Traction drives

MULTIQUADRANT MULTIQUADRANT DYNAMICS IN THE DYNAMICS IN THE SPEED-TORQUE SPEED-TORQUE

PLANEPLANE

►A motor can operate in two modes A motor can operate in two modes

►Motoring- it converts electrical energy Motoring- it converts electrical energy into mechanical energy which support into mechanical energy which support its motionits motion

►Braking- it converts mechanical Braking- it converts mechanical energy into electrical energy which energy into electrical energy which opposes its motionopposes its motion

►For multiquadrant operation of drives, For multiquadrant operation of drives, it is useful to establish suitable it is useful to establish suitable conventions about the signs of torque conventions about the signs of torque and speedand speed

►The power developed by a motor is The power developed by a motor is given by the product of speed and given by the product of speed and torque.torque.

►FIRST QUADRANTFIRST QUADRANT►Operation is motoringOperation is motoring►Motor draws electrical power to drive the Motor draws electrical power to drive the

mechanical load coupled to it.mechanical load coupled to it.

►SECOND QUADRANTSECOND QUADRANT►Operation is brakingOperation is braking►Direction of rotation is positive and Direction of rotation is positive and

torque is negativetorque is negative►The machine operates as a generator The machine operates as a generator

developing a counter torque which developing a counter torque which opposes motionopposes motion

►THIRD QUADRANTTHIRD QUADRANT

►The operation is motoringThe operation is motoring►Motor action in the reverse direction Motor action in the reverse direction

both speed and torque have negative, both speed and torque have negative, so power is positive.so power is positive.

►Operation is similar to first quadrant Operation is similar to first quadrant operation, with direction of rotation is operation, with direction of rotation is reversed.reversed.

►FOURTH QUADRANTFOURTH QUADRANT

►Operation is braking.Operation is braking.►Both torque and speed have opposite Both torque and speed have opposite

signs positive and negative signs positive and negative respectively.respectively.

►The motor tends to attain highspeedThe motor tends to attain highspeed►The motor must develop a torque The motor must develop a torque

which opposes the acceleration due to which opposes the acceleration due to load.load.

►So the motor acts as a brakeSo the motor acts as a brake

Four quadrant operation of a Four quadrant operation of a motor driving a Hoist Loadmotor driving a Hoist Load

Classification of LoadsClassification of Loads

►Continuous constant modeContinuous constant mode►Continuous variable type modeContinuous variable type mode►Pulsating loadsPulsating loads► Impact loadsImpact loads►Short time intermittent loadsShort time intermittent loads►Short time loadsShort time loads

Speed Control of Induction Speed Control of Induction drivesdrives

►Stator voltage control methodStator voltage control method►Stator frequency control methodStator frequency control method►Rotor resistance controlRotor resistance control►Slip power recovery controlSlip power recovery control►Pole changing method of controlPole changing method of control►Eddy current couplingEddy current coupling

Stator voltage controlStator voltage control

► By reducing the stator voltage speed of a high By reducing the stator voltage speed of a high slip induction motor can be reduced by an slip induction motor can be reduced by an amount which is sufficient for speed control.amount which is sufficient for speed control.

► Supply frequency kept constant.Supply frequency kept constant.► Torque is proportional to the square of the Torque is proportional to the square of the

voltage.voltage.► It is an excellent method for reducing starting It is an excellent method for reducing starting

current and increasing efficiency during light load current and increasing efficiency during light load conditions.conditions.

► This method is applicable for speed control below This method is applicable for speed control below rated speed.rated speed.

AC voltage controllerAC voltage controller

►On-off control (or) Internal cycle On-off control (or) Internal cycle controlcontrol

►Phase angle controlPhase angle control

AdvantagesAdvantages The control circuit is very simpleThe control circuit is very simple More compact and less weightMore compact and less weight Quick responseQuick response

DisadvantagesDisadvantages The input power factor is very lowThe input power factor is very low Voltage and current waveforms are highly Voltage and current waveforms are highly

distorted due to harmonics, which affects distorted due to harmonics, which affects the efficiency of the machine.the efficiency of the machine.

Operating efficiency is low as resistance Operating efficiency is low as resistance losses are highlosses are high

►F increases : N increases : Tmax F increases : N increases : Tmax decreasesdecreases

►V increases : Tmax increasesV increases : Tmax increases

►By combining the above two features to By combining the above two features to achieve a control design by which the achieve a control design by which the speed increase and the torque is kept speed increase and the torque is kept the same.This is known as Voltage the same.This is known as Voltage /Frequency control./Frequency control.

