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10thInternational Symposium

Topical Problems in the Field of Electrical and Power Engineering

Prnu, Estonia, January 10-15, 2011

Simulation Study of Inverter-Fed Motor Drives

Mikhail Egorov

Tallinn University of Technology

mikhail.egorov@ttu.ee

Abstract

The paper presents models developed to study

modulation systems of inverters that supply the

induction motor drives. A modelling technique and

simulation results are proposed. The benefits of the

discontinuous space vector modulation algorithms

for the six-step, pulse-width, and continuous space

vector control methods are described.

Keywords

Power electronics, electric drive, modelling,

simulation, Simulink, modulation

1. IntroductionAs the complexity of the motor drive circuits and

systems increases, the role of simulation as an

analysis and verification tool has expanded

significantly. It has become a cost-efficient way to

design many complex circuits in this way. Both

large- and small-signal simulations are performed onthe drive motors, power converters, and control

circuits to better predict and verify projects [1].

Several simulation toolkits are propagated such as

Simulink, Multisim, Spiceand PSim[2], [3]. Among

them, Matlab is in high demand in the motor drive

study [4]. This high-level 4th generation

programming language and interactive environment

enables users to perform intensive calculation-based

tasks very fast. The toolbox allows matrix

manipulation, functions and data plotting, algorithm

implementation, creation of the user interfaces, and

interfacing with programs in other languages. It hasbeen widely adopted for over 25 years in the

academic community, industry and research centers.

The toolkit provides the users with a large collection

of toolboxes and modules for a variety of

applications in many fields of interest. Its interactive

graphical superstructure Simulink [5] was added to

Matlab to make the modeling and simulation of

various systems as easy as connecting predefined

and designed building blocks. Simulink contains

many block sets that are used in different

applications, such as the communication block,

signal-processing block, etc [6], [7].

The objective of this study was to develop and study

Simulink models of the control blocks of inverters

that supply and adjust induction motors of

asynchronous motor drives. This problem is

significant because the control method governs the

voltage and current harmonics, torque ripple,

acoustic noise emitted from the motor as well as

electromagnetic interference. To this end, the new

models of the motor drive inverter control systems

developed are described in this paper.

Several techniques are known in the drive controlpractice: low commutation simple six-stepmodulation, high commutation pulse-widthmodulation (PWM) and progressive space vectormodulation (SVM).

The six-step modulation algorithm generatessignificant harmonic distortion as its alternatingcurrent (ac) waveforms are the low frequencyrectangles. Here, the reference sinusoidal voltage isapproximated by the six discrete voltage levelsavailable at direct current (dc) supply.Implementation of this type of modulation isrelatively simple and does not require high switching

speed, which makes it suitable for converters.Though the six-step inverters have found occasionaluse in motor drives, simulation of this technique isimportant for understanding open-loop voltage-frequency control [8].

Intensive switching PWM techniques are veryvaluable for drive performance. Simulation of thiscontrol method is beneficial to study voltage andcurrent harmonics, torque ripple, acoustic noiseemitted from an induction motor, and alsoelectromagnetic interference [9]-[11].

An SVM method is the most advanced computation-intensive control approach for generating afundamental sine wave providing a higher voltage tothe motor and lower switching losses than both thesix-step and PWM methods. This is known today asthe best among all the power converter controltechniques in the field of variable drive applications.Several SVM algorithms have been reported in[12]-[15].

The models proposed in this paper help to analyze

and compare different modulation techniques from

the ripple and switching loss point of view.

Advantages and disadvantages of different

modulation methods are illustrated by the simulation

results of the inverter performance in ac driveapplications. The developed models can be

effectively applied in the development and study of

open-loop and closed loop motor drives.

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2. Modelling and ExperimentationArrangements

To perform numerical simulation, a set of models

was developed where a controlled induction motor

drive was investigated in the Matlab/Simulink

toolbox. The models designed in the simulation

environment were accomplished with an inductionmotor, loading device, and independent bridge-

connected IGBT simulators (Fig. 1). The control

circuits which are focused in the paper are enveloped

by the bold contours in Fig. 1. Simulation was

executed for the voltage/frequency control mode of

an induction motor drive using the six-step, PWM

and SVM continuous and discontinuous techniques.

Power converter M Load

Gate circuits

ControllerInputs

References

Supply

Controls

Feedbacksfrom

sensors

Fig. 1. Generalized block diagram of a motor drive

A model of a squirrel cage induction motor

M33AA 132S of ABB series was used for

simulation. To measure its torque, currents and

speed of rotation, the Simulink machine

measurement block was used.

To develop the control algorithms, the three-phasebridge inverter shown in Fig. 2 consisting of the

three-leg IGBTs VT1and VT4, VT2and VT5, and

VT3 and VT6 with the freewheeling diodes was

studied.

VT4

VD1 VD3

VD4 VD5 VD6

VT1 VT3

VT5 VT6

UdM

C

C

VD2VT2

N

L1

L2L3

+

Fig. 2. Three-phase bridge inverter

To this aim, the Simulinkuniversal bridge block was

used built on the Semikron SKM 145 GB 124 DN

transistor simulators. The switches were examined in

eight different combinations designated by the binary

variables 100, 110, 010, 011, 001, 101, 111, and

000, which indicate whether the switch is under the

positive (1) or negative (0) supply, thus defining all

possible switching states. During the modulating

period 1 / = 1 / (2fm), a phase voltage sequentially

changed its values depending on which of theswitches were on-state. Here, is an angular motor

speed, fmis the modulating frequency (max 50 Hz),

and Ud is the dc supply voltage. To simulate a

dc link, the Simulinkdc voltage source was used. For

voltage and current measuring and tracing, the

Simulink voltage and current measurement blocks

and scopes were connected. The Simulink series

RLC branches were used to reach the dc link and

load neutral points.

