electronic starters and variable speed drives

..........................................................................

Cahier technique no. 208

Electronic starters andvariable speed drives

D. Clenet

Collection Technique

"Cahiers Techniques" is a collection of documents intended for engineersand technicians, people in the industry who are looking for more in-depthinformation in order to complement that given in product catalogues.

Furthermore, these "Cahiers Techniques" are often considered as helpful"tools" for training courses.They provide knowledge on new technical and technological developmentsin the electrotechnical field and electronics. They also provide betterunderstanding of various phenomena observed in electrical installations,systems and equipments.Each "Cahier Technique" provides an in-depth study of a precise subject inthe fields of electrical networks, protection devices, monitoring and controland industrial automation systems.

The latest publications can be downloaded from the Schneider Electric internetweb site.Code: http://www.schneider-electric.comSection: Experts' place

Please contact your Schneider Electric representative if you want either a"Cahier Technique" or the list of available titles.

The "Cahiers Techniques" collection is part of the Schneider Electric’s"Collection technique".

ForewordThe author disclaims all responsibility subsequent to incorrect use ofinformation or diagrams reproduced in this document, and cannot be heldresponsible for any errors or oversights, or for the consequences of usinginformation and diagrams contained in this document.

Reproduction of all or part of a "Cahier Technique" is authorised with thecompulsory mention:"Extracted from Schneider Electric "Cahier Technique" no. ....." (pleasespecify).

no. 208Electronic starters andvariable speed drives

ECT 208 first issue November 2003

Daniel CLENET

Graduated from the Brest Ecole Nationale d’Ingénieurs in 1969.Following his first appointment working in drive systems at Alstom,he joined Telemecanique’s variable speed drive group in 1973 as adesign engineer. He has developed variable speed drives for DCmotors for the machine tools market and drives for materials handlingtrucks as well as some of the first variable speed drives forasynchronous motors.His application experience comes from dealing with end users andhis role as a project manager within Schneider Electric’s IndustrialApplications Division. He was responsible for the launch of the Altivardrive in the USA during the years 1986 to 1990.

Cahier Technique Schneider Electric no. 208 / p.2


Electronic starters andvariable speed drives

The most common way of starting asynchronous motors is directly on theline supply. This technique is often suitable for a wide variety of machines.However, it sometimes brings with it restrictions that can be inconvenientfor some applications, and even incompatible with the functions requiredfrom the machine:

c The inrush current on start-up can interfere with the operation of otherdevices connected on the same line supply

c Mechanical shocks during starting that cannot be tolerated by themachine or may endanger the comfort and safety of users

c Acceleration and deceleration cannot be controlled

c Speed cannot be controlled

Starters and variable speed drives are able to counter these problems.Electronic technology has made them more flexible and has extended theirfield of application. However, it is still important to make the right choice.The purpose of this “Cahier Technique” is to provide more extensiveinformation about these devices in order to make it easier to define themwhen designing equipment or when improving or even replacing a motorswitchgear assembly for control and protection.

Table of contents

1 Brief history and reminders 1.1 Brief history p. 41.2 Reminders: The main functions of electronic starters p. 4and variable speed drives

2 The main operating modes and 2.1 The main operating modes p. 6main types of electronic drive 2.2 The main types of drive p. 8

3 Structure and components of 3.1 Structure p. 10electronic starters and drives 3.2 Components p. 11

4 Variable speed drive/regulator for DC motor 4.1 General principle p. 14

4.2 Possible operating modes p. 15

5 Frequency inverter for asynchronous motor 5.1 General principle p. 16

5.2 V/f operation p. 17

5.3 Vector control p. 18

5.4 Voltage power controller for asynchronous motor p. 21

5.5 Synchronous motor-drives p. 23

5.6 Stepper motor-drives p. 23

6 Additional functions of variable speed drives 6.1 Dialog options p. 25

6.2 Built-in functions p. 25

6.3 Option cards p. 26

7 Conclusion p. 27


1 Brief history and reminders

1.1 Brief history

Originally, rheostatic starters, mechanical drivesand rotating sets (Ward Leonard in particular)were used for starting electric motors andcontrolling their speed. Later, electronic startersand drives came to the fore as a modern, cost-effective, reliable and maintenance-free solutionfor industrial applications.

An electronic drive or starter is an energyconverter, which modulates the electrical energysupplied to the motor.Electronic starters are used solely forasynchronous motors. They are a type of voltagecontroller.Variable speed drives ensure gradualacceleration and deceleration and enable speedto be matched precisely to operating conditions.Controlled rectifier type variable speed drives areused to supply power to DC motors andfrequency inverters are used for AC motors.

Historically, drives for DC motors appeared first.Reliable and cost-effective frequency invertersappeared as a result of advances in powerelectronics and microelectronics. Modernfrequency inverters can be used to supply powerto standard asynchronous motors withperformance levels similar to those of the bestDC variable speed drives. Some manufacturerseven offer asynchronous motors with electronicvariable speed drives housed in a custom-madeterminal box. This solution is designed forreduced power assemblies (only a few kW).

Recent developments in variable speed drivesand information about current manufacturertrends appear at the end of this “CahierTechnique”. These developments aresignificantly expanding the drives on offer andtheir options.

1.2 Reminders: The main functions of electronic startersand variable speed drives

Controlled acceleration

Motor speed rise is controlled using a linear or Sacceleration ramp. This ramp is usually adjustableand therefore enables a speed rise time that isappropriate for the application to be selected.

Speed control

A variable speed drive cannot be a regulator atthe same time. This means that it is a rudimentarysystem where the control principle is developedon the basis of the electrical characteristics ofthe motor using power amplification but without afeedback loop and is described as “open loop”.

The speed of the motor is defined by an inputvalue (voltage or current) known as thereference or setpoint. For a given referencevalue, this speed may vary depending ondisturbances (variations in supply voltage, load,temperature).

The speed range is defined in relation to thenominal speed.

Speed regulationA speed regulator is a controlled drive(see Fig. 1 ). It features a control system withpower amplification and a feedback loop and isdescribed as “closed loop”.

The speed of the motor is defined by areference.

The value of the reference is continuouslycompared with a feedback signal, which is animage of the motor speed. This signal is suppliedeither by a tachogenerator or by a pulsegenerator connected at the motor shaft end.

If a deviation is detected following speedvariation, the values applied to the motor(voltage and/or frequency) are automaticallycorrected in order to restore the speed to itsinitial value.

Speed measurement

Motor

Regulator

ComparatorSpeed reference +

-

Fig. 1 : Principle of speed regulation


The feedback control renders the speed virtuallyimpervious to disturbances.

The precision of a regulator is usually expressedas a % of the nominal value of the value to becontrolled.

Controlled deceleration

When a motor is switched off, it deceleratessolely on the basis of the resistive torque of themachine (natural deceleration). Electronicstarters and drives can be used to controldeceleration via a linear or “S” ramp, which isusually independent of the acceleration ramp.