Voltage / Frequency Voltage / Frequency control(V/F)control(V/F)

►The low frequency operation at a The low frequency operation at a constant V/F ratio is not satisfactory constant V/F ratio is not satisfactory particularly at very low frequency, due to particularly at very low frequency, due to this low frequency, the starting torque this low frequency, the starting torque and breakdown torque decreases and no and breakdown torque decreases and no load current increases.load current increases.

►The air gap flux cannot be maintained The air gap flux cannot be maintained constant at low frequency.constant at low frequency.

►The V/F control method, one must be The V/F control method, one must be careful not to increase the voltage careful not to increase the voltage magnitute beyond the rating of the magnitute beyond the rating of the motor.motor.

► Increased voltage can cause damage Increased voltage can cause damage to insulation of the motor, winding, to insulation of the motor, winding, leading to short and internal fault.leading to short and internal fault.

►The main features of V/F control The main features of V/F control scheme arescheme are

The maximum torque should be The maximum torque should be constantconstant

The starting current is also constantThe starting current is also constant

►Open loop:Open loop:►Used in low performance applications Used in low performance applications

where precise speed control is not where precise speed control is not necessarynecessary

►Problems in the operation of open loop Problems in the operation of open loop drivedrive

►The speed of the motor cannot be The speed of the motor cannot be controlled precisely because the rotor controlled precisely because the rotor speed will be less than synchronous speed will be less than synchronous speedspeed

►The slip speed, being the difference The slip speed, being the difference between the synchronous and between the synchronous and electrical rotor speed cannot be electrical rotor speed cannot be maintained.maintained.

Closed loopClosed loop

►Problem in open loop is overcome in Problem in open loop is overcome in this methodthis method

Slip power Recovery systemSlip power Recovery system

►The method of speed control by voltage The method of speed control by voltage control and resistance control have poor control and resistance control have poor efficiency, particularly at low speed and efficiency, particularly at low speed and find limited application.find limited application.

►The slip power is wasted in rotor The slip power is wasted in rotor resistance.resistance.

► If slip power can be returned to the mains, If slip power can be returned to the mains, the system can be made efficient and the the system can be made efficient and the capacity of the drive can be increased. capacity of the drive can be increased.

►The conventional methods of slip The conventional methods of slip power employ rotating machines such power employ rotating machines such as rotary converters, alternators, dc as rotary converters, alternators, dc machines, etc. in the rotor circuit to machines, etc. in the rotor circuit to convert into power at conventional convert into power at conventional schemes.schemes.

►Two controls Two controls

►Scherbius control systemScherbius control system►Kramer control systemKramer control system

Slip power recovery schemeSlip power recovery scheme

► Either the slip power is fed back to the supply or used to supply an Either the slip power is fed back to the supply or used to supply an additional motor which is mechanically coupled to the main motor.Thus additional motor which is mechanically coupled to the main motor.Thus methodology is termed as slip power recovery scheme it improves the methodology is termed as slip power recovery scheme it improves the overall efficiency of the system to an extremely large extentoverall efficiency of the system to an extremely large extent

► Kramer control system Kramer control system

1.Conventional kramer method1.Conventional kramer method

2.Static kramer method2.Static kramer method► Scherbius control systemScherbius control system

1.Conventional scherbius method1.Conventional scherbius method

2.Static scherbius method2.Static scherbius method

KRAMER CONROL SYSTEMKRAMER CONROL SYSTEM

The kramer control system The kramer control system is only applicable for sub synchronous is only applicable for sub synchronous speed operation that means speed speed operation that means speed below the synchronous speed.below the synchronous speed.

Conventional kramer system Conventional kramer system

► If the mechanical losses in cascade system If the mechanical losses in cascade system are neglected the shaft power output of the are neglected the shaft power output of the slip ring induction motor is slip ring induction motor is

► PPmm=(1-s)P=(1-s)Pinin► This method is only used for large motor of This method is only used for large motor of

4000KW or above.4000KW or above.► Advantages of conventional version of the systemAdvantages of conventional version of the system

The main advantage is that any speed, within the The main advantage is that any speed, within the working range can be obtained.working range can be obtained.

If the rotary converter is over excited, it will take a If the rotary converter is over excited, it will take a leading current which compensates for the leading current which compensates for the lagging current drawn by slip ring induction motor lagging current drawn by slip ring induction motor and hence improves the power factor of the and hence improves the power factor of the system.system.

Modified kramer systemModified kramer system

► In that, the rotary converter is In that, the rotary converter is replaced by a 3 phase diode bridge replaced by a 3 phase diode bridge rectifier or 3 phase controlled rectifier.rectifier or 3 phase controlled rectifier.