To obtain sufficient resolution, the modulating

period was presented by the six sectors, each dividedinto 20 sampling periodsTc. Therefore, the sampling

frequency was 120fm (max 6 kHz). All the schemes

assume digital implementation therefore calculations

were performed at the beginning of each sampling

period, based on the value of the reference voltage

and speed at that instant. In this way, the reference

was updated at every sampling intervalTc.

To test the proposed models, an experimental

workplace was organized at the Department of

Electrical Drives and Power Electronics of Tallinn

University of Technology. It unites two electric

drives ACS800 series, the testing drive and theloading one. Each drive has the same structure,

consisting of an induction motor, power converter,

remote console, as well as the cabinet, housing,

measuring, and cabling equipment. The motor shafts

of the drives were mechanically coupled to provide

their joint rotation. Both power converters include

the line-side active rectifier and the motor-side

inverter connected via the dc link. The tested motor

M33AA 132S has the following characteristics: rated

power 5.5 kW, voltage 400 V, current 11 A, speed

1460 r/min, torque 36 Nm, and the moment of inertia

0.038 kgm2. The loading motor M33AA 160L has

the rated power 15 kW, voltage 400 V, current 29 A,

speed 1460 r/min, torque 98 Nm, and the moment of

inertia 0.102 kgm2. The power converters ACS800

enable two modes of operation: voltage / frequency

(U/f) control and direct torque control (DTC) with

direct and indirect measuring of the motor speed,

torque, and current. Their technical data are as

follows: input voltage 400 V, output voltage 0 to

415 V, output frequency from 8 to 300 Hz, and

output power 15 kW with the speed and torque

scalar and vector control, flux and mechanical

braking, acceleration and deceleration ramps.

During the verification stage of research the

differences of the transient and steady-state

characteristics between the tested and the simulated

drive were studied. Both the half-speed, half-rated

running (18 Nm) and the rated speed nominal

loading (36 Nm) operations were compared. In this

way, the model correctness and adequacy were

confirmed.

3.Six-Step Control ModelTo generate the six-step control signals, six Simulink

pulse generators were connected directly to the

IGBT gates (Fig. 3). Thus, switching of the threeinverter legs supplied by the dc voltage is phase-

shifted by 120 and each phase L1, L2, L3 is kept

under the current during half a modulating period

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and open in another half-period. In this way, specific

phase is alternately switched from the positive pole

to the negative one being sequentially in series with

the remaining two parallel-connected phases. The

traces of the gate signals, the phase-to-ground,

phase-to-neutral and line-to-line voltages and the

phase current are shown in Fig. 4 for the full-loaded

drive running with nominal speed.

Fig. 3. Six-step control system

Fig. 4. Simulation results of six-step control

4.PWM Control ModelA model developed to simulate the PWM controlleris presented in Fig. 5. It includes three sine-wavegenerators accompanied by speed-amplitude andmodulating index reference blocks, a carriergenerator, three comparators and gate drivers. In thisway, the symmetrical triangle double-sided wave ofthe carrier frequency is compared with themodulating wave thereby generating the controlpulses. The control output represents the chain ofconstant magnitude pulses, the duration of which ismodulated to obtain the sinusoidal waveform. The

traces of the gate signals, the phase-to-ground,phase-to-neutral and line-to-line voltages and thephase current are shown in Fig. 6 for the full-loadeddrive running with nominal speed.

Fig. 5. PWM control system

Fig. 6. Simulation results of PWM control

5.SVM Control ModelAn SVM controller model was designed on the basis

of the Simulinkembedded Matlabfunction block to

compose and study both continuous and

discontinuous SVM algorithms (Fig. 7). Here, the

switching sequence of pulses is searched in each

sampling interval, the time durations of which are

computed in real on the basis of the value of the

reference voltage and speed at the beginning of each

sampling period. Three inputs of the function block

carry the reference amplitude, speed and clock

signals. The traces for the continuous SVM are

practically the same as for the PWM-fed drive. For

the discontinuous SVM, the gate signals, the phase-

to-neutral and line-to-line voltages as well as the

phase current are shown in Fig. 8 for the full-loaded

drive running with nominal speed. Some additionalcurrent distortion was registered here in contrast to

the continuous SVM as a result of non-linear inverter

adjustment.

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Fig. 7. SVM control system

6.ConclusionThree models of the motor drive control systems

were developed in the Matlab/Simulink

environment and studied using the proposed

models: low commutation simple six-stepmodulation, high commutation PWM, and

progressive space vector modulation SVM. Studies

focused on the impact of the modulation method on

the voltage and current harmonics, torque ripple,

acoustic noise emitted from the motor and

electromagnetic interference was studied.

Advantages and drawbacks of different control

methods were illustrated in the steady-state and

dynamic modes of the converter performance in ac

drive applications. Simulation revealed an

improvement in the switching performance of the

motor drive converter from the discontinuous SVM

operation when compared to the six-step

modulation, PWM and continuous SVM

techniques.

Acknowledgements

This paper is supported by Project DAR8130

II Doctoral School of Energy and

Geotechnology. Also, the author expresses

gratitude to his supervisor Professor Valery

Vodovozov for essential contribution to this work.

Fig. 8. Simulation results of SVM control

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