
Gh
'
= cooling time constant=
'
s
If motor where disconnected from supply during cooling, there would be no losses taking place and
hence, final temperature reached will be the ambient temperature.
There fore, =0 and hence equation (4) becomes
f
1
'
e
i
Cooling time constant
'
, 0.368
At t=
i
Cooling time constant is, therefore, defined as the time required cooling the machine down to
0.368 times the initial temperature rise above ambient temperature.
Fig.1.2 Heating and cooling time curves
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1.6 CLASSES OF MOTOR DUTY
various load time variations encountered into eight classes as
(i) continuous duty
(ii) short time duty
(iii) intermittent periodic duty
(iv) intermittent periodic duty with starting
(v) intermittent periodic duty with starting & braking
(vi) continuous duty with intermittent periodic loading
(vii) continuous duty with starting & braking
(viii) Continuous duty with periodic speed changes.
TL
t
Fig-1 (a) t (b)
Ө
TL
t
Fig-2 (a) (b)
TL
Ө
Fig3 (a) t t
Ө (b)
t t
Fig 4 (a) (b)
TL Ө
Fig 1.3 Classes of Motor Duty
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Where,
TL – Load torque in N-M,
Ө- Temperature in Deg.centigrade,
t- Time in seconds.
1. Continuous duty:
This type drive is operated continuously for a duration which is long enough to reach its steady
state value of temperature.
This duty is characterized by constant motor torque and constant motor loss operation.
Depicted in fig.1 (a) & (b).
This type of duty can be accomplished by single phase/ three phase induction motors and DC
shunt motors.
Examples:
Paper mill drives ,
Compressors
Conveyors,
Centrifugal pumps and
Fans ,
2. Short time duty:
In this type drive operation, Time of operation is less than heating time constant and motor
is allowed to cool off to room temperature before it is operated again.
Here the motor can be overloaded until the motor temperature reaches its permissible
limit. Depicted in fig.2 (a) & (b).
This type of duty can be accomplished by single phase/ three phase induction motors and
DC shunt motors, DC series motors, universal motors.
Examples:
Crane drives ,
Drives for house hold appliances
Turning bridges
Sluice gate drives
Valve drives and
Machine tool drives.
3. Intermittent periodic duty:
In this type drive operation, It consists of a different periods of duty cycles
I.e. a period of rest and a period of running, a period of starting, a period of braking.
Both a running period is not enough to reach its steady state temperature and a rest period
is not enough to cool off the machine to ambient temperature.
In this type drive operation, heating due to starting and braking is negligible.
Depicted in fig.3 (a) & (b).
This type of duty can be accomplished by single phase/ three phase induction motors and
DC shunt motors, universal motors.
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Examples:
Pressing
Cutting
Drilling machine drives.
4. Intermittent periodic duty with starting:
This is intermittent periodic duty where heating
Due to starting can‟t be ignored.
It consists of a starting period; a running period, a braking period & a rest period are being
too short to reach their steady state value.
In this type of drive operation, heating due to braking is negligible.
Depicted in fig.4 (a) & (b).
This type of duty can be accomplished by three phase induction motors and DC series
motors, DC compound motors, universal motors.
Examples:
Metal cutting,
Drilling tool drives,
Drives for forklift trucks,
Mine hoist etc.
5. Intermittent periodic duty with starting & braking:
This is an intermittent periodic duty where heating during starting & braking can‟t be
ignored.
It consists of a starting period, a running period; a braking period & a rest period are
being too short to reach their steady state temperature value.
Depicted in fig.5 (a) & (b).
This type of duty can be accomplished by single phase/ three phase induction motors and
DC shunt motors, DC series motors, DC compound motors, universal motors.
Examples:
Billet mill drive
Manipulator drive
Ingot buggy drive
Screw down mechanism of blooming mill
Several machine tool drives
Drives for electric suburban trains and
Mine hoist
6. Continuous duty with intermittent periodic loading:
This type of drive operation consists a period of running at constant load and a period of
running at no load with normal voltage to the excitation winding in separately excited
machines.
Again the load and no load periods are not enough to reach their respective temperature
limits.
This duty is distinguished from intermittent periodic duty by running at no load instead of
rest period.
This type of duty can be accomplished by single phase/ three phase induction motors and
DC compound motors, universal motors.
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Examples:
Pressing
Cutting
Shearing and
Drilling machine drives.
7. Continuous duty with starting & braking:
It consists a period of starting, a period of running & a period of electrical braking.
Here period of rest is negligible.
This type of duty can be accomplished by single phase/ three phase induction motors.