This ramp can be adjusted in order to produce atime for deceleration from the steady state speedto an intermediate speed or zero speed:

c If the required deceleration is faster than thenatural deceleration, the motor must develop aresistive torque that can be added to theresistive torque of the machine. This is describedas electrical braking, which can be achievedeither by restoring energy to the line supply orvia dissipation in a braking resistor.

c If the required deceleration is slower than thenatural deceleration, the motor must develop amotor torque greater than the resistive torque ofthe machine and continue to drive the load untilthe motor comes to a stop.

Reversal of operating direction

The majority of today’s drives support thisfunction as standard. The order of the motorsupply phases is inverted automatically either by

inverting the input reference, or via a logiccommand on a terminal, or via informationtransmitted via a line supply connection.

Braking to a standstill

This type of braking stops a motor withoutactually controlling the deceleration ramp.For starters and variable speed drives forasynchronous motors, this is achievedeconomically by injecting direct current into themotor with a special power stage function. As allthe mechanical energy is dissipated in themachine rotor, this braking can only beintermittent. On a drive for a DC motor, thisfunction will be provided by connecting a resistorto the armature terminals.

Built-in protection

Modern drives generally provide thermalprotection for motors and self-protection.A microprocessor uses the current measuredand speed data (if motor ventilation depends onits speed of rotation) to calculate thetemperature rise of the motor and sends analarm signal or trigger signal in the event of anexcessive temperature rise.

Drives, and in particular frequency inverters, arealso often fitted with protection against:c Short-circuits between phases and betweenphase and groundc Overvoltages and voltage dropsc Phase unbalancec Single-phase operation


2 The main operating modes and main types of electronic drive

2.1 The main operating modes

Depending on the electronic converter, variablespeed drives can either be used to operate amotor in a single direction of rotation (in whichcase they are known as “unidirectional”) or tocontrol both directions of rotation (in which casethey are known as “bidirectional”).

Drives that are able to regenerate energy from themotor operating as a generator (braking mode)can be “reversible”. Reversibility is achieved eitherby restoring energy to the line supply (reversibleinput bridge) or by dissipating the energyregenerated via a resistor with a braking chopper.

Figure 2 illustrates the four possible situations inthe torque-speed diagram of a machinesummarized in the corresponding table.

Please note that when the machine is operatingas a generator, a driving force must be applied.This state is used in particular for braking.The kinetic energy then present on the machineshaft is either transferred to the line supply ordissipated in the resistors or, for low powerratings, in the machine losses.

Fig. 2 : The four possible situations of a machine in its torque-speed diagram

Q1

F

1

F

1

F

2

F

2

Q2

Q4Q3

Torque

Speed

MG

GM

Unidirectional drive

This type of drive is most often non-reversibleand is used for:

c A DC motor with a direct converter (AC => DC)comprising a mixed diode and thyristor bridge(see Fig. 3a next page)

c An AC motor with an indirect converter (withintermediate DC transformation) comprising adiode bridge at the input followed by a frequencyinverter, which forces the machine to operate inquadrant 1 (see Fig. 3b next page). In somecases, this assembly can be used in bidirectionalconfigurations (quadrants 1 and 3).

Direction of Operation Torque Speed Product Quadrantrotation -T- -n- T.n1 (CW) As a motor yes yes yes 1

As a generator yes 22 (CCW) As a motor yes 3

As a generator yes 4


An indirect converter comprising a brakingchopper and a correctly dimensioned resistor isthe ideal solution for instantaneous braking(deceleration or on lifting gears when the motormust generate a downward braking torque inorder to hold the load).A reversible converter is essential for long-termoperation with a driving load as the load is thennegative as, for example, on a motor used forbraking on a test bench.

Bidirectional drive

This type of drive can be a reversible or non-reversible converter.If it is reversible, the machine operates in all fourquadrants and can tolerate significant braking.If it is non-reversible, the machine only operatesin quadrants 1 and 3.

Operation at constant torque

Operation is described as being at constanttorque when the characteristics of the load aresuch that, in steady state, the torque required isapproximately the same regardless of the speed(see Fig. 4 ). This operating mode is found onconveyors and kneaders. For this type ofapplication, the drive must be able to supply ahigh starting torque (at least 1.5 times the

nominal torque) in order to overcome staticfriction and to accelerate the machine (inertia).

Operation at variable torque

Operation is described as being at variabletorque when the characteristics of the load aresuch that, in steady state, the torque requiredvaries with the speed. This is the case inparticular with helical positive displacementpumps on which the torque increases linearlywith the speed (see Fig. 5a ) or centrifugalmachines (pumps and fans) on which thetorque varies with the square of the speed(see Fig. 5b ).

Fig. 3 : Simplified schematics: [a] direct converter with mixed bridge; [b] indirect converter with (1) input diodebridge, (2) braking device (resistor and chopper), (3) frequency inverter

Ma M

1 2 3

a

a - b -

P. T%

P

T

N%0

0

50

100

150

50 100 150

Fig. 4 : Operating curve at constant torque Fig. 5 : Operating curves at variable torque

P. T%

PT

N%0

0

50

100

150

50 100 150

P. T%

PT

N%0

0

50

100

150

50 100 150

a -

b -


For a drive designed for this type of application,a lower starting torque (usually 1.2 times thenominal motor torque) is sufficient. The driveusually has additional functions such as theoption to skip resonance frequencies caused bythe machine vibrating inadvertently. Operationabove nominal frequency is impossible due tothe overload this would impose on the motor andthe drive.

Operation at constant power

This is a special case of variable torque.Operation is described as being at constantpower when the torque supplied by the motor isinversely proportional to the angular speed(see Fig. 6 ). This is the case, for example, for awinder with an angular speed that must reduceas the winding diameter increases when thematerial is wound on. It is also the case forspindle motors on machine tools.

The operating range at constant power is by itsnature limited, at low speed by the current

supplied by the drive and at high speed by theavailable motor torque. As a consequence, theavailable motor torque with asynchronousmotors and the switching capacity ofDC machines must be checked carefully.

P.T%

P

T

N%0

0

50

100

150

50 100 150

Fig. 6 : Operating curve at constant power

2.2 The main types of drive

Only the most up-to-date drives and standardtechnological solutions are referred to in thissection.

There are numerous types of schematic forelectronic variable speed drives:subsynchronous cascade, cycloconverters,current commutators, choppers, etc.Interested readers will find an exhaustivedescription in the following publications:“Entraînement électrique à vitesse variable”(work by Jean Bonal and Guy Séguier describingvariable speed electrical drive systems) and“Utilisation industrielle des moteurs à courantalternatif” (by Jean Bonal describing AC motorsin industrial applications).

Controlled rectifier for DC motorThe rectifier supplies direct current from a single-phase or three-phase AC line supply where theaverage voltage value is controlled.

Power semiconductors are configured as single-phase or three-phase Graetz bridges (seeFig. 7 ). The bridge can be diode/thyristor(mixed) or thyristor/thyristor (full). This lattersolution is the most common as it improves theform factor of the current supplied.

The DC motor usually has separate excitation,except for low power ratings, where permanentmagnet motors are quite common.

