PowerPoint Presentation

Utilization of Electrical Energyby

Dr. K V S R Murthy,Professor,Department of E. E. E.Aditya Engineering Collge,Surampalem.

11Electrical DriveElectrical drive is an electric motor along with its controlling devices, transmission equipment and mechanical load The combination of a prime-mover , speed control device, transmission equipment and mechanical load is called as a driveFactors Governing the Selection of Electric Drive1. Nature of electric supply a) AC supply b) Pure DC supply c) Rectified DC supply

2. Nature of drive a) Individual drive b) Group drivec) Multi motor driveFactors Governing the Selection of Electric Drive3. Electrical characteristics

a) Operating or Running characteristics

b) Starting characteristics

c) Speed control

d) Braking characteristicsFactors Governing the Selection of Electric Drive Type of enclosure

Type of bearing

Type of transmission of drive

Noise level

Type of cooling system4. Mechanical considerations

Factors Governing the Selection of Electric DriveWhether the motor is to work on Continuous dutyIntermittent dutyVariable load cycle duty5. Size and rating of the motor Whether the motor is adequate for Over-load capacityPull-out torqueFactors Governing the Selection of Electric DriveCapital Cost 6. Cost of the motor Running Cost Power factorLossesMaintenance CostDepreciationComparison between AC & DC drives AC DriveNo commutator problemSpeed and design rating have no upper limits.Large power/weight ratio.Speed control variation is limited .Solid state converters decides cost .

DC DriveHeavier size due to commutator.Speed and design is limited.Small power/weight ratio.Wide range and smooth control of speed .costly.

Elements of an Electric Drive Electric motor

Speed control device

Power transmission equipment

Mechanical load Elements of an Electric DriveMechanical loadMPower Transmission by CouplingSpeed ControllerPower SupplyFeed backElectric motorAdvantages of Electric DriveIt is quite clean due to the absence of fuel, fumes etc..Electric motors are available over a wide range of power i.e., few watt (5W) to mega watt.It is more flexible and can be moved to any place.No need of fuel storage and fuel transportation.Electrical energy is transmitted easily. It is more economicalNo hazardous fuel is requiredThe maintenance is easy.The starting and stopping of electric motors is easyThe noise level of electric drive is lessIt occupies less floor area as compared to other devices

Advantages of Electric Drive contd..1Electric devices provide a wide range of torques over a wide range of speeds.It can be remote controlled.The operating characteristics of electric drive can be easily modified.It is a reliable drive.The life of this drive is more.Advantages of Electric Drive contd..2Disadvantages of Electric DriveNon- availability of drive on supply failure.They require continuous power supply.The drive cannot be used in far off places where electricity is not available.Problems of saturation of iron.Classification of Electric Drives Electric drives are classified into three types

Group drive

Individual drive

3. Multimotor driveWhen several machines are loads are run on one shaft and the shaft is driven by a single motor, the system is called group drive

Group drive can also be called as shaft drive.

Group DriveThe cost of single large motor is less, than the cost of many motors of same aggregate ratingHigh over load capacityOnly one motor can be usedThe rating of the motor can be less than sum of all loads usedIt is suited for textile mills where stoppage of one operation necessitates stoppage of sequence of operations

Advantages of Group DriveDisadvantages of Group Drive

A fault in the motor will leave all loads idle.If only small load is to be used, the motor has to work with light load. This results in reduced efficiency.Power transmission through belts and pulleys is inefficient. So efficiency is less. The system is not very safeIndividual DriveIf one motor is used to drive one load that drive is called individual driveIn individual drive each load has its own motor

Advantages of Individual DriveA fault in the motor will not idle the other loads.It eliminates all shafting and belting.Machines can be installed at any desired place.The system is very safe.The noise level is less.Motors operate at good power factorDisadvantages of Individual DriveThe fixed or initial cost is high.It is not suited for a sequence of operations.The over load capacity of each motor will be less (about 5 to 10% only).

Multimotor DriveIn multi-motor drive, separate motors are used for operating different parts of the same mechanism.

