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New DTC Control Scheme for the Induction Motor

Fed with a Three-Level Inverter

X. del Toro*, M.G. Jayne*, P. A. Witting*, V.M. Sala**, A. Arias**, J.L. Romeral**

*School of Electronics, University of Glamorgan, Pontypridd, Wales, United Kingdom.

**Electronic Engineering Department, Universitat Politcnica de Catalunya, Terrassa, Catalunya, Spain.

Abstract In this paper a new scheme based on Direct

Torque Control strategy is presented. The new controller

is used to control an induction motor fed with a three-level

inverter. The new control scheme avoids the use of the

classical hysteresis block and the look-up table. Instead,

the Torque and Stator flux errors are used together with

the motor speed to generate a reference voltage that can be

synthesized using different techniques afterwards.Experimental results show some improvement regarding

the reduction of torque ripple, stationary mean torque

error, THD in stator currents and switching frequency

when compared to the Classical DTC scheme.

I. INTRODUCTION

Direct Torque Control (DTC) has emerged over the

last decade to become one possible alternative to thewell-known Vector Control of Induction Machines. Its

main characteristic is the good performance, obtaining

results as good as the classical vector control but with

several advantages based on its simpler structure and

control diagram [1][2][3].

In DTC it is possible to control directly the stator flux

and the torque by selecting the appropriate inverter

state. Its features are the following:

Direct control of flux and torque.

Indirect control of stator currents and voltages.

Approximately sinusoidal stator fluxes and stator

currents.

High dynamic performance, even at zero speed.

Variable switching frequency and harmonic content influx and current waveforms depend on flux and torque

hysteresis bands and the operative point.

However, some disadvantages are present such as:

Possible problems during starting and low speed

operation and during changes in torque command.

Requirement of torque and flux estimators, implying

the consequent parameters identification (the same as

for other vector controls).

Variable switching frequency caused by the hysteresis

controllers. Inherent torque and stator flux ripple.

Flux and current distortion caused by sector changes

of the flux position.

A higher harmonic distortion of the stator voltage and

current waveforms.

A variety of techniques have been proposed to

overcome the drawbacks present in classical DTCstrategy [4]. Some of the different solutions proposed

include: DTC with Space Vector Modulation (SVM)

[5]; the duty ratio controller based on a simplemodulation [6][7][8]; artificial intelligence techniques,

such as Neuro-Fuzzy controllers [9]; different inverter

topologies, such as the three-level Neutral Point

Clamped (NPC) inverter [10][11][12][13][14][15][16].

In this work a new control scheme has been devised to

overcome some of the drawbacks present in the classical

DTC scheme. The main objective when designing the

new control is to keep the simplicity of the system and

to obtain a general solution that could be applied to

different converter topologies, such as three-levelVoltage Source Inverters (VSI).

II. NOVEL DTCSCHEME

In order to design a novel controller based on the DTC

principle some considerations have been made that may

lead to the improvement of the system behaviour:

Better quantification of the input variables will also

lead to a more accurate control.

The Operating point of the motor, which is not taken

into account in Classical DTC, must be considered

among the inputs of the control system. DTCperformance is especially poor at low speed.

Some kind of modulation can be also introduced to

obtain lower harmonic distortion, fixed switching

frequency and a more accurate control.

If the control algorithm does not involve using

induction motor parameters the robustness of the

control system is improved, as these parameters

change during the motor operation.

Using more complex inverter topologies such as the

three-level NPC topology that provide a higher

number of voltage vectors will enhance the possibilities of the control system and improve the

performance. It is therefore desirable to use a control

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structure that can be easily applied to any kind of

converter.

The general structure of the novel controller is shown in

Fig. 1

Fig. 1 Novel DTC scheme

This novel controller has a modular structure that

generates a reference voltage, following the DTC basic

principle instead of choosing the inverter state as in the

classical DTC. The inputs of the controller will be the

same and additionally the operating point ( m ) will

also be taken into account, to improve the behaviour at

different speeds, especially at low speed where DTC is

less satisfactory. The reference voltage generated as acontrol action can be synthesised using different

techniques with different degree of complexity from

choosing the nearest vector available, emulating a look-

up table, to using advanced modulation techniques to

balance the undesired capacitor unbalance present in

NPC inverters. This structure of controller can beapplied to any topology because the type of inverter will

only affect the way the reference voltage has to be

synthesised.

The controller is based on the basic principle that

desired decoupled control of the stator flux modulus and

torque is achieved by acting on the radial and tangential

components respectively of the stator flux-linkage space

vector. These two components are directly proportional

(considering RS=0) to the components of the same

voltage space vector in the same directions. Therefore,

when calculating of the reference voltage (in -coordinates fixed to the Stator Flux phasor) the

tangential component () will depend on the Torque

error, while the radial component () will depend on the

Stator Flux error. The initial expressions of the

controller showing this dependency can be described as:

'ref sV e (1)

'ref eV e (2)

Starting from these basic equations several

modifications have been made to tune the controller.Initially a preliminary reference vector in -

coordinates is calculated, where some scaling factorshave been introduced.

'ref s sV K e = (3)

'ref e m mV e K = + (4)

sK is a factor to scale the weights of the Stator Flux

error and the Torque error. As it can be seen a feed-forward action depending on the speed of the motor isadded to the Torque error in order to calculate the

tangential component of preliminary vector reference.

mK is used to scale this action. The objective is to

take into account the operating point and add somequantity to the tangential component that will make theTorque increase. The module of this action isproportional to the speed.

After that, a Cartesian to polar transformation is

calculated to obtain an expression with module andargument.