This type of drive is suitable for use in allapplications. The only restrictions are thoseimposed by the DC motor, in particular thedifficulty of reaching high speeds and themaintenance required (the brushes must bereplaced). DC motors and associated drives

were the first industrial solutions. Their use hasbeen declining over the past decade asfrequency inverters take center stage.Asynchronous motors are in fact more ruggedand more economical than DC motors. UnlikeDC motors, asynchronous motors arestandardized in an IP55 enclosure and are alsovirtually unaffected by environmental conditions(dripping water, dust, hazardousatmospheres, etc.).

Frequency inverter for asynchronous motor

The inverter supplies a variable frequency three-phase AC rms voltage from a fixed frequencyAC line supply (see Fig. 8 next page). A single-phase power supply can be used for the drive atlow power ratings (a few kW) and a three-phasepower supply at higher ratings. Some low-powerdrives can tolerate single-phase and three-phasepower supplies equally. The output voltage ofthe drive is always three-phase. In fact, single-phase asynchronous motors are not particularlysuitable for power supply via a frequency inverter.

MDCa

Fig. 7 : Diagram of a controlled rectifier for a DC motor


Frequency inverters can supply power to standardcage motors with all the advantages associatedwith these motors: standardization, low cost,ruggedness, ingress protection, no maintenance.As these motors are self-cooled, their onlyoperating restriction is long-term use at low speeddue to the reduction in this ventilation. If this typeof operation is required, a special motor fitted witha separate forced ventilation unit must be used.

U

W

VMotor

Rectifier Filter Inverter

Voltage controller for starting asynchronousmotorsThe controller supplies, from an AC line supply,a fixed frequency alternating current equal to theline supply current where control of the rmsvalue of the voltage is achieved by modifying thetrigger delay angle a of the powersemiconductors - two thyristors connected headto tail in each motor phase (see Fig. 9 ).

Fig. 8 : Simplified schematic of a frequency inverter

Fig. 9 : Asynchronous motor starter and form of power supply current

M3 ϕ

α

α

I


3 Structure and components of electronic starters and drives

3.1 Structure

Electronic starters and variable speed drivescomprise two modules, which are usuallyhoused in a single enclosure (see Fig. 10 ):

c A control module, which manages theoperation of the device

c A power module, which supplies power to themotor in the form of electrical energy

The control moduleOn modern starters and drives, all functions arecontrolled by a microprocessor, which uses thesettings, the commands sent by an operator orby a processing unit and the results ofmeasurements such as speed, current, etc.

Along with dedicated circuits (ASICs), the micro-processors’ calculation functions have made itpossible to perform extremely high-performancecontrol algorithms and in particular to recognizethe parameters of the machine being driven. Themicroprocessor uses this information to managethe deceleration and acceleration ramps, for speedcontrol and current limiting as well as to controlpower components. Protection and safety measuresare processed by dedicated circuits (ASICs) orcircuits integrated in power modules (IPMs).

Speed limits, ramp profiles, current limits andother settings are defined using the integrated

keypads, or via PLCs (over fieldbuses) or PCs.Similarly, the various commands (run, stop,brake, etc.) can be sent via HMIs, PLCs or PCs.

Operating parameters and alarm and fault datacan be displayed using indicators,electroluminescent diodes, segment displays orLCDs. Alternatively they can be displayedremotely to supervisors via fieldbuses.

Relays, which are usually programmable,provide the following data:

c Fault (line supply, thermal, product, sequence,overload, etc.)

c Monitoring (speed threshold, pre-alarm, end ofstarting)

The voltages required for all measurement andcontrol circuits are supplied via a power supplythat is integrated into the drive and electricallyisolated from the line supply.

The power moduleThe main components of the power module are:

c Power components (diodes, thyristors, IGBTs,etc.)

c Interfaces for measuring voltages and/orcurrents

c In most cases, a fan unit

Motor

Rectifier

Converter

Firing

Feedback

Feedback security

Power supplyAdjustment

Commands

Control module Powermodule

Status display

Data processing

Thermal memory

Mic

ropr

oces

sor

Relay

Power interface

Safety interface

Fig. 10 : Structure of an electronic variable speed drive


3.2 Components

The power components (see Fig. 11 ) arediscrete semiconductors and as such can belikened to static switches which can take one oftwo states: on or off.

These components, combined in a powermodule, form a converter that supplies power toan electrical motor at a variable voltage and/orvariable frequency from a fixed voltage fixedfrequency line supply.

Power components are the keystone of speedcontrol and progress made in recent years hasled to the development of cost-effective variablespeed drives.

Reminder

Semiconductor materials such as silicon have aresistivity between that of conductors and that ofinsulators. Their atoms have 4 peripheralelectrons. Each atom associates with 4 adjacentatoms to create a stable 8-electron structure.

A P type semiconductor is obtained by adding topure silicon a small proportion of a substancewhose atoms have 3 peripheral electrons.Another electron must therefore be added tocreate a structure with 8 electrons, which resultsin a surplus of positive charges.

An N type semiconductor is obtained by addinga substance whose atoms have 5 peripheralelectrons. This therefore creates a surplus ofelectrons, i.e. a surplus of negative charges.

The diode

The diode is a non-controlled semiconductorcomprising 2 regions, P (anode) and N(cathode), which will only permit current to beconducted in one direction, from the anode tothe cathode.

It conducts current when the anode voltage is ata higher positive value than that of the cathodeand therefore behaves like a closed switch. Itblocks the current and behaves like an openswitch if the voltage at the anode becomes lesspositive than that at the cathode.

The main characteristics of the diode are asfollows:

c In the on state:v A drop in the voltage composing a thresholdvoltage and that due to an internal resistancev A maximum permissible continuous current(order of magnitude up to 5000 A rms for themost powerful components)

c In the off state, a maximum permissiblevoltage that may exceed 5000 V peak

The thyristor

This is a controlled semiconductor comprisingfour alternate layers: P-N-P-N.

I

+

-

I

+ –

Diode NPNtransistor

Thyristor

IGBT MOSGTO

Fig. 11 : Power components

It behaves like a diode in sending an electricalpulse on a control electrode known as a “gate”.This closing (or firing) is only possible if the anodeis at a voltage more positive than the cathode.

The thyristor changes to the off state whencurrent ceases to pass through it.

The firing energy to be supplied to the gate isindependent of the current to be switched. It isnot necessary either to maintain a current in thegate while the thyristor is conducting.

The main characteristics of the thyristor are asfollows:

c In the on state:v A composite voltage drop from a thresholdvoltage and an internal resistancev A maximum permissible continuous current(order of magnitude up to 5000 A rms for themost powerful components)

c In the off state:v A maximum permissible reverse and forwardvoltage (may exceed 5000 V peak). Forward andreverse voltages are usually identicalv A recovery time that is the minimum timeduring which, if a positive anode cathode voltagewas applied to the component, it would refirespontaneouslyv A gate current that will fire the componentSome thyristors are designed to operate atthe line supply frequency and others, knownas “high-speed” thyristors, will operateat several kHz using an extinction circuit.Some high-speed thyristors haveasymmetrical forward and reverse cut-off voltages.