Multi-motor drive contains several individual drives, each drive is used to operate its own mechanism

Multi-motor drives are used in machine tools, rolling mills, overhead cranes etc.Comparison between group drive and individual drive Group DriveInitial cost is less.Single large motor is used.Future expansion is very difficult.Machines should be installed nearby shaft.The speed control is cumbersome .

Individual Drivemore.No. of motors equal to No. of machines.Easy.

Machines can be installed at convenient place.Easy.

Types of loads according to Speed Torque CharacteristicsLoads requiring constant torque at all speedsLoads requiring torque which may increase in direct proportion to the speedLoads requiring torque, which may increase with the square of the speedLoads requiring torque which may decrease with the increase in speed

Speed-Torque Characteristics of different loadsFig:1Curve I Load torque is constant with speedCurve II Load torque is increasing with speedCurve III Load torque is increasing with the square of the speedCurve IVLoad torque is decreasing with the speedSPEEDLOAD TORQUEIIIIIIIV Loads Requiring Constant Torque at all Speeds ExamplesCranes during hoistingHoist winchesMachine tool feed mechanismsPiston pumps operating against a constant pressure head Conveyors handling a constant weight of the material per unit time.

Loads Requiring Torque which may Increase in Direct proportion to the Speed

Example Fluid friction where lubricant is used

Loads Requiring Torque which may Increase with the Square of the Speed

ExamplesBlowersFansCentrifugal PumpsShip propellers

Loads Requiring Torque which may Decrease with the Increase in Speed

ExamplesWhere deformation of material takes place such asLathesBoring machinesMilling machinesMetal cutting machine toolsSteel mills

Classification of Loads with Respect to TimeContinuous constant loadsContinuous variable loadsPulsating loadsImpact loadsShort time intermittent loadsShort time loads

Continuous Constant LoadsThe load on the motor remains same for a longer time

Examples: Fans and PumpsContinuous Variable LoadThe load on the motor varies continuously over a period of time, but occurs repetitively for a longer duration

ExamplesConveyers Metal cutting lathes Pulsating Loads Certain types of loads exhibit a torque behaviour which can be thought of as a constant torque superimposed by pulsation.

ExamplesCompressorsReciprocating pumpsFrame saws Textile looms All loads having crank shafts.Impact LoadsThe load on the motor increases to a maximum level suddenly These peak loads occur at regular intervals of time. Motors driving these loads are equipped with fly wheels for load equalizationExamplesForging hammersRolling mills Shearing machinesPresses etc

Short Time LoadsThe load on the motor occurs periodically in identical duty cycle, each consisting of a period of application of load and rest.

ExamplesExcavatorsCranesRoller trainsHoists.

Short Time LoadsThe load on the motor remain constant and occurs for a short time and then remain idle or off for a sufficiently long time.

ExamplesMixiesBattery chargersMost of the household equipmentsClassification of Loads with Respect to DutyContinuous dutyContinuous duty with constant loadContinuous duty with variable load cycleShort time dutyIntermittent duty cycleSelection of Motor for Continues Duty with Constant Load CycleThe motor should have the rating sufficient to drive the load without exceeding the specified temperatureThe rating of the motor selected for this duty is called its continuous rating Continuous rating means that it is the maximum load that the motor can give continuously over a period of time without exceeding the temperature limit

The motor selected for this duty should be able to with stand momentary over load capacity.The motor selected to this duty will have high efficiency because they will be operating at its full load and good power factor

Selection of Motor for Continues Duty with Constant Load CycleExamples of this duty Centrifugal pumps Fans etc

Selection of Motor for Continuous Duty with Variable Load CycleIf load variations are slight, highest load may be chosenas the rating of the motorIf variation in the load are large, selection of motor involves thermal calculation

The selection of motor for this duty may be based on average power or average currentShort Duty LoadIn this class of duty constant load occurs on the motor for a short interval of time and then off for sufficiently long time.

During off time the motor is allowed to cooled down to ambient temperature.

The examples of this duty areMixies Crane drivesLock gatesBridges

timeSelection of Motor for Short DutyThe motor selected for this duty may be of smaller capacity such that during the operation the temperature should not exceed the limits

Intermittent or Periodic Intermittent DutyThe load on the motor is a sequence of identical duty cycles.The motor is loaded for some time and shut off for some time.

