Switching Pwm for Motor

7/30/2019 Switching Pwm for Motor

1/12

MITSUBISHIELECTRIC In Depth Tutorial

SEMICONDUCTORS Loc LE COZ 26/04/01

TECHNOLOGY FOR LIFE MITSUBISHI ELECTRIC Page 1 of 12

Evolution of Industrial Motor Control

Loic LeCozEmail: [email protected]

MITSUBISHI ELECTRIC EUROPE B.V.Travellers LaneHatfield, Hertfordshire

AL10 8XBUKhttp://wwww.mitsubishichips.com

2001 Embedded System Show


2/12




Abstract

Over more than a century, many different types of motors have been developed, with most usuallydedicated to a particular application. The reason for this was that there was no easy way of regulatingthe supply voltage or frequency to control thespeed, and designers had therefore to find ways of providing this control within the motor itself.

All this changed in the 1960s, first with the thyristor,providing a relatively cheap and easily controlledvariable-voltage supply for DC motors and later inthe 1970s with the development of variable-frequency inverters suitable for induction motors.

These major developments resulted in thediscontinuation of many of the special motors,leaving the majority of applications to only fewtypes for example; DC and Brushless DC motors;Induction motors and recently Switched Reluctancemotors.

Complexity has now shifted from the motor itself tothe external drives and control circuit.

Choosing a complete drive system requires not onlyknowledge about motors, but also the associatedpower electronics and the control options, thispaper will examine first five fundamental machinesused in industrial drives: DC, Brushless DC and

AC, Induction and Switched Reluctance motors.The topics such as how each motor and drivesystem works and for which application they aresuitable will then be considered.

In the second section, the focus will be on two mainsolutions offering a high level of performance. Thecontrol of induction machines will be explainedstarting with the basics. The different types of PWMwill be described, mentioning their practicalimplementation using microprocessors, andexplanation of the famous Space Vector Modulation and Vector control will be provided.Finally, as switched reluctance drives are rapidlyexpanding into new applications, the developmentof a closed loop system solution will be presented.


3/12




Introduction

Electric motors now form a big part in our daily lifeespecially with domestic applications like hair-dryers, fans, mixers and drills that we do notreally pay any attention to them. We just expect allthese items to do their job without even giving athought to the motor itself and its rather complexelectronics.

Because of this, and considering the fact thatelectric motors use 60% of the world wide energygenerated, this paper will provide the reader firstwith a basic understanding of how the differenttechnologies operate and what kind of applicationseach one can be associated with. This paper isthen targeted for all engineers wishing to get ageneral understanding of the main technologies onthe market.

Assumptions

To generate the torque required to producerotation, most motors use the force created on acurrent-carrying conductor placed in a magneticfield.

This paper assumes that the reader already knows

that when a current-carrying carrying conductor isplaced in a magnetic field, it experiences a forcedepending on the current in the wire, the intensity of the magnetic field as shown below.

Fig 1: Electromagnetic force created on a current- carrying conductor in a magnetic field

1- Motor technologies

The following diagram gives an overview of thedifferent families of electric motors.

Fig2: Classification of electrical motors

1-1 DC Motors

Although DC motors (also called universal motors)now tends to be less popular than inverter-fedinduction motors, there are still a number of applications such as cranes, fork lift truckswhere DC motors are still used. When it isimportant to hold a load, customers still appreciatethe accurate and high torque at zero speed of the

classical solution: DC Motor + Drive system.

The basic principle is described in Fig 3.Basically, the torque is produced by the interactionbetween the axial current-carrying conductors onthe rotor and the radial magnetic flux generated bythe stator.

A universal motor can be divided into 3 main parts: Stator : Permanent magnets (in Fig3) or field-

windings are used to generate the radialmagnetic flux.

Rotor : Coils wound in the rotor (armature)supplied in current via the carbon brushes

Carbon Brushes : Mechanical commutatorslinking DC power supply and rotor windings

Current

MagneticField

Force

ELECTRIC MOTORS

DC AC

Synchronous Asynchronous

InductionBrushless

DC

ReluctanceBrushless

AC


4/12




Fig 3: DC motor principle

Regarding the power topologies used for DCmotors, the two main solutions (chopper or thyristor bridge) are shown below in Fig 4.

