Lecture notes for Electrical drives and Control

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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 3 R.RAJAGOPAL, S.SATHYAMOORTHI,AP/EEE 2015-16 EE 6361- ELECTRICAL DRIVES & CONTROL II/III MECHANICAL 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 4 R.RAJAGOPAL, S.SATHYAMOORTHI,AP/EEE 2015-16 EE 6361- ELECTRICAL DRIVES & CONTROL II/III MECHANICAL 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 5 R.RAJAGOPAL, S.SATHYAMOORTHI,AP/EEE 2015-16 EE 6361- ELECTRICAL DRIVES & CONTROL II/III MECHANICAL 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 6 R.RAJAGOPAL, S.SATHYAMOORTHI,AP/EEE 2015-16 EE 6361- ELECTRICAL DRIVES & CONTROL II/III MECHANICAL 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 7 R.RAJAGOPAL, S.SATHYAMOORTHI,AP/EEE 2015-16 EE 6361- ELECTRICAL DRIVES & CONTROL II/III MECHANICAL 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 8 R.RAJAGOPAL, S.SATHYAMOORTHI,AP/EEE 2015-16 EE 6361- ELECTRICAL DRIVES & CONTROL II/III MECHANICAL  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. 9 R.RAJAGOPAL, S.SATHYAMOORTHI,AP/EEE 2015-16 EE 6361- ELECTRICAL DRIVES & CONTROL II/III MECHANICAL 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). 10 R.RAJAGOPAL, S.SATHYAMOORTHI,AP/EEE 2015-16 EE 6361- ELECTRICAL DRIVES & CONTROL II/III MECHANICAL 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 11 R.RAJAGOPAL, S.SATHYAMOORTHI,AP/EEE 2015-16 EE 6361- ELECTRICAL DRIVES & CONTROL II/III MECHANICAL  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 12 R.RAJAGOPAL, S.SATHYAMOORTHI,AP/EEE 2015-16 EE 6361- ELECTRICAL DRIVES & CONTROL II/III MECHANICAL 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. 13 R.RAJAGOPAL, S.SATHYAMOORTHI,AP/EEE 2015-16 EE 6361- ELECTRICAL DRIVES & CONTROL II/III MECHANICAL 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 14 R.RAJAGOPAL, S.SATHYAMOORTHI,AP/EEE 2015-16 EE 6361- ELECTRICAL DRIVES & CONTROL II/III MECHANICAL 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 15 R.RAJAGOPAL, S.SATHYAMOORTHI,AP/EEE 2015-16 EE 6361- ELECTRICAL DRIVES & CONTROL II/III MECHANICAL 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. 16 R.RAJAGOPAL, S.SATHYAMOORTHI,AP/EEE 2015-16 EE 6361- ELECTRICAL DRIVES & CONTROL II/III MECHANICAL 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. 17 R.RAJAGOPAL, S.SATHYAMOORTHI,AP/EEE 2015-16 EE 6361- ELECTRICAL DRIVES & CONTROL II/III MECHANICAL 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 18 R.RAJAGOPAL, S.SATHYAMOORTHI,AP/EEE 2015-16 EE 6361- ELECTRICAL DRIVES & CONTROL II/III MECHANICAL  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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