Selection of Motor for Intermittent DutyA motor of smaller capacity would be sufficient due to rest periods in between successive loading

During rest period the motor is allowed to cool down to ambient temperature

Selection of the Motor for given LoadIn the selection of the motor the basic problem is of matching the mechanical output of the motor with the load requirement Practically all electric motors, shunt wound, series wound, compound wound d.c. motors ,squirrel cage and slip ring induction motors and a.c commutator motors have drooping speed-torque characteristics i.e. their speed falls as the load torque increase. Selection of the Motor for the Fan LoadFor Example for driving a fan , whose load torque varies as the square of the speed, induction motor among ac motors and d.c shunt motor among d.c motors will be suitable.Load EqualizationIt is usual to see the changes in load on any machine in the industry.The fluctuations in load vary widely within a span of few seconds.The process of smoothening these load fluctuations is called Load Equalization.The energy is saved during the interval of light load. Necessity of Load EqualizationIn industries there are many applications where load variations are large . A best example is the steel rolling mill operation The load is very large during the rolling operation and very little as the charge passes by.

The Following Figure Shows Load Curve Of A Rolling MillTP>^A Rolling mill will draw a heavy current from the supply during the intervals of peak load.This Produces large voltage drop in the system and it requires cables and wires of large size.It is desirable to smooth out these load fluctuations and so load equalization is very necessary Load EqualizationThe most common method of the load Equalization is by the use of Flywheel.

A flywheel is a big wheel mounted on the same shaft of the motor whose speed is to be controlled.

It may be defined as a heavy rotating body which acts as a reservoir for absorbing and redistributing the kinetic Energy

FlywheelFlywheel for a Stationary Engine

Twin Flywheel for a Stationary Engine

A Tractor Fitted with a Fly Wheel

Fly wheelFig:2

FlywheelIts main function is to keep the speed of a motor within the specified limits while doing a variable work .During the light load period the flywheel accelerates and stores the excessive energy drawn from the supply During peak load period the flywheel decelerates and supplies some of its stored energy to the load in addition to the energy supplied from the supply .In this way the speed of the motor will be maintained within the required limits.FlywheelContd..

Fig:3>^Variation of Speed with Respect to the Load Torque The thick lines represent the speed variation without flywheel The dashed lines represent the speed variation with fly wheel.

It is clear that by using a Fly wheel the speed fluctuations of the motor can be reduced.Electric BrakingIf an electric motor is to be stopped, the motor should be disconnected from the supplyThe motor takes a long time to stop because of inertiaIt depends upon the kinetic energy it has gained during the running period58Electric Braking contd..If the kinetic energy is more the motor takes long time to stopIf the kinetic energy is less, it takes less timeIn order to stop the motor at once to avoid accidents and also to stop as and when required for a particular operation of a jobSome system must be used called braking system 5960Electrical BrakingKinetic energy of moving parts is converted in to electrical energy

Kinetic energy of moving parts is opposed by producing opposite energy61Advantages of Electric BrakingNo need of replacement of brake shoesNo maintenance cost of brake shoesSystem capacity can be increasedPart of energy is returned to the supplyFailure of brake is not possibleNo metal dust is produced62Braking torque is easily controllableThe system is quickThe system is more reliableHigher rate of retardationLong lifePassenger of lifts and trains does not feel any discomfort Advantages of Electric Brakingcontinued63Disadvantages of Electric BrakingExtra friction brake is requiredThere must be suitable braking characteristicsHigh initial costRequires additional components64Types of BrakingPlugging or Counter- current braking

Dynamic or Rheostatic braking

Regenerative braking65PluggingEither the field or armature terminals are reversed suddenly during motor operationA strong braking torque is developedThe system will come to restA special device will cut-off the supplyThe motor will be stopped66Advantages of PluggingGreater braking torque