In standard schematics, they are usuallyassociated with a diode connected back-to-backand semiconductor manufacturers use thisspecial feature to increase the forward voltagethat the component can tolerate in the off state.Today, these components have been replacedcompletely by GTOs, power transistors and inparticular by IGBTs (Insulated Gate BipolarTransistors).

The GTO (Gate Turn Off) thyristor

This is a special type of high-speed thyristor thatcan be turned off by its gate. A positive currentsupplied to the gate will cause thesemiconductor to start conducting if the voltageat the anode is more positive than at thecathode. The gate current must be maintained ifthe GTO is to continue conducting and thevoltage drop is to be limited. The thyristor isblocked by reversing the polarity of the gatecurrent. GTOs are used on very high-powerconverters as they are able to control highvoltages and currents (up to 5000 V and5000 A). However, as IGBTs continue todevelop, GTO market share is declining.

The main characteristics of the GTO thyristor areas follows:

c In the on state:v A composite voltage drop from a thresholdvoltage and an internal resistancev A holding current designed to reduce drops inthe forward voltagev A maximum permissible continuous currentv A cut-off current to block the current

c In the off state:v Maximum permissible reverse and forwardvoltages, often asymmetrical as with high-speedthyristors and for the same reasonsv A recovery time that is the minimum timeduring which the extinction current must bemaintained to prevent spontaneous refiringv A gate current that will fire the componentGTOs can operate at frequencies of several kHz

The transistorThis is a controlled bipolar semiconductorcomprising 3 alternating regions, P-N-P or N-P-N.It only permits current to be conducted in onedirection: from the emitter to the collector forP-N-P semiconductors and from the collector tothe emitter for N-P-N semiconductors.N-P-N type transistors, often configured as“Darlington” type transistors, are capable ofoperating at industrial voltages.

The transistor can operate as an amplifier. Thevalue of the current passing through it is thendetermined by the control current circulating inits base. However, it can also function as a

discrete static switch: open when there is nobase current, closed when saturated. Thissecond operating mode is the one used in powercircuits on drives.Bipolar transistors can be used for voltages up to1200 V and support currents that may reach800 V.

This component has today been replaced inconverters by IGBTs.

In terms of the type of operation in which we areinterested, the main characteristics of the bipolartransistor are as follows:

c In the on state:v A composite voltage drop from a thresholdvoltage and an internal resistancev A maximum permissible continuous currentv A current gain (to maintain saturation of thetransistor, the current injected in the base mustbe greater than the current circulating in thecomponent, divided by the gain)

c In the off state, a maximum permissibleforward voltage

The power transistors used in speed control canoperate at frequencies of several kHz.

The IGBT

This is a power transistor controlled by a voltageapplied to an electrode called a “gate” that isisolated from the power circuit, hence the nameInsulated Gate Bipolar Transistor (IGBT).This component requires minute levels of energyin order to generate the circulation of highcurrents.

Today, this component is used as a discreteswitch in most frequency inverters up to highpower ratings (several MW). Its voltage/currentcharacteristics are similar to those of bipolartransistors, although its performance levels interms of control energy and switching frequencyare significantly higher than those of othersemiconductors. The characteristics of IGBTsare improving all the time and high-voltage(> 3 kV) and high-current (several hundredamps) components are now available.

The main characteristics of the IGBT are asfollows:

c A control voltage enabling the component tobe switched on/off

c In the on state:v A composite voltage drop from a thresholdvoltage and an internal resistancev A maximum permissible continuous current

c In the off state, a maximum permissibleforward voltage

c IGBTs used in speed control can operate atfrequencies of several tens of kHz


The MOS transistor

The operating principle of this component differssignificantly from those listed above due to themodification of the electrical field in asemiconductor obtained by polarizing an isolatedgate, hence the name “Metal OxideSemiconductor”. Its use in speed control islimited to low-voltage (battery-powered variablespeed drives) or low-power applications becausethe silicon surface required to obtain a high cut-off voltage with a negligible voltage drop in theon state is too expensive to implement.

The main characteristics of the MOS transistorare as follows:

c A control voltage enabling the component tobe switched on/off

c In the on state:v An internal resistancev A maximum permissible continuous current

c In the off state, a maximum permissibleforward voltage (may exceed 1000 V)

MOS transistors used in speed control canoperate at frequencies of several hundred kHz.They are found in virtually all switch mode powersupply stages in the form of discretecomponents or as an integrated circuitcomprising the power (MOS) and the command-control circuits.

The IPM (Intelligent Power Module)Strictly speaking, this is not a semiconductor buta series of IGBT transistors. This module (seeFig. 12 ) combines, in a single compact housing,

P

N

B

U

V

W

+

–

Brakingresistor

Tomotor

IncomingDC

Fig. 12 : IPM (Intelligent Power Module)

an inverter bridge with IGBT transistors and thelow-level electronics for controllingsemiconductors:

c 7 x IGBT components (six for the inverterbridge and one for braking)

c The IGBT control circuits

c 7 x freewheel power diodes associated withthe IGBTs in order to enable the current tocirculate

c Protection against short-circuits, overcurrentsand excessive temperatures

c The electrical isolation for this module

The diode rectifier bridge is usually integratedinto this same module.This assembly is the best way to deal with thewiring and control restrictions of IGBTs.


4 Variable speed drive/regulator for DC motor

4.1 General principle

The Ward Leonard set was the first variablespeed drive for DC motors.

This set, which comprised a drive motor (usuallyasynchronous) and a variable excitationDC generator, supplied power to one or moreDC motors. Excitation was controlled by anelectromechanical device (Amplidyne, Rototrol,Regulex) or by a static system (magneticamplifier or electronic regulator). Today, thisdevice is totally obsolete and has been replacedby semiconductor variable speed drives capableof performing the same operations statically withsuperior levels of performance.

Electronic variable speed drives are suppliedwith power at a fixed voltage via an AC linesupply and provide the motor with a variableDC voltage. A diode bridge or a thyristor bridge(usually single-phase) powers the excitationcircuit.

The power circuit is a rectifier. As the voltage tobe supplied has to be variable, this rectifier mustbe a controlled rectifier, i.e. it must comprisepower components whose conductivecharacteristics can be controlled (thyristors).The output voltage is controlled by limiting to agreater or lesser extent the conduction timeduring each alternation. The longer the triggeringof the thyristor is delayed in relation to the zeroof the alternation, the lower the average voltagevalue and therefore the lower the motor speed(remember that a thyristor will shut downautomatically when the current crosses zero).

For low-power drives or drives powered by abattery pack, the power circuit, which maycomprise power transistors (chopper), will varythe DC output voltage by adjusting theconduction time. This operating mode is knownas PWM (Pulse Width Modulation).