Fig4: Power Topologies

The benefits offered by this technology are: Separate control of Speed and Torque Torque at 0 speed Widely used in the industry

However the disadvantages are: Expensive Maintenance required(brushes)

Commutations

Sparks

As previously mentioned, DC motors are used inhigh power applications such as lifts, cranes up

around 100KW up to few Mega Watts for industrialpumps or rollers. They are used as well in electricvehicle and machine tools in a range of power up to

50KW.

1-2 Brushless DC Motors

Brushless DC motors are part of the synchronousmotors family. They are also called permanentmagnet motors due to the structure of the rotor.

Although they do not operate from a DC voltagesource, their name comes from the fact theyoperate in a similar way as universal DC motors butturned inside out.

Fig5: Brushless DC principle

Regarding its architecture presented in Fig5, aBrushless DC motor has: Rotor : Permanent magnets

Stator : Windings arranged to produce an air-gap flux density wave having a trapezoidalshape.

The windings being separate poles, they areenergised in a pattern rotating around the stator inorder to produce rotation. The rotor magnets arethen lead by the excited windings to the alignmentposition where the next commutation will occur.

As shown in Fig6, the motor is fed from an inverter producing rectangular currents waveforms. Theswitching pattern is easily obtained using generally

three Hall effect sensors associated with logicgates.

N

S

Stator

Rotor

Windin s

Brushes

PermanentMagnet

DC

DC

VariableDuty-cycle

Phase 1: ON

Stator

PermanentMagnet

Windings

SN

Rotor


5/12




Though Brushless DC motors retain some of universal motors benefits, they offer distinctiveadvantages as well like: Brushless technology (maintenance free and no

spark)

High torque to speed ratio High speed (up to 70000rpm)

The potential disadvantages of this technologycome from the structure itself: Expensive( cost of the rotor permanent

magnets) Commutated Torque ripples

Brushless DC motors are really popular inconsumer applications (vacuum cleaners), whitegoods (washing machines) in a range of power below 20KW.

1-3 Brushless AC Motors

Brushless AC motors are also known asSynchronous motors. When looking at their structure, the difference with Brushless DC motorsmight not be obvious, especially the permanentmagnet version.

As previously mentioned, a very similar structure isthen used for both rotor and stator. The maindifference is a different organisation of the stator windings. Instead of having different poles as seenbefore, they are arranged in a sinusoidaldistribution .

Fig7 presents the basic drive principle for synchronous motors. They are driven with sinewave voltages, and whether permanent magnets or windings are used on the rotor, the motor rotatessynchronously with the stators rotating magnetic

field.

In variable speed applications, the power topologywill often be the one shown above. Though theinverter structure remains the same, the drivingstrategy will change to use now PWM signals. Thedifferent ways to produce PWM signals will bedetailed in part two.

This type of motor has mainly the same advantagesas Brushless DC motors. Another characteristicmakes them even attractive: Speed is independentof the load, i.e. it only relates to current. The downside is if the maximum load is reached, the motor will suddenly stall.

These motors are used for example in applicationslike trains, ships, or pumps in a range of power going up to few Mega Watts.

BDC

T1

T4

T2

T5

T3

T6

3 Hall Effectsensors

LogicGates

T1

T2

T3

T4

T5

T6

Fig 6: Brushless DC Drive principle


6/12




1-4 Induction Motors

Induction motors play a key role in industrial societyas they convert to mechanical energy roughly 30%[1] of all the electricity generated.

Using windings on both stator and rotor(instead of mechanical commutators), they are one of the mostcost-effective solutions on the market.

Fig8 presents the structure of an inductionmachine.

Fig8: Induction motor principle

Stator : Windings arranged in a sinusoidaldistribution. When connected to a 3 phase ACvoltage power supply, a rotating magnetic fieldis then generated

Rotor : Windings as well

Like a DC motor, the torque is created by theinteraction of a radial magnetic field produced bythe stator and axial currents induced on the rotor.The rotor is dragged around by the rotating field of the stator but it can never run as fast as the field, it

just slips as the field rotates.

The drive principle is not presented here but it isreally similar to brushless AC one. The power topology generally consists in three half-bridgesdriven by PWM signals.

Induction machines main benefits are: Robust mechanical structure Cheaper than most other technologies

The weak points are the following: Torque & speed are dependant Tends to slow down when overloaded

Induction machines are applicable in a wide rangeof power, from a few hundred Watts up to a fewMega Watts. Although previously used in constantspeed applications (pumps, fans), thanks to newcontrol strategies such as vector control, inductionmachines can now be used in variable speedapplications (Trains, machine tools).