'refV M=

(5)Where:

2 2( ) ( )s s e m mM K e e K = + + (6)

1tan e m m

s s

e K

K e

+ =

(7)

Finally this preliminary reference vector in -

coordinates is rotated an angle equal to the Stator Flux

angle s and the final voltage reference vector is

obtained in D-Q fixed coordinates. Additionally, the

module is multiplied by another scaling factor, VK ,

which is used to scale the reference voltage according tothe voltage that can be delivered by the inverter. ASaturation block is used to limit the maximum value

that the reference voltage can take.

( )ref V S V K M = +

(8)

Initially, the novel controller has been developed usingthe simplest technique to synthesize the referencevoltage calculated, which is to choose the nearest vectorto this reference voltage. This keeps the simplicity ofthe system very close to Classical DTC. The internal

implementation of the new controller is shown in Fig. 2.

Fig. 2 New controller implementation.

Instead of choosing the nearest vector some modulationtechniques can be used like SVM, or some active-null

modulation like in a duty ratio controller.

The advantages of the novel controller are:

Simple structure and tuning. Possible use of modulation techniques to synthesize

the reference voltage calculated.

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TABLE I. MOTORPARAMETERS

Rated power1.1kW

(Y: 380V/4.43A)Poles 4

Nominal speed1415rpm

(148.17rad/s)

Rated torque 7.4Nm

Nominal Flux 0.96Wb

Stator Resistance (Rs) 9.21

Rotor Resistance (Rr) 6.644

Magnetising Inductance(Lm) 0.44415H

Stator Leakage

Inductance (Ls)0.03207H

Rotor Leakage

Inductance (Lr)0.00847H

Fig. 3 Experimental Setup

Operating point is taken into account.

There is no motor parameter dependency.

III. EXPERIMENTALSETUPThe novel controller has been tested in an experimental

setup. The workbench used contains the following partsas shown in Fig. 3:

A dSpace DS1103 board that performs the controltasks. This board contains aPowerPCand aDSP.

A 30A three-level NPC inverter with IGBTs tosupply an induction motor.

A 1.1kW induction motor to be controlled. TABLE Ishows the parameters of this motor.

A permanent magnet synchronous motor attached tothe induction motor shaft and supplied with anindustrial rectifier and inverter. Torque load can be

controlled with this equipment.

The control algorithm is created in Matlab/Simulink.

IV. EXPERIMENTAL RESULTS

Some experimental results have been obtained for theClassical DTC control system with a two-level inverter(DTC2L) and the new control technique with a three-level inverter (NDTC3L) in order to establish acomparison. Fig. 4 and Fig. 5 show the steady-state

stator currents, the stator flux and torque responses at200rpm and nominal torque conditions (7.4Nm). The

sample time used is 100s. In order to assess the performance of both systems, the torque standarddeviation and RMS error are calculated to measure the

torque ripple and steady-state errors. Additionally theflux standard deviation and the Total HarmonicDistortion (THD) of the stator current are calculated.The mean switching frequency of the inverter switcheshas also been calculated. These results are presented inTABLE II.

As it can be seen torque and flux ripple is reduced usingthe new controller and the three-level inverter. The

stator current distortion is also lower for the NDTC3Lsystem. Switching frequencies and THD are reduced

and therefore a higher efficiency is achieved.

Fig. 6 and Fig. 7 show the phase voltages in both

systems and the increased number of voltage steps whenusing a three-level inverter.

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Fig. 4 Torque response (left), stator flux response (centre) and stator currents (right) DTC2L at 200 rpm and nominal load

Fig. 5 Torque response (left), stator flux response (centre) and stator currents (right) NDTC3L at 200 rpm and nominal load

Fig. 6 Stator phase Voltage for DTC2L (five voltage levels) Fig. 7 Stator phase Voltage for NDTC3L (nine voltage levels)

TABLE II. SUMMARY OF THE EXPERIMENTAL RESULTS AT 200 RPMAND NOMINAL LOAD

SYSTEM DTC2L NDTC3L

Torque Standard Deviation 0.462Nm 0.221Nm

Torque RMS error 0.617Nm 0.236Nm

Flux Standard Deviation 0.0059Wb 0.0029Wb

Stator current THD 2.62% 1.17%

Mean Switching

Frequency4078Hz 1977Hz

V. CONCLUSIONS

A novel controller based on the DTC principle is presented. It is shown that it can be easily applied to

three-level inverters. One of the aims during the designis to keep the system simplicity as close as possible tothe Classical DTC system. The new controller

incorporates the motor speed in order to improve the performance at different operating points. Using thespeed it is possible to eliminate the steady-state error of

the mean torque at high speeds. The novel controller

equations do not involve the use of motor parameters providing high robustness. It is demonstrated that the

novel structure also provides the possibility of easily

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incorporate modulation techniques due the output of the

controller, which is a reference voltage vector. Initiallya very simple technique choosing the nearest vector tothe reference voltage vector is applied.

Results of the novel DTC scheme show a reduction intorque and flux ripple, harmonic distortion in stator

currents and switching frequency, when compared toclassical DTC with a two-level inverter. Additionally,the elimination of the steady-state error in the mean

torque is achieved.

LIST OF SYMBOLS

e : Electromagnetic torque.

_e ref : Torque reference.

_e est : Estimated torque.

_s ref : Stator flux module reference.

_s est : Estimated stator flux module reference.

s : Stator Flux angle.

ee : Torque error.

se : Stator flux angle.

m : Induction motor mechanical speed.

1, 2, 3S S S : State (on/off) of the three inverter legs.

ACKNOWLEDGMENT

The authors would like to acknowledge the economicsupport received from the Ministerio de Ciencia y

Tecnologa de Espaa for realising this work under the

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