RegulationRegulation is the precision maintenance of thevalue imposed in spite of disturbances (variationof resistive torque, power supply voltage,temperature). However, during acceleration or inthe event of an overload, the current must notreach a value that may endanger the motor or

the power supply device. An internal control loopin the drive maintains the current at anacceptable value. This limit can be accessed inorder to be adjusted as appropriate for thecharacteristics of the motor.

The reference speed is determined by an analogor digital signal supplied via a fieldbus or anyother device, which provides a voltage image ofthis required speed. The reference may be fixedor vary during the cycle.

Adjustable acceleration and deceleration rampsgradually apply the reference voltagecorresponding to the required speed. This rampcan follow any profile. The adjustment of theramps defines the duration of the accelerationand deceleration.

In closed loop mode, the actual speed ismeasured continuously by a tachogenerator or apulse generator and compared with thereference. If a deviation is detected, the controlelectronics will correct the speed. The speedrange extends by several revolutions per minuteuntil the maximum speed is reached. In thisvariation range, it is easy to achieve precisionrates better than 1% in analog regulation andbetter than 1/1000 in digital regulation, taking intoaccount all possible variations (no-load/on-load,voltage variation, temperature variation, etc.).

This type of regulation can also be implementedusing the motor voltage measured taking intoaccount the current passing through the motor.In this case, performance levels are slightlylower, both in the speed range and in terms ofprecision (several % between no-load operationand on-load operation).

Reversal of the operating direction andregenerative brakingIn order to reverse the operating direction, thearmature voltage must be inverted. This can bedone using contactors (this solution is nowobsolete) or statically by reversing the outputpolarity of the variable speed drive or the polarityof the excitation current. The use of this lattersolution is rare due to the time constant of thefield coil.


4.2 Possible operating modes

MDCa

Fig. 13 : Schematic of a drive with reversal andregenerative braking for a DC motor

Operation at “constant torque”With constant excitation, the speed of the motoris determined by the voltage applied to the motorarmature. Speed control is possible betweenstandstill and the nominal voltage of the motor,which is selected on the basis of the AC supplyvoltage.The motor torque is proportional to the armaturecurrent and the nominal torque of the machinecan be obtained continuously at all speeds.

Operation at “constant power”

When the machine is supplied with power at itsnominal voltage, its speed can still be increasedby reducing the excitation current. In this case,the variable speed drive must feature a controlledrectifier bridge that powers the excitation circuit.The armature voltage will remain fixed and equalto the nominal voltage and the excitation currentis adjusted in order to reach the required speed.

The power is expressed as

P = E x IwhereE is the supply voltage andI is the armature current.

For a given armature current the power willtherefore be constant throughout the speedrange, but the maximum speed is limited by twoparameters:

c The mechanical limit associated with thearmature and in particular the maximumcentrifugal power that can be tolerated by thecommutator

c The machine’s switching options, which are, ingeneral, more restrictive

The motor manufacturer must therefore be urgedto select the correct motor, in particular inrespect of the speed range at constant power.

If controlled braking is required or necessitatedby the nature of the load (driving torque), energymust be fed back to the line supply. Duringbraking, the drive acts as an inverter or, in otherwords, the current circulating is negative.

Drives capable of performing these two functions(reversal and regenerative braking) feature twobridges connected back-to-back (see Fig. 13 ).Each of these bridges can be used to invert thevoltage and current as well as the sign for theenergy circulating between the line supply andthe load.


5 Frequency inverter for asynchronous motor

5.1 General principle

The frequency inverter, which is powered atfixed voltage and frequency via the line supply,provides a variable voltage and frequencyAC power supply to the motor as appropriate forits speed requirements.

Constant flux must be maintained in order tofacilitate the supply of power to an asynchronousmotor at constant torque regardless of speed.This requires the voltage and frequency toincrease simultaneously in equal proportions.

Composition

The power circuit comprises a rectifier and aninverter, which uses the rectified voltage toproduce a variable amplitude voltage andfrequency (see Fig. 8).

In order to meet the requirements of the EC(European Community) directive and associatedstandards, a “line supply” filter is installedupstream of the rectifier bridge.

The rectifier is usually fitted with a diode rectifierbridge and a filter circuit comprising one or morecapacitors depending on the power rating.A limitation circuit controls the current on drivestart-up. Some converters use a thyristor bridgeto limit the inrush current of these filter capacitors,which are loaded to a value that is approximatelyequal to the peak value of the line supply sinewave (approx. 560 V at 400 V three-phase).

Note: Although discharge circuits are fitted,these capacitors may retain a dangerous voltageonce the line voltage has been disconnected.Work must only be carried out on this type ofproduct by trained personnel with knowledge ofthe essential precautions to be taken (additionaldischarge circuit or knowledge of waiting periods).

The inverter bridge connected to thesecapacitors uses six power semiconductors(usually IGBTs) and associated freewheel diodes.

This type of drive is designed to powerasynchronous cage motors. Telemecanique’sAltivar brand can be used to create a miniatureelectrical supply network providing a variablevoltage and frequency capable of supplyingpower to a single motor or to several motors inparallel. It comprises:

c A rectifier with filter capacitor

c An inverter with 6 IGBTs and 6 diodes

c A chopper, which is connected to a brakingresistor (usually external to the product)

c IGBT transistor control circuits

c A control unit based around a microprocessor,which is used to control the inverter

c Internal sensors for measuring the motorcurrent, the DC voltage at the capacitorterminals and in some cases the voltages at theterminals of the rectifier bridge and the motor aswell as all values required to control and protectthe motor-drive unit

c A power supply for low-level electronic circuits

This power supply is provided by a switchingcircuit connected to the filter capacitor terminalsin order to make use of this energy reserve.Altivar drives use this feature to avoid the effectsof transient line supply fluctuations, therebyachieving remarkable performance levels on linesupplies subject to significant disturbances.

Speed control

The output voltage is generated by switching therectified voltage using pulses with a duration,and therefore a width, which is modulated sothat the resulting alternating current will be assinusoidal as possible (see Fig. 14 ). Thistechnique, known as PWM (Pulse WidthModulation), conditions regular rotation at lowspeed and limits temperature rises.

t t

U motor I motor

Fig. 14 : Pulse width modulation


The modulation frequency selected is acompromise: it must be high enough to reducecurrent ripple and acoustic noise in the motorwithout significantly increasing losses in therectifier bridge and in the semiconductors. Tworamps control acceleration and deceleration.

Built-in protectionThe drive provides self-protection and protectsthe motor against excessive temperature risesby disabling it until the temperature falls back toan acceptable level.

It also provides protection against any type ofdisturbance or problem that may affect theoperation of the unit, such as overvoltages orundervoltages or the loss of an input or outputphase.In some ratings, the rectifier, the inverter, thechopper, the control and protection againstshort-circuits are housed in a single IPM.