T1

T4

T2

T5

T3

T6

ControlUnit

B.AC

T1

T2

T3

T4

T5

T6

Stator

Windings

Rotor

Windings

Fig 7: Brushless AC Drive principle


7/12




1-5 Switched Reluctance Motors

Switched Reluctance Motors (SRMs) have recently

attracted much attention due to their potentiallywide-ranging applications as a result of advances inmicroelectronics. These advances, together withthe motors intrinsic mechanically robust andthermally stable structure, have fuelled themomentum of using SRMs in favour of other motor types in many cost-sensitive and competitiveindustrial and consumer markets.

SRMs have the simplest mechanical structurecompared with other types of electrical machines,making them one of the most interesting from aneconomic point of view.

Fig9: 3 Phase 6/4 symmetrical SRM

Fig9 presents the structure of a SRM:

Stator : Windings only located on the stator teeth. Quantity determined by the number of phases.

Rotor : Steel laminations stacked onto the shaft.

The fact that the rotor is only made with steellaminations is the main mechanical difference withconventional motors such as DC or inductionmotors having either rotor windings or permanentmagnets .Indeed, if we consider these machines on theprinciple of how the torque is produced, they can be

classified into two different classes:

In the first category, which includes Induction andDC motors, the torque is generated by the

interaction of two magnetic fields, one on the rotor and one on the stator. Then, these machines couldbe differentiated by the geometry used, and on the

different method of generating the two fields, withpermanent magnets, energised windings or withinduced currents.

In the second one, with SRMs, the rotation of therotor is created by the tendency of the motor teethto align with the excited stator teeth. This isbecause when a stator winding is energised, areluctance torque is produced as the rotor moves toits minimum reluctance position. As the first excitedphase makes the rotor teeth move to the alignedposition, then, the next phase to be excited ischosen to be the most aligned stator teeth, with

respect to the required position.

As far as the working principle is concerned we candraw comparison between the phenomenoninvolved in the production of torque for AC & DCmotors with the one that makes like poles of baremagnets repel. We can also compare thereluctance torque with the force that attracts iron topermanent magnets.

Mechanical simplicity is probably the main SRMsadvantage but we could sum up its most interestingcharacteristics as being:

Cheaper than the other motors Motor virtually maintenance free As there is neither permanent magnet nor

winding on the rotor, very high speeds can bereached without risk of damage, relatively tocomparable motors

Tolerant to high temperatures Size.However, SRM drives suffer from differentproblems. First the rotating field theory is notapplicable here, and because of its structure, thismotor has highly non-linear characteristics. Thedisadvantages for that motor could be summarisedin the following points[2]: Difficult to control Often noisy Torque ripples Not available yet as standard of the shelf

motors.

Most of these problems can be compensated by abetter understanding of SRM mechanical designand the development of specific algorithms. SRMsare not very common on the actual market placecompared with traditional DC and AC motors.

However, industries are getting more and moreinterested in this technology because of theadvantages shown above.

Stator

Windings

Rotor

Flux


8/12




2- Introduction to Induction andSwitched Reluctance Motor Control

For these last few years, two motor technologieshave attracted a lot of interest: Induction andSwitched Reluctance motors. The reason is thatthese two solutions seem to be the most costeffective on the market.The aim of this part will be to explain the basics of the key strategies in motor control.

2-1 Induction motor control

When talking about induction motor control, a lot of people have already heard the expressionsPWM, SVM or Vector Control but are stillconfused about their real meaning. The target for this part is to clarify each of these expressions toprovide the reader a good overall understanding.

2-1-a Pulse Width Modulation (PWM)

For any analog or digital industrial drive, the desiredvoltage across the motor phases relates to theoutput signals of the controller. Pulse WidthModulation is a technique to recreate thesedesired waveforms from a DC voltage using adirect converter. The control technique consists infirst, changing the turn on and turn off time of thisconverters power switches but also in controllingthe strategy modifying these times.

Although PWM is a very flexible technique, it tendsto generate many harmonics. The minimisation of these harmonics will have to be considered in any

industrial development.

Here are the three main PWM techniques:

i ) Triangular wave modulation This strategy is illustrated in Fig 11.

