Embedded controlled four switch three phase inverter fed induction motor.

Saravanasundaram, S. ; Thanushkodi, K.

Introduction

In recent years, the alarming increase of the emissions of green house gases associated with the drain in time of fuel reserves make it urgent the substitution of thermal propulsion systems by electric or hybrid propulsion ones. The latter are currently considered as a mature technology as far as the electric propulsion systems are penalized by the short autonomy of the battery pack. Nevertheless, hybrid propulsion systems require further improvements in order to reach a large-scale market integration. Our particular interest is to reduce the cost.

Many Research and development projects focusing the cost reduction of hybrid propulsion systems are presently developed. Within this trend, the paper deals with the implementation of the rotor flux--oriented control (RFOC) in the induction motor fed by a current--regulated four--switch three--phase voltage inverter (CR-FSTPI). The reduction of the number of power switches from six to four improves the cost--effectiveness, the volume--compactness and the reliability. The principle of field orientation as applied to new transvector closed loop system is presented in [2] and [3]. The concept of vector controlled AC drive is presented in [4]. New space vector PWM strategy for a low cost three phase inverter fed induction motor drive is given in [5]. Flux tracking methods for direct field orientation is presented in [6]. The principles of vector control and field orientation are furnished in [7]. A new synchronous current regulator and analysis of PWM regulator is depicted in [8]. Comparative investigation of three phase induction motor under different control options is given in [9].

Following a brief recall of the formulation of the RFOC, the principle of operation of the Four Switch Three Phase Inverter (FSTPI) is described. Then, the implementation scheme of the RFOC in the CR-FSTPI is presented. In order to model as accurately as possible the drive considering its integration in automotive applications, an electric equivalent circuit of the battery pack supplying the FSTPI is taken into account. For the sake of comparison, the dynamic and steady state performance of the FSTPI-fed induction motor drive are referred to the ones of the conventional six-switch three-phase inverter (SSTPI)-fed induction motor drive. In the literature [1] to [9], the Matlab simulation model and embedded implementation of FSTPI are not presented. In the present work, Matlab simulation model is developed and the same is used for simulation. The hardware implementation of FSTPI is also presented.

[FIGURE 1 OMITTED]

Problem formulation

The induction motor model is expressed assuming that the direct axis of the ([d.sub.1]q) plane is held by the rotor flux ([[PHI].sub.r]=[[PHI].sub.dr] and [[PHI].sub.qr] = 0). In this case, the control of the induction motor turns to be simple similar to the DC motor one.

Under RFOC strategy, induction motor electrical equations are expressed as follows by [2], [3], [6] and [7].

Voltage Equations:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

Flux Equations:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

The expression of the electromagnetic torque [T.sub.em] is reduced to By [2],[3],[6],and[7].

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)

Substituting the direct rotor current [i.sub.dr] of equation (2) into the expression of [[PHI].sub.r] given by equation (1) yields

By [2],[3],[6] and [7].

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (4)

Where [T.sub.r], is the rotor time constant, such that:

[T.sub.r] = [l.sub.r]/[r.sub.r] (5)

and [rho] is the Laplace operator.

The Stator electric angle [[theta].sub.s] can be expressed as:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (6)

Where [[OMEGA].sub.m] is the mechanical speed and P is the pole pair number

The rotor angular frequency [[omega].sub.r] can be expressed is terms [[PHI].sub.r] and [i.sub.qs] as follows By [2],[3],[6] and [7].

[[omega].sub.r] = M/[T.sub.r] [i.sub.qs]/[[phi].sub.r] (7)

FSTPI: Principle of Operation

[FIGURE 2 OMITTED]

The circuit of FSTPI is shown in Fig. 2. The FSTPI consists of a two-leg topology. Such a reduced structure inverter is fed by a battery pack which is divided into two equal parts, so that two phases of the motor are fed by the legs of the inverter while the third one is connected to the middle voltage point of the battery pack [9].

In this paper, we consider a simulation and implementation of FSTPI-fed induction motor drive. This drive can be used in variable speed applications like hoists and cranes.

Let as assume that the states of the four power switches are denoted by the binary variables [K.sub.1] to [K.sub.4] where the binary "1" corresponds to an ON state and the binary "0" indicates an OFF state. The states of the upper and lower switches of a leg are complementary, which yields [4], [5].

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (8)

Le us consider a Y-connected induction motor, therefore, the terminal voltages [V.sub.as], [V.sub.bs] and [V.sub.cs] can be expressed as a function of the stages of the upper switches as follows [4].

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (9)

This inverter has reduced hardware since four transistors are sufficient. Less Driver chips are required since lesser transistors are used. Reactive energy can be stored in the split capacitors.