Quick reversal

Rapid 67Disadvantages of PluggingMotor draws energy from supply

Kinetic energy is wasted

Motor draws very high current

Causes shock to the motor and equipmentDynamic or Rheostatic BrakingThe supply in the armature of the motor is disconnectedThe motor is made to work as generatorThe kinetic energy of moving parts is converted into electrical energyWhich is dissipated in resistance connected as electrical load, during braking period68Regenerative BrakingThe motor is made to work as generator and the electrical energy generated is fed back in to the supplyMagnetic drag is produced due to generating action offers the required braking torqueWhen ever the value of back emf Eb becomes greater than supply voltage, VThe direction of the current in the armature gets reversed, the direction of field is same69Regenerative BrakingThe torque produced in the opposite directionThe back emf is made greater than the supply voltage byReducing the supply voltageIncreasing the field excitationMaking the motor to run at a speed greater than the no load speed70 Normal Working of D.C Shunt Motor

Figure :1 Plugging applied to D.C Shunt Motor

Figure 2Plugging to D.C Shunt MotorThe direction of torque developed in the motor can be reversed by reversing the current Current is reversed either in the armature or In the field windingReversal of field current is rarely usedThe armature current is normally reversed

Braking Torque DerivationDuring braking armature current V+Eb RaIa =Where V = supply voltage Eb = back emf Ra = armature resistance The armature resistance is very small The motor draws very heavy current Braking resistance (Rb) is inserted(1)The current at the instant of braking Ia =V+EbRa+RbElectric braking torque, Tb IaWhere = Flux Tb = K1 Ia .. (3)Substitute equation(2) in equation(3)(2)Tb = K1(V+EbRa+Rb)........(4)Braking Torque Derivation continuedWe know the back emf of d.c motorEb NWhere N = speed in r.p.m Eb= K2 N ..(5)Substitute equation (5) in equation (4)Tb = K1 V+ K2 N Ra+Rb()Tb =K1 VRa+RbK1K22N Ra+Rb +(6)Braking Torque Derivation continuedAssume

K1 VRa+Rb= K3 = ConstantK1K2 Ra+Rb= K4 = ConstantTb = K3 + K42N In d.c shunt motor flux is constant K3 = K5 K4 = K6 Braking torque, Tb = K5 + K6N(7)(8)Braking Torque Derivation continued The direction of torque developed in the motor can be reversed by reversing the current either in the armature or in the field winding Reversal of field current is rarely used The armature current is normally reversed During plugging a series resistance is included in the circuit to limit the currentPlugging of D.C Series Motor Normal Working Of D.C Series Motor

Fig 3 Plugging Applied To D.C Series Motor

Fig 4Braking Torque DerivationDuring braking

Armature current, V+Eb RaIa =Where V = supply voltage Eb = back emf Ra = Armature resistance Rb = Braking resistance Ib = Braking current The armature resistance is very small The motor draws very heavy current Braking resistance (Rb) is insertedThe current at the instant of braking Ib =V+EbRa+RbElectric braking torque, Tb IbWhere = Flux Tb = K1 Ib .. (2)Substitute equation(1) in equation(2)(1)Tb = K1(V+EbRa+Rb)........(3)Braking Torque Derivation continuedWe know the back emf of d.c motorEb NWhere N = speed in r.p.m Eb= K2 N ..(4)Substitute equation (4) in equation (3)Tb = K1 V+ K2 N Ra+Rb()Tb =K1 VRa+RbK1K22N Ra+Rb +Braking Torque DerivationcontinuedAssume

K1 VRa+Rb= K3 = ConstantK1K22N Ra+Rb= K4 = ConstantTb = K3 + K42N In d.c series motor flux is proportional to armature current Braking torque, Tb = K5Ia + K6Ia2NBraking Torque Derivation continuedPlugging to 3 Induction MotorThe direction of torque developed in the motor can be reversed by reversing the direction of rotating magnetic field

Direction can be reversed by interchanging of any two stator phases

The reverse direction of magnetic field produces torque in reverse direction

The motor will be stoppedUnder BrakingThe rate of cutting of the flux with the rotor conductors gets doubled