Fig 11: Triangular wave modulation [3]-V/2

V/2

V/2

V/2

PWM output

Carrier Wave ReferenceVoltage

ControlUnit

T1

T2

T3

T4

T5

T6

Fig 10: SRM Drive principle


9/12




For the controller to generate the correct PWMoutput that mimics a sine wave for example, it mustuse a carrier wave. In motor control applications,

the carrier wave is the signal used to generate thePWM signal (either triangular or sawtooth). ThePWM signal results from the comparison betweenthe carrier wave and the reference signal . Thistechnique has been implemented on Mitsubishi 8and 16 bit microcontrollers .

ii ) Pre-calculated modulation This PWM is pre-calculated relating to therequirements of a specific application such as theminimisation of particular harmonics. The pre-calculated pattern is then periodically applied on theswitches.

iii) Space Vector Modulation

Fig 12: Power Topology for Induction motors

This kind of modulation is based on the fact that thethree wanted voltages could be represented in realtime by only one vector. During each modulationperiod, this vector is approximated, using the sixpower switches commutation sequences. Thevector co-ordinates are given by CLARKEtransform.

This transformation consists in obtaining theCLARKE components:V', V', Vo' (Vo'=0) using the real components:V A', VB' and V C'(the three desired voltages across

the motor).These voltages are given by:

='

'

'

'

'

232302 / 12 / 11

32

C

B

A

V

V

V

V V

V', V' are the vector coordinates, that representsthe three phases system.

According to the eight configurations of theinverters power switches, six main vectors with thesame modulus define six different sectors (see Fig13).

Fig13: Space Vector diagram [4]

The rotation speed of this vector is ( =2 f 0, withf 0 the output voltages frequency). A 2 rotation of this vector represents one rotor revolution.

Fig 14 shows how the vector 'V is going to besampled in the first sector. k is the number of modulation periods per sector. (k=3 in thisexample).

Fig 14: Sector sampling

During a modulation period, each sampled vector can be expressed to a base of four vectors.

V2

V1

V1,1

V1,2

1 st modulationperiod

V1,3

VV2V3

V A VB VC

Induction Motor

U

K1

K1

K2 K3

K2 K3

V

V

2

13

4

5

6

V2

V1

V6V5

V4

V3

V

t

(1)

'

''

V

V V =


10/12




Therefore, knowing the six switchesconfigurations for each vector, a switchingsequence can be expressed by:

'V .Tc = '0V .ta + iV ' .tb + 1' +iV .tc + '7V .td

The switching times(t a , tb, t c, t d) for the ith

sector are then calculated with :

tb = T c.r.23

sin (3

-t)

tc = T c.r.23

sin t

ta = t d = 21

. [T c-tb-tc]2-1-b Vector control (PWM)

Vector control is not a PWM technique. It is amodern regulation technique used to controlinduction machines. This fairly recent controlhas to be compared to an older one:V/Fcontrol.With this technique, for a sensed or evensensorless system, very high performance canbe reached with inductance motors. For example, in open loop, the nominal torque canbe obtained down to very load speeds (around30rpm) when having a 1% speed accuracy. Inclosed loop, the nominal torque is still availableat zero speed with an overall 0.01% accuracyon speed.Vector control consists of building amathematical model of the motor and use thedifferent equations to obtain a separate controlof flux and torque. The fairly complex inductionmachine normally has flux and torque relatingto each other, so can then be simply driven likea conventional DC motor where these twoparameters are independently controlled.

V/F is a simpler control used mainly inapplications where constant torque is required.Because this technique is independent fromthe motor characteristics constantly changing,the same performance, i.e. response time,accuracy can not be reached.

2-2 SRM control

2-2-a Global principle

The dynamic control of a motor depends a loton the torque control. A simple expression of

the SRM torque T is:

Where- Current ( i ),- Inductance ( L )- Position ( )

The global principle used to control and drivethe SRM is then based on a goodsynchronisation between the time when aphase is excited (i > 0) and the rotor position.In a motoring sequence where a positivetorque is required (T >0), each phase has to beexcited when the inductance increases asdL/d is positive as shown in Fig 15. Fromequation (1), constant torque can be obtainedby maintaining constant current, provided if dL/d is constant.

Fig 15: Basic Principle for torque control in one phase

This part is now going to present the speedcontrol of a three phase SRM implementedwith a 16-bit microcontroller, the MITSUBISHIM16C-62. A solution to make the motor run inopen loop will be presented. The resultsobtained in open loop will then be used to drawconclusions about that method. Finally, byimplementing the position and speed control,the implementation of the closed loop programwill be detailed.