5.2 V/f operation

In this type of operation, the speed referenceimposes a frequency on the inverter andconsequently on the motor, which determinesthe rotation speed. There is a direct ratiobetween the power supply voltage and thefrequency (see Fig. 15 ). This operation is oftendescribed as operation at constant V/f or scalar

1.75

1.50

1.25

10.95

0.75

0.50

0.25

0 50 100 150 200 % 0

1a

TorqueT/Tn

Finverter

Fline supply

1b

3

2

operation. If no compensation is applied, theactual speed varies with the load, which limitsthe operating range. Summary compensationcan be used to take account of the internalimpedance of the motor and to limit the on-loadspeed drop.

Fig. 15 : Torque characteristics of a drive (Altivar 66 – Telemecanique)1 – continuous useful torque self-cooled motor (a) and forced-cooled motor (b)2 – transient overtorque (< 1.7 Tn during 60 s)3 – overspeed torque at constant power


5.3 Vector control

Performance levels can be significantly increasedby using control electronics based on flux vectorcontrol (FVC) (see Fig. 16 ). The majority oftoday’s drives feature this function as standard.Knowing or estimating the machine parametersenables the speed sensor to be omitted from themajority of applications. In this case, a standardmotor can be used subject to the usual restrictionin relation to long-term operation at low speed.The drive generates information from the valuesmeasured at the machine terminals (voltage andcurrent).This control mode enables acceptable levels ofperformance to be achieved without increasingcosts.To achieve these levels of performance, someknowledge of the machine parameters is required.On commissioning, the machine troubleshootermust in particular apply the characteristicsindicated on the motor rating plate to the driveadjustment parameters.These include:UNS: Nominal motor voltageFRS: Nominal stator frequencyNCR: Nominal stator currentNSP: Nominal speedCOS: Motor cosine

1 / g

(d,q)

(a,b,c)

(a,b,c)

(d,q)

VcVbVa

Ia, Ic

θs

θs

Speed loop

Quadrature current loop

Forward current loop

Speed estimate

Current limitsIdlim Iqlim

Speed referenceΩcons

Ωest

Quadrature current referenceIqref

Quadrature voltage referenceVqref

Forward voltage reference

VdrefMagnetizing current

Speed correction

Slip compensation

ΩcomΩcor

Forward current reference

Idref

Voltage limitsVdlim Vqlim

Voltage reference generator

Forward and quadrature currents

Id

Iq

Phase angleθs

Motor

Fig. 16 : Simplified schematic of a drive with flux vector control

The drive uses these values to calculate therotor characteristics (Lm, Tr).

Drive with sensorless flux vectorcontrol

On power-up, a drive with sensorless flux vectorcontrol (such as Telemecanique’s ATV58F)performs auto-tuning to determine the statorparameters Rs, Lf. This measurement can betaken with the motor connected to the mechanism.The duration will vary from 1 to 10 s depending onthe motor power. These values are stored and canbe used by the product to derive control ratios.

The oscillogram in Figure 17 next page illustratesthe acceleration of a motor loaded to its nominaltorque and powered by a sensorless drive. Youwill note that the nominal torque is reached quickly(in less than 0.2 s) and that the acceleration islinear. Nominal speed is reached in 0.8 s.

Drive with flux vector control in closed loopmode with sensor

Another option is flux vector control in closedloop mode with sensor. This solution uses Parktransformation and can be used to control thecurrent (Id) that provides the flux in the machineand the current (Iq) that provides the torque


independently (equal to the product Id x Iq).The motor is controlled in the same way as aDC motor. This solution (see Fig. 18 ) meets therequirements of complex applications: highdynamics in the event of transient phenomena,speed precision, nominal torque on stopping.

The maximum transient torque is equal to 2 or3 times the nominal torque depending on thetype of motor. In addition, the maximum speedoften reaches double the nominal speed or moreif permitted by the motor mechanics.

This type of control also permits very highpassbands and performance levels comparablewith and even superior to the best DC drives.On the other hand, the motor used is not astandard design due to the presence of a sensorand, where appropriate, forced ventilation.

The oscillogram in Figure 19 next pageillustrates the acceleration of a motor loaded toits nominal torque powered by a drive with fluxvector control with sensor. The time scale is0.1 s per division. Compared with the sameproduct without a sensor, the increase inperformance levels is significant. Nominal torqueis reached after 80 ms and the speed rise timeunder the same load conditions is 0.5 s.

(d,q)

(a,b,c)

(a,b,c)

(d,q)

VcVbVa

Calculation of voltages and current loops

Speed regulation

Speed ramp

Speed calculationSlip estimateCalculation of angle of rotation

Current and torque limits

Encoder

Estimation and regulation of flux

Motor

Speed referenceΩref

Speed reference Ωsetp

Quadrature current referenceIqsetp

Forward current reference Idref

Speed measuredΩm

Phase angleθs

Forward and quadrature currentsId, Iq

Forward and quadrature voltagesVd, Vq

Flux reference (internal reference)Φsetp

Id, Iq

Vd, Vq

Ia, Ic

Ωm

Ωm

Ωm Φsetp

Φsetp

θs

θs

Id, Iq

Iqsetp

Fig. 18 : Simplified schematic of a drive with flux vector control with sensor

0 0.2 1

1

2

3

t (s)

1 - motor current

2 - motor speed

3 - motor torque

Fig. 17 : Characteristics of a motor on power-up via adrive with sensorless flux vector control(Telemecanique ATV58F type)


By way of conclusion, the table in Figure 20compares the respective performance levels of adrive in the three possible configurations.

Reversal of operating direction and braking

The operating direction is reversed by sendingan external command (either to an inputdesignated for this purpose or by a signal on acommunication bus), which reverses theoperating sequence of the inverter components,thereby reversing the operating direction of themotor. A number of operational scenarios arepossible.

c Scenario 1: Immediate reversal of the controldirection of the semiconductors

If the motor is still rotating when the operatingdirection is reversed, this will produce significantslip and the current in the drive will rise to itsmaximum possible level (internal limiting). Thebraking torque is low due to the significant slipand the internal regulation will reduce the speedreference considerably. Once the motor reacheszero speed, the speed will reverse by following

the ramp. The surplus energy not absorbed bythe resistive torque and the friction is dissipatedin the rotor.

c Scenario 2: Reversal of the control direction ofthe semiconductors preceded by decelerationwith or without ramp

If the resistive torque of the machine is such thatnatural deceleration is faster than the ramp setby the drive, the drive will continue to supplyenergy to the motor. The speed will graduallydecrease and reverse.

In contrast, if the resistive torque of the machineis such that natural deceleration is slower thanthe ramp set by the drive, the motor will act as ahypersynchronous generator and restore theenergy to the drive. However, because thepresence of the diode bridge prevents theenergy being fed back to the line supply, thefilter capacitors will charge, the voltage will riseand the drive will lock. To avoid this, a resistormust be connected to the capacitor terminals viaa chopper in order to limit the voltage to anappropriate value. The braking torque will thenonly be limited by the capacities of the drive,meaning that the speed will gradually decreaseand reverse.

For this type of application, the drivemanufacturer supplies braking resistorsdimensioned in accordance with the motorpower and the energy to be dissipated. As inmost cases the chopper is included as standardwith the drive, only the presence of a brakingresistor will single out a drive capable ofcontrolled braking. Therefore, this type ofbraking is particularly economical. It follows thatthis type of operation can be used to deceleratea motor to standstill without necessarily havingto reverse the direction of rotation.