Examples:
The main drive of a blooming mill.
8. Continuous duty with periodic speed changes:
It consists a period of running in a load with a particular speed and a period of running at
different load with different speed which are not enough to reach their respective steady state
temperatures.
Further here is no period of rest.
This type of duty can be accomplished by single phase/ three phase induction motors and DC
series motor in traction.
Examples:
All variable speed drives.
1.7 SELECTION OF POWER RATING OF MOTORS
From the point of view of motor rating for various duty cycles in section 1.6 can be broadly classified
as:
Continuous duty and constant load
Continuous duty and variable load
Short time rating
1.7.1 Continuous duty and constant load
If the motor has load torque of T N-m and it is running at radians/seconds, if efficiency in
, then power rating of the motor is
T
P = KW
1000
Power rating is calculated and then a motor with next higher power rating from commercially
available rating is selected.
Obviously, motor speed should also match load‟s speed requirement .It is also necessary to
check whether the motor can fulfill starting torque requirement also.
1.7.2 Continuous duty and variable load
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The operating temperature of a motor should never exceed the maximum permissible
temperature, because it will result in deterioration and breakdown of insulation and will
shorten the service life of motors.
It is general practice to base the motor power ratings on a standard value of temperature, say
35 c.
Accordingly, the power given on the name plate of a motor corresponds to the power which the
motor is capable of delivering without overheating at an ambient temperature of 35 c. the
duty cycle is closely related to temperature and is generally taken to include the environmental
factors also.
The rating of a machine can be determined from heating considerations.
However the motor so selected should be checked for its overload capacity and starting torque.
This is because, the motor selected purely on the basis of heating may not be able to meet the
mechanical requirements of the basis of heating may not be able to meet the mechanical
requirements of the load to be driven by it.
The majority of electric machines used in drives operate continuously at a constant or only
slightly variable load.
The selection of the motor capacity for these applications is fairly simple in case the
approximate constant power input is known
In many applications, the power input required for a motor is not known before hand and
therefore certain difficulties arise in such cases.
For the determination of ratings of machines whose load characteristics have not been
thoroughly studied, it becomes necessary to determine the load diagram i.e., diagram shown
the variation of power output versus time.
The temperature of the motor changes continuously when the load is variable. On account of this, it
becomes difficult to select the motor rating as per heating.
The analytical study of heating becomes highly complicated if the load diagram is irregular in
shape or when it has a large number of steps.
Therefore it becomes extremely difficult to select the motor capacity through analysis of the
load diagram due to select the motor capacity through analysis of the load diagram due to lack
of accuracy of this method.
On the other hand it is not correct to select the motor according to the lowest or highest load because
the motor would be overloaded in the first case and under loaded in the second case. Therefore it
becomes necessary to adopt suitable methods for the determination of motor ratings.
Methods used
The four commonly used methods are:
Methods of average losses
Equivalent current method
Equivalent torque method
Equivalent power method
19 R.RAJAGOPAL, S.SATHYAMOORTHI,AP/EEE 2015-16
EE 6361- ELECTRICAL DRIVES & CONTROL II/III MECHANICAL
1. Methods of average losses
The method consists of finding average losses Q in the motor when it operates
av
according to the given load diagram.
These losses are then compared with the Q , the losses corresponding to the continuous
duty of the machine when operated at its normal rating.
The method of average losses presupposes that when Q = Q , the motor will
av nomn
operate without temperature rise going above the maximum permissible for the particular
class of insulation.
The figure shows a simple power load diagram and loss diagram for variable load
conditions.
The losses of the motor are calculated for each portion of the load diagram by referring to
the efficiency curve of the motor.
Power
Time
Fig 1.4 Average Load Losses
The average losses are given by
Q t Q t Q t ............... Q t
1 1 2 2 3 3 n n
Q
av
t t ........ t
1 2 n
In case ,the two losses are equal or differ by a small amount ,the motor is selected .if the losses
differ considerably ,another motor is selected and the calculations repeated till a motor having
almost the same losses as the average losses is found.
Iit should be checked that the motor selected has a sufficient overload capacity and starting
torque.
The method of average losses dopes not take into account, the maximum temperature rise
under variable load conditions .However, this method is accurate and reliable for determining
the average temperature rise of the motor during one work cycle.
The disadvantage of this method is that it is tedious to work with and also many a times the
efficiency curve is not readily available and the efficiency has to be calculated by means of
empirical formula which may not be accurate.
20 R.RAJAGOPAL, S.SATHYAMOORTHI,AP/EEE 2015-16
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