2-2-b Implementationi) Open loop sytem

As previously mentioned, the principle chosento drive the motor is similar to the one used todrive stepper motors. The first objective was tocreate a simple program to drive the motor

(2)

(3)

(5)

(4)

(9)

ddL

i21

T =

InductanceLa

Lu

RotorpositionCurrent

ON OFF


11/12




phase after phase, being able to adjust and reverse the speed.The idea has been to create a LUT where theoutput pattern is stored, i.e. different

combinations of the state of the power switches(i.e. on or off). The advantage of usinga LUT is that it becomes easier to compensateproblems like overlapping. Then, with a pointer pointing to that table, the different values arecopied from memory to the output port. Toincrease or decrease the speed of rotation, theincrementation speed of the pointer is modifiedchanging a timer value. This value is obtainedfrom the result of the Analogue/Digitalconversion of the potentiometers outputvoltage.

Keeping the same basis the program can beimproved using the DMA(Direct Memory

Access) instead of a pointer to transfer datafrom memory to the output port. The benefit of using the DMA is that it takes no CPU time.

Although it is not a problem in the open loopsystem, this time can become important inclosed loop where many tasks have to be donewithin a short time. Fig 16 shows the principleof the DMA transfer.

Fig 16: Data transfer using the DMA

For practical reasons, i.e. to avoid too muchcurrent overlapping, it is interesting to modifythe LUT to allow each phase current todecrease before starting to supply the nextphase as shown on Fig 17.

Fig 17: Phase current

Overall, the results obtained with the three-phase motor in open loop were not as good as

expected, especially the torque quality. Themain reason for this is the lack of positionfeedback. This parameter appears to be verycritical as contrary to conventional stepper motors, the torque generation depends on theinductance profile ( L( ) ). Then, if we consider the rotor position, it is possible to define anoptimal configuration to supply the motor, thisconfiguration being the objective of theposition. Fig 17 illustrates this.

Fig18: position areas to supply the motor

From the previous diagram, a condition on theposition can be raised for an optimised torquegeneration: Each phase could be supplied onlyon 1/3 rd of the electrical period. The dwellangle ( ON - OFF ) will then have to be smaller than 120 electrical degrees to avoid anyoverlapping.

As this condition is not checked in open loop,the worst case can be reached if on one periodthe average torque is positive(T>0) and on thenext one it will be negative(T


12/12




Position controlThe control principle is based on the fact thateach phase should not be excited on morethan 1/3 rd of the inductance profile period.Changing the commutation speed modifyingthe data transfer speed to match a positioncondition is the first operation.

Speed controlThe technique used is called Bang-Bangcontrol. Basically, it consists in fully supplyingthe different phases until the motor reachesthe reference speed, and then stop, repeatingalways these sequences of On-Off, On-Off;On-Off...

Fig 18: Speed control principle

Fig 18 illustrates the global principle of theSRM controller implemented.

Conclusion

Choosing a motor and the drive associated is afairly complex process. It does not consist onlyof checking that parameters such as nominalspeed, torque or power match the applicationsrequirements. This is only the first step. Thesecond step will be to study the dynamicalperformance required by the application(acceleration time, response time, maximumtorque and speed allowed). And finally, theenvironment where the system will be used willhelp choosing the right technology to matchspecific needs like no spark, low harmonics, or even maintenance free. ..

Generally, for low power applications, all typesof machines can be considered. In high power,the choice will be reduced to universal motors

or brushless technologies (synchronous or asynchronous motors). For very low power,DC motors remain the first. Recently, thanks tohigh performance controllers the mediumpower market has evolved a lot. New highperformance technologies like Switchreluctance motors or Inductance machinesusing vector control offer low cost solutions for industrial motor control.Mitsubishi Electric offer a wide range of microcontrollers ideally suited for motor controlapplications, especially the M16C family.

Application notes and demo-systems areavailable for induction, brushless DC andswitched reluctance motors.

References

[1] Austin Hughes, Electric Motors andDrives, Butterworth Heineman, SecondEdition 1990, pp 152-154

[2] T.J.E. Miller, Switched Reluctance Motorsand their Control, Oxford SciencePublications, 1993

[3] Guy Grellet, Guy Clerc, ActionneursElectriques, Principe, Modeles, Commande,Eyrolles Edition,1997, pp 211

[4] Guy Grellet, Guy Clerc, ActionneursElectriques, Principe, Modeles, Commande,Eyrolles Edition,1997, pp 218-220

N/3 samples

N samples

Inductance profile inhase 1

Output

Driving signalPhase 1

Phase 2

Phase 3

TA1 modified by position info

ref

Documents

Switching Pwm for Motor