At standstill the rotor e.m.f is two times higher than running value

The rotor draws very high current

Rotor windings require additional insulation Normal Working of 3 Induction Motor

Fig 5

Fig 6Plugging applied to 3 Induction Motor Torque-Slip CharacteristicsDuring braking the slip of induction motor is greater than 1

The synchronous speed (Ns) is negative

The characteristic can be modified by adding external resistance in slip ring induction motor

The brake must be applied before it approaches the stand still speed

At the same time the power supply must be switched off Torque-Slip Characteristics

Fig 7Torque-Current Characteristics The rotor current can be calculated from this characteristic

Rotor current, I2 = SE2/(R22+SX22)

Where, E2 e.m.f induced in rotor at stand still R2 rotor resistance X2 rotor reactance at stand still S - slip Torque-Current Characteristics

Fig 8Dynamic Braking

Fig 1vNormal working of D.C series motor Dynamic Braking applied to D.C Series Motor

Fig 2Dynamic Braking to D.C Series MotorThe motor is disconnected from the supply and field connections are reversedThe motor is connected in series with a variable resistance RThe machine runs as generatorThe direction of current in the field is same as beforeIt produces a braking torqueTB = k2.2.N = k3.Ia2.N

Advantages of Dynamic BrakingGreater braking torque

Motor draws no energy from supply

Rapid braking i.e. motor comes to rest rapidlyDisadvantages of Dynamic BrakingKinetic energy is wasted

It is dependent on supply

In case of induction motor it requires separate d.c excitation Dynamic Braking

Fig 3Normal working of D.C. Shunt MotorDynamic Braking Applied to D.C Shunt Motor

(b)

Fig 4(a)Dynamic Braking applied to D.C Shunt MotorThe armature is disconnected from the supplyA braking resistance is connected across the armature The field winding is left across the supplyThe motor acts as a generator driven by stored kinetic energy

Dynamic Braking applied to D.C Shunt MotorThe stored kinetic energy is dissipated as heatThe motor will be stopped

TB = k2.2.N TB N as the flux is constant in D.C shunt motor

Regenerative Braking

Regeneration takes place when Eb in the armature, greater than applied voltage.

Motor run as Generator

Speed falls until Eb = V

Mechanical brakes are applied

Advantages Motor draws no energy from supply

Energy is fed back to supply

Speed can be controlledDisadvantagesPossible in only drives carrying overhauling loads.

Requires additional equipmentApplicationsSeparately excited d.c motors

D.C Series motor

D.C Shunt motor

D.C Compound motor

Induction motorsRegenerative Braking of D.C Series Motor

Fig 1When the motor is driving a overhauling load

The speed of the motor increases

The back e.m.f is also increases

The direction of armature current is reversed

Regenerative Braking of D.C Series Motor Special arrangements are made to reverse field winding connections.

The motor acts as generator and supplies energy.

This will reduce the speed

The motor comes to normal operation

Regenerative Braking of D.C Series Motor

Regenerative Braking of D.C Shunt Motor Regenerative braking is an inherant characteristics of shunt motorWhen the motor is driving a overhauling load the speed of the motor increasesThe back e.m.f Exceeds (Eb>V) the supply voltageThe motor acts as generator and energy will be fed into the supply system.

Regenerative Braking of D.C Shunt Motor The machine will start supplying current to the lineThe direction of armature current Ia and the torque is reversedThis will reduce the speed until Eb < VThe motor comes to rest

Regenerative Braking of D.C Series Motor Regenerative Braking of 3- Induction MotorWhen the motor is driving a overhauling load

The speed of the motor increases

The motor acts as induction generator and supplies energy

This will reduce the speed

The motor comes to normal operationMethods Used to Operate Induction Motor Above Synchronous Speed By using Frequency controlled induction motors

During Downward motion of a loaded hoisting mechanism.

Change over to a large number of poles from a smaller one in multi-speed squirrel cage induction motor.Torque-Slip Characteristics

In above all cases, the slip and torque developed become negativeThe machine acts as a generatorIf the load drives the motor above synchronous speedNo switching operation is requiredThe braking operation starts and motor comes to normal positionRegenerating Braking of 3 Phase Introduction MotorTHANK YOU