Dynamic DC injection brakingEconomical braking can be achieved easily byoperating the output stage of the drive as achopper, which injects direct current into thewindings. The braking torque is not controlledand is fairly ineffective, particularly at highspeeds. Therefore, the deceleration ramp is notcontrolled. Nevertheless, this is a practicalsolution for reducing the natural stopping time ofthe machine. As the energy is dissipated in therotor, this type of operation is, by its nature, rare.

0 0.2 1

1

2

3

t (s)1 - motor current2 - motor speed3 - motor torque

Fig. 19 : Oscillogram for the acceleration of a motorloaded to its nominal torque powered by a drive withflux vector control (Telemecanique ATV58F type)

Scalar control With sensorless With flux vectorflux vector control control and sensor

Speed range 1 to 10 1 to 100 1 to 1000Passband 5 to 10 Hz 10 to 15 Hz 30 to 50 HzSpeed precision ± 1 % ± 1 % ± 0.01 %

Fig. 20 : Respective performance levels for a drive in the three possible configurations(Telemecanique ATV58F type)


Possible operating modes

c Operation at “constant torque”As the voltage supplied by the drive can varyand insofar as flux in the machine is constant(constant V/f ratio or even better with flux vectorcontrol), motor torque will be approximatelyproportional to the current and it will be possibleto obtain the nominal torque of the machinethroughout the speed range (see Fig. 21 ).However, long-term operation at low speed isonly possible if the motor is provided with aforced ventilation unit, and this requires a specialmotor. Modern drives feature protection circuits,which create a thermal image of the motor as afunction of the current, the operating cycles andthe rotation speed, thereby protecting the motor.

c Operation at “constant power”When the machine is powered at its nominalvoltage, it is still possible to increase its speedby supplying it with a frequency greater than thatof the line supply. However, because the outputvoltage of the inverter cannot exceed that of theline supply, the available torque decreases ininverse proportion to the increase in speed (seeFig. 21). Above its nominal speed, the motorceases to operate at constant torque andoperates at constant power (P = Cw) insofar as

this is permitted by the natural characteristic ofthe motor.

The maximum speed is limited by two parameters:

v The mechanical limit associated with the rotor

v The available torque reserve. For anasynchronous machine powered at constantvoltage, whereby the maximum torque varieswith the square of the speed, operation at“constant power” is only possible in a limitedspeed range determined by the characteristic ofthe machine’s own torque.

T

Tn

0 10 10050 F (Hz)

a b

Fig. 21 : Torque of an asynchronous motor at constantload powered by a frequency inverter [a] - operatingzone at constant torque, [b] - operating zone atconstant power

5.4 Voltage power controller for asynchronous motor

This voltage control device, which can be usedfor lighting and heating, can only be used withresistive cage or slip-ring asynchronous motors(see Fig. 22 ). The majority of these asynchronousmotors are three-phase, although some are single-phase for low power ratings (up to approx. 3 kW).

Often used as a soft start/soft stop unit, providedthat a high starting torque is not required, apower controller can be used to limit the inrush

Fig. 22 : Available torque for an asynchronous motor powered at variable voltage and a parabolic resistive torqueload (fan) [a] - squirrel cage motor, [b] - resistive cage motor

current, the resulting voltage drop and themechanical shocks caused by the suddenoccurrence of torque.

The most common applications of this type arestarting centrifugal pumps and fans, beltconveyors, escalators, car wash gantries,machines fitted with belts, etc. and in speedcontrol on very low power motors or universalmotors such as those in electrolifting tools.

T

Tr linear

Tr = kN2 T

a - b -

N1

NS

NS

N2

N N

N max

0

Un = 100%

U

U4 N max

∆ua Un

Un

4

3

2

1

U2 = 85%

U1 = 65%

00


However, for some applications, such as speedcontrol on small fans, power controllers have allbut been replaced by frequency inverters, whichare more economical during operation.

In the case of pumps, the soft stop function canalso be used to eliminate pressure surges.

However, some caution must be exercised whenselecting this type of speed control. When a motorslips, its losses are actually proportional to theresistive torque and inversely proportional to thespeed. A power controller works on the principleof reducing the voltage in order to balance theresistive torque to the required speed. Theresistive cage motor must therefore be able, atlow speed, to dissipate its losses (small motorsup to 3 kW are usually suitable for theseconditions). Above this, a forced-cooled motor isusually required. For slip-ring motors, theassociated resistors must be dimensioned inaccordance with the operating cycles. Thedecision is left to the specialist, who will selectthe motor according to the operating cycles.

Three types of starter are available on the market:starters with one controlled phase in low powerratings, starters with two controlled phases (thethird being a direct connection), or starters withall phases controlled. The first two systems mustonly be used for non-severe operating cyclesdue to the increased harmonic ratio.

General principle

The power circuit features 2 thyristors connectedhead to tail in each phase (see Fig. 9). Voltagevariation is achieved by varying the conductiontime of these thyristors during each alternation.The longer triggering is delayed, the lower thevalue of the resulting voltage.

Thyristor triggering is controlled by amicroprocessor, which also performs thefollowing functions:

c Control of the adjustable voltage rise and fallramps; the deceleration ramp can only befollowed if the natural deceleration time of thedriven system is longer

c Adjustable current limit

c On starting torque

c Controlled braking via DC injection

c Protection of the drive against overloads

c Protection of the motor against overheatingdue to overloads or frequent starting

c Detection of phase unbalance, phase failure orthyristor faults

A control panel, which displays various operatingparameters, provides assistance duringcommissioning, operation and maintenance.

Some power controllers such as the Altistart(Telemecanique) can control starting andstopping of:

c A single motor

c A number of motors simultaneously subject torating limits

c A number of motors in succession by means ofswitching. In steady state, each motor ispowered directly from the line supply via acontactor.

Only the Altistart features a patented device thatcan be used to estimate the motor torque,thereby enabling linear acceleration anddeceleration and, if necessary, limiting the motortorque.

Reversal of operating direction and brakingThe operating direction is reversed by invertingthe starter input phases. Counter-current brakingis then applied and all the energy is dissipated inthe machine rotor. Therefore, operation is by itsnature intermittent.

Dynamic DC injection braking

Economical braking can be achieved easily byoperating the output stage of the starter as arectifier, which injects direct current into thewindings. The braking torque is not controlledand braking is fairly ineffective, particularly athigh speeds. Therefore, the deceleration ramp isnot controlled. Nevertheless, this is a practicalsolution for reducing the natural stopping time ofthe machine. As the energy is dissipated in therotor, this type of operation is also rare.


Fig. 23 : Photograph of a synchronous motor-drive(Schneider Electric Lexium servodrive + motor)

A common DC source powers this driveassembly in parallel.

This type of installation enables the energygenerated by the braking of one of the axes tobe made available to the assembly.