Simulation Results

The simulation was done using Matlab simulink and the results are presented. Space vector modulation involves three phase to two phase conversion. Four driving pulses are generated from the two signals. The block diagram of pulse generation module in Fig.1. Four switch 3[PHI] inverter circuit is shown in Fig. 2. Charged capacitors are represented as DC sources. Measurement circuit is shown in Fig.3. The simulated 3[PHI] currents are shown in Fig4. Pulse width modulated 3[PHI] voltages are shown in Fig. 5. There is a slight unbalance in the output voltages and currents. This unbalance is due to the capacitors in the first leg. Rotor Speed curve is shown in Fig. 6. The rotor speed increases and settles at 158 rps. From the simulation results of conventional 3[PHI] inverter fed induction motor, the THD is found to be 24.69%. The THD for FSTPI system is found to be 8.17%. Thus using FSTPI system reduces the THD.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

Experimental Results

A 1 KW laboratory model of FSTPI fed induction motor drive system was designed and fabricated. Experimental verification of FSTPI fed induction motor drive system was done by testing the hardware. Atmel Microcontroller 89C2051 was used to generate driving pulses for the MOSFET switches. Port1 of the atmel micro controller is used for determining the gate pulses. Timer is used for producing the delay required for Ton and Toff periods. The pulses from the microcontroller are amplified using IR 2110. It amplifies the pulses to the level of 10V. The hardware module is shown in Fig.7. The pulses applied to the gate of the MOSFET are shown in Fig.8. The PWM inverter output voltage is shown in Fig.9.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

Conclusion

The modeling of PWM controlled 4 switch 3 phase inverter has been done using Matlab simulink. The conventional 6 Switch 3[PHI] inverter drive system and FSTPI fed induction motor drive system are simulated and their results are compared.

This comparison shows that FSTPI system has reduced harmonics when compared to the multilevel inverter system. The hardware was tested by giving the pulses from an embedded microcontroller. The experimental results closely agree with the simulation results. The present work indicates that FSTPI fed induction motor drive is an economical drive with reduced harmonics. The hardware count is reduced since it uses only four switches. Each driver IC can amplify two pulses. Two driver ICs are sufficient to amplify the driving pulses since FSTPI has only four MOSFETs.

References

[1] Bad, A. (2000), Electrochemical Methods, Fundamental and Applications, 2nd ed., Wiley, New York, NY.

[2] Blaschke, F. (1972), "The Principle of field orientation as applied to the new transvector closed loop control system for rotating field machines", Siemens for rotating field machines", Siemens Review, Vol. 34, pp. 217-20.

[3] Boldea, I. and Nasar, S.A. (1992), Vector Control of AC Drives, CRC Press, Boca Raton, FL.

[4] Elbadsi, B., Guermazi, A. and Masmoudi, A. (2005), "New space vector PWM strategy intended for a low--cost four-switch three-phase inverter-fed induction motor drive", paper presented at CD-ROM of the Third IEEE International Conference on Systems, Signals and Devices, Sousse.

[5] Jacobina, C.B., Silva, E.R.C., Lima, A.M.N. and Riberio, R.L.A. (1995), "Vector and scalar control of a four switch three phase inverter", Proceedings of the IEEE Industry Applications Society Annual Meeting, Orlando, FL, Vol. 3. pp. 2422-30

[6] Manjrekar, M.D., Lipo, T.A., Chang, S.G. and Kim, K.S. (1988), "Flux tracking methods for direct field orientation", paper presented at International Conference on Electric Machines, Istanbul, m Vol.2, pp. 1022-9.

[7] Novotony, D.W. and Lipo, T.A. (1985), "Principles of vector control and field orientation", paper presented at IEEE Industry Application Society Annual Meeting, Toronto, pp. 2.0-2.65.

[8] Rown, T.R. and Kerkman, R.L. (1986), "A new synchronous current regulator and an analysis of current--regulated PWM inverter", IEEE Transactions on Industry Application, Vol. 22, pp. 678-90.

[9] Van Der Broeck, H.W. and Van Wyh, J.D. (1984), "A comparative investigation of a three-phase induction machine drive with a component minimized voltage--fed inverter under different control options" IEEE Transactions on Industry Applications, Vol. 20 No.2, pp. 309-20.

S. Saravanasundaram (1) * and K. Thanushkodi (2)

(1) * Research Scholar, Hindusthan College of Engineering and Technology, Coimbatore, India.

(2) Principal, Coimbatore Institute of Engineering and Information Technology, Coimbatore, India

* Corresponding author: S.Saravanasundaram Tel : 00 91 98947 56463 email: [email protected]