► In this method slip ring induction In this method slip ring induction motor speed can be controlled from motor speed can be controlled from zero to around synchronous speed, by zero to around synchronous speed, by varying the firing angle of the thyristor varying the firing angle of the thyristor rectifier.rectifier.

Static kramer SystemStatic kramer System► Instead of wasting the slip power in the rotor Instead of wasting the slip power in the rotor

circuit resistance, it can be converted to circuit resistance, it can be converted to 50Hz ac and pumped back to the line.Here, 50Hz ac and pumped back to the line.Here, the slip power can flow only in one direction. the slip power can flow only in one direction. This kind of drive is called static kramer This kind of drive is called static kramer drive.drive.

►This system offers speed control only for sub This system offers speed control only for sub synchronous speed i.e. control of speed is synchronous speed i.e. control of speed is possible only when it is less than the possible only when it is less than the synchronous speed.synchronous speed.

► It is economicalIt is economical

Closed loop control of slip Closed loop control of slip power Recovery schemepower Recovery scheme

Sherbius SystemSherbius System

►Only difference in the scherbius Only difference in the scherbius system as compared to kramer system system as compared to kramer system is that in the kramer system the is that in the kramer system the feedback is mechanical and in the feedback is mechanical and in the scherbius system the feedback return scherbius system the feedback return power is electrical.power is electrical.

► It is controlling the speed both below It is controlling the speed both below and above synchronous speed.and above synchronous speed.

Conventional Scherbius Conventional Scherbius SystemSystem

► Instead of coupling the dc motor Instead of coupling the dc motor directly to the main motor ,it is directly to the main motor ,it is mechanically coupled to the AC gen mechanically coupled to the AC gen which converts mech power into which converts mech power into electric power and returns to main electric power and returns to main supply linesupply line

Static Scherbius systemStatic Scherbius system►Replacing the all rotary machines by Replacing the all rotary machines by

SCR converter and inverter systems.SCR converter and inverter systems.►This static scherbius system is quite This static scherbius system is quite

similar to static kramer systemsimilar to static kramer system►Only diode bridge is replaced by SCR Only diode bridge is replaced by SCR

bridgebridge

►Motor can be operated in both Motor can be operated in both synchronous and super synchronous synchronous and super synchronous regions.regions.

►Sub synchronous speed SCR Bridge 1 Sub synchronous speed SCR Bridge 1 operates in the rectifier mode and SCR operates in the rectifier mode and SCR Bridge 2 operates in the inverter mode Bridge 2 operates in the inverter mode so slip power is returned to the supplyso slip power is returned to the supply

►At sub synchronous speed SCR 1 At sub synchronous speed SCR 1 operates in the rectifier mode and SCR 2 operates in the rectifier mode and SCR 2 operates in the inverter mode slip power operates in the inverter mode slip power is returned to the supply.is returned to the supply.

►At supersynchronous speed SCR 2 should At supersynchronous speed SCR 2 should operate in Rectifier mode and SCR 1 operate in Rectifier mode and SCR 1 should operate in inverter mode so that should operate in inverter mode so that slip power is pumped to the rotor.slip power is pumped to the rotor.

►Machine can be controlled from 50% Machine can be controlled from 50% above and below the synchronous speed.above and below the synchronous speed.

Closed Loop Control of Static Closed Loop Control of Static Scherbius SystemScherbius System

Cycloconverter Scherbius Cycloconverter Scherbius DriveDrive

► In this model, the dual bridge converter In this model, the dual bridge converter system is replaced by a 3 phase controlled system is replaced by a 3 phase controlled line commutated cycloconverter as shown. line commutated cycloconverter as shown. The Slip power flow in either direction.The Slip power flow in either direction.

►Modes of OperationModes of OperationSub synchronous motoring Sub synchronous motoring Super synchronous motoringSuper synchronous motoringSub synchronous regenerationSub synchronous regenerationSuper synchronous regenerationSuper synchronous regeneration

Cycloconverter in the rotor Cycloconverter in the rotor circuit static scherbius circuit static scherbius

systemsystem

Advantages of scherbius systemAdvantages of scherbius system

The cycloconverter is to be controlled so The cycloconverter is to be controlled so that its output frequency tracks preciously that its output frequency tracks preciously with the slip frequencywith the slip frequency

The problem of commutation near The problem of commutation near synchronous speed disappears in this synchronous speed disappears in this method.method.

Disadvantage of scherbius systemDisadvantage of scherbius system The cycloconverter cost is highThe cycloconverter cost is high The control of the scherbius drive is The control of the scherbius drive is

complexcomplex

Applications of static scherbius systemApplications of static scherbius system

Multi MW,Variable speed pumps or Multi MW,Variable speed pumps or generatorsgenerators

Flywheel energy storage systemsFlywheel energy storage systems