As in frequency inverters, a braking resistorassociated with a chopper can be used todissipate the excess braking energy.

The electronics servocontrol functions, lowmechanical and electrical time constants, permitaccelerations and more generally passbandsthat are very high, combined with simultaneoushigh speed dynamics.

5.6 Stepper motor-drives

General principleStepper motor-drives combine power electronicssimilar in design to a frequency inverter with a

Fig. 24 : Simplified schematic of a drive for a bipolar stepper motor

+ DC

- DC

Motor

Q1a Q2a Q3a Q4a

Q2b Q1b Q4b Q3b

stepper motor (see Fig. 24 ). They operate inopen loop mode (sensorless) and are designedfor use in position control applications.

5.5 Synchronous motor-drives

General principle

Synchronous motor-drives (see Fig. 23 )combine a frequency inverter and a permanentmagnet synchronous motor fitted with a sensor.

These motor-drives are designed for specificmarkets such as robots or machine tools, wherea low volume of motors, high-speed accelerationand an extended passband are required.

The motor

The motor rotor is fitted with rare earth permanentmagnets in order to achieve increased fieldstrength in a reduced volume. The statorfeatures three-phase windings. These motorscan tolerate significant overload currents in orderto achieve high-speed acceleration. They arefitted with a sensor in order to indicate theangular position of the motor poles to the drive,thereby ensuring that the windings are switched.

The drive

In design terms, the drive operates in the sameway as a frequency inverter.

It also features a rectifier and an inverter withpulse width modulation (PWM) transistors, whichrestores an output current in sine form.

It is common to find several drives of this typepowered by a single DC source. Therefore, ona machine tool, each drive controls one of themotors connected to the machine axes.


The motor

The motor can be a variable reluctance motor, apermanent magnet motor or a combination of thetwo (see “Cahier Technique” no. 207“Introduction aux moteurs électriques”).

The drive

In design terms, the drive is similar to afrequency inverter (rectifier, filters and bridgecomprising power semiconductors).

However, in terms of operation, it isfundamentally different insofar as its purpose isto inject a constant direct current into thewindings. Sometimes, it uses pulse widthmodulation (PWM) to improve performancelevels, in particular the rise time of the current(see Fig. 25 ), which enables the operatingrange to be extended.

Micro-step operation (see Fig. 26 ) can be usedto artificially multiply the number of possiblepositions of the rotor by generating successivesteps in the coils during each sequence. Thecurrents in the two coils therefore resemble twoalternating currents offset by 90°. The resultingfield is the vector composition of the fieldscreated by the 2 coils. The rotor therefore takes

t

t

Uu

i

In

U/R

1 step

Fig. 25 : Current form resulting from PWM control

0.86

0.5

B1 I1B1

B2

I2B2

t

t

Fig. 26 : Diagram, current curves and step principle for micro-step control of a stepper motor-drive

all possible intermediate positions. The diagrambelow illustrates the power supply currents ofcoils B1 and B2 and the positions of the rotor arerepresented by the vector.


6 Additional functions of variable speed drives

6.1 Dialog options

In order to ensure that the motor operatescorrectly, the drives are fitted with a number ofsensors for monitoring the voltage, the “motor”currents and the thermal state of the motor. Thisinformation, which is essential for the drive, canbe useful for operation.

The latest drives and starters feature dialogfunctions based on fieldbuses. This provides ameans of generating information that is used bya PLC and a supervisor to control the machine.The PLC also uses the same channel to providecontrol information in the same way.

The information transmitted includes:c Speed referencesc Run or stop commandsc Initial drive settings or modifications of thesesettings during operationc The drive status (run, stop, overload, fault)c Alarmsc The motor status (speed, torque, current,temperature)These dialog options are also used in connectionwith a PC in order to simplify settings on start-up(download) or to archive initial settings.

6.2 Built-in functions

In order to be compatible for use in a largenumber of applications, the drives feature asignificant number of adjustments and settings,including:

c Acceleration and deceleration ramp times

c Ramp profiles (linear, S or U)

c Ramp switching, which can be used to obtaintwo acceleration or deceleration ramps in order,for example, to permit a smooth approach

c Reduction of the maximum torque controlledusing a logic input or a reference

c Jog operation

c Management of brake control for liftingapplications

c Choice of preset speeds

c The presence of summed inputs, which can beused to sum speed references

c Switching of references present at the driveinput

c The presence of a PI regulator for simpleservocontrol (speed or flow rate for example)

c Automatic stop following loss of line supplyenabling the motor to brake

c Automatic catching a spinning load withdetection of motor speed for catch on the fly

c Thermal protection of the motor using animage generated in the drive

c Option to connect PTC thermal sensorsintegrated into the motor

c Skipping of the machine resonance frequency,the critical speed is skipped in order to preventoperation at this frequency

c Time-delayed locking at low speed in pumpingapplications where the fluid is used to lubricatethe pump and prevent seizing

These functions are increasingly being includedas standard on sophisticated drives(see Fig. 27 ).

Fig. 27 : Photograph of a drive featuring numerousbuilt-in functions (Telemecanique ATV58H)


6.3 Option cards

For more complex applications, manufacturerscan supply option cards, which can be usedeither for special functions, e.g. flux vectorcontrol with sensor, or as dedicated application-specific cards. These types of card include:

c “Pump switching” cards as a cost-effectivemeans of setting up a pumping stationcomprising a single drive that supplies power toa number of motors in succession

c “Multi-motor” cards

c “Multi-parameter” cards, which can be used toautomatically switch preset drive parameters

c Special cards developed to meet a specificuser requirement

Some manufacturers also offer PLC cards builtinto the drive, which can be used for simpleapplications. This provides the operator withprogramming instructions and inputs and outputsfor setting up small automated systems wherethe presence of a PLC cannot be justified.


7 Conclusion

As the selection of a variable speed drive isinextricably linked with the type of load drivenand the target performance levels, the definitionand selection of any variable speed drive mustinclude an analysis of the operationalrequirements of the equipment and performancelevels required of the motor itself.

Constant torque, variable torque, constantpower, flux vector control, bidirectional drive, etc.are all terms that feature heavily in manufacturerdocumentation. In essence, this is all the datayou will need in order to identify the mostappropriate drive.

Selecting the wrong drive can result indisappointing operation. Equally, it is essential toconsider the required speed range in order toselect the most suitable motor/drive combination.

The information in this “Cahier Technique” willensure that you have all the necessary data tohand to help you make the right choice whenconsulting manufacturers’ documentation or- a more reliable option - when seeking specialistadvice in order to select the drive that will giveyou the best price/performance ratio.

Schneider Electric Direction Scientifique et Technique,Service Communication TechniqueF-38050 Grenoble cedex 9Fax: 33 (0)4 76 57 98 60E-mail : [email protected]

Transl: Lloyd International - Tarporley - Cheshire - GB.DTP: AXESS - Valence (26)Edition: Schneider Electric- 20 € -

© 2

03 S

chne

ider

Ele

ctr ic

11-03xxxxx

electronic starters and variable speed drives

Documents