Space-vector Pwm Inverter Feeding A Small Im

Proceedings of International Conference on Mechatronics Kumamoto Japan, 8-10 May 2007

TuAl -C-3

Space-Vector PWM Inverter Feeding a Small Induction Motor A. M. Kheireldin

A. Maamoun, A. M. Soliman

Electronics Research Institute El-Tahrir Street, Dokki, Cairo EGYPT Postal Code: 12622, Fax: 202-3351631

Faculty of Engineering Ain Shams University Cairo EGYPT

maamoun(ieri.sci.eg

Abstract The paper presents a space-vector pulse width modulation (SVPWM) inverter feeding a small three-phase induction motor. The SVPWM inverter enables to feed the motor with a higher voltage with low harmonic distortions than the conventional sinusoidal PWM inverter. The voltage/frequency control method is used for open loop speed control of induction motor with a reasonable degree of accuracy. The performance of the proposed drive system is simulated. The advantages of the proposed drive system are confirmed by the simulation results. -

Vdc

motor phhases

I INTRODUCTION The space-vector pulse width modulation (SVPWMI) technique has become a popular pulse width modulation technique for three-phase voltage-source inverters in the control ofAC motors [1-6]. The SVPWMI voltage-source inverters for variable voltage variable frequency (VVVF) drives of small induction motors are widely used both in industrial and household applications [7, 8]. The SVPWMI inverter is used to offer 15% increase in the dc-link voltage utilization and low output harmonic distortions compared with the conventional sinusoidal PWM inverter. The control strategy of the inverter is the voltage/frequency control method, which is based on the spacevector modulation technique.

Figure 1 Three-phase voltage source inverter diagram

If the inverter operation starts by state (100) to be state 1, it is possible to compute the voltage space vectors for all inverter states which are shown in the complex spacevector plane in Fig.2. The six active voltage space vectors are of equal magnitude and mutually phase displaced by 600. The general expression for the eight voltage vectors is [5]:

2Vdc exp Kj

Vs,kw= 13

The paper presents a SVPWM inverter feeding a small threephase induction motor. The voltage/frequency control is used for open loop speed control of induction motor with a reasonable degree of accuracy.

dc

k

i

0

)

where

31

where

=1,61>6

(1)

k= 0,7

The maximum fundamental phase voltage magnitude that link voltage

may be produced by the inverter for a given dc occurs under six-step operation, and is given by:

SPACE-VECTOR MODULATION TECHNIQUE A diagram of the power circuit of a three-phase voltage source inverter (VSI) is shown in Fig. 1, where Va, Vb, and Vc are the output voltages applied to the star-connected motor windings, and where Vdc is the continuous inverter input voltage. Qi through Q6 are the six power transistors those shape the output, which are controlled by a, a\, b, b\, c, and c. When an upper transistor is switched on (when a, b, or c is 1), the corresponding lower transistor is switched off (the corresponding a b\, or c\ is 0). There are eight different combinations of switching states as follows (000), (100), (110), (010), (011), (001), (101), and (111). The first and last states do not cause a current to flow to the motor, and hence, the line - to - line voltages are zero. The other six states can produce voltages to be applied to the motor terminals. II

VI, six -step

2

=-Vdc

(2)

On the other hand, the maximum achievable fundamental phase voltage magnitude for conventional sinusoidal modulation is [6]:

,

1-4244-1184-X/07/$25.00©2007 IEEE

k

Isin

-pwm

2

(3)

From equations (2) and (3), only 78.5% of the inverter capacity is used. 1

01 S \tor 3 Sector 4

Sector 1 V Re S Ax Sector 6

\ 9/Sector \AV1o (001)V \ Figure 3 Synthesis of desired voltage space vector using realizable voltage vectors

Figure 2 Voltage space vectors for a three-phase voltage source inverter .S

ni v

v

X

g

x

not produce the desired voltage directly. It is possible to decompose it into two vectors, Vx and V , that lie on the two active inverter vectors on either side of the reference vector. Therefore, in space-vector notation:

The reference voltage space vector is given by:

Vs

=v exp

(jmt)

M

2c exp

jc m t

)

(4)

Vs

The modulation index M is defined as the ratio ofthe desired peak fundamental phase voltage magnitude to halfthe dc link voltage (maximum achievable fundamental phase voltage magnitude for conventional sinusoidal PWM). V1

M

(Vdc I 2)

Xd

(5)

Vs

(6)

Vsa

+

TsVs,m

+

Ts Vs,m+l

(9)

states:

Tzero =To+T7 =Ts -Tm-Tm+i

(10)

Having computed the active and zero state times for a particular modulation cycle, it is possible to produce the switching signals, PWMa, PWMb, and PWMc to be applied to the inverter. The total zero time is most often divided equally between the two zero states. It is possible to satisfy the above restrictions by the use of symmetrical pulses as shown in Fig. 4. The cycle begins in state 0, (000) , with each inverter pole being successively toggled until state 7, (111), is obtained. The pattern is them reversed in order to complete the modulation cycle.

The desired voltage space vector at any particular instant may be written in Cartesian co-ordinates as: =

(8)

where Tm and Tm+i are the times spent at adjacent active inverter states, Vs,m and Vs,m+l . The remainder of the switching cycle is subdivided between the zero

which corresponds to a maximum modulation index Mmax 1.15. From equations (2) and (6), about 90.6% ofthe inverter capacity is used. This represents 15% increase in maximum voltage compared with the conventional sinusoidal modulation.

Fs

vx + vy

where the vectors, V. and V , are obtained by operating at the relevant inverter states, Vsl and Vs2, for suitable portions of the switching period, T,. In general, when operating in sector m, the reference vector may be decomposed according to [5]:

The largest possible phase voltage magnitude that may be achieved using the space-vector modulation strategy corresponds to the radius of the largest circle that can be inscribed within the hexagon of Fig.2. Thus, the maximum fundamental phase voltage magnitude that may be achieved is:

Visvpwm

=

j(7)

Consider the example depicted in Fig.3, in which the desired voltage is found to lie in Sector 1. Although, the inverter can 2

.T tI 1-

V

PWMA

11-

Sec

(a) V

Figure 4 Inverter switching signals for SVM in Sector

200 1

120

SVPWM INVERTER FEEDING AN INDUCTION MOTOR The space-vector PWM technique is used to produce the switching control signals to be applied to the three-phase inverter circuit given in Fig. 1. The SVPWM inverter is used to offer 15% increase in thee dc link voltage utilization and low output harmonic distortions compared with the conventional sinusoidal PWM inverter. The control strategy of the SVPWM inverter is the voltage/frequency control method, which based on the space-vector modulation technique. For constant torque output, the air gap flux in the motor is maintained constant by operating on a constant voltage/frequency supply. However, the analysis above assumes negligible winding resistance, whereas, in practice, at low frequencies the resistive voltage drop becomes significant compared with the induced voltage. This voltage drop causes a reduction in the air gap flux and motor torque. In order to maintain the low-speed torque, the voltage/frequency ratio must be increased at low frequencies. The voltage/frequency control method is used for open loop speed control of induction motor with a reasonable degree of III

a80

I1

So000

iS 000

10000

20000

Hz

(b) Figure 5 Line voltage of the motor at fL=25 Hz and M=0.85 (a) voltage waveform (b) voltage spectrum (rms)

A , , | , ~~~~~~ l~~ ~~~

~~~

~~~

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

2

1 ...

.................. ............................................ .2

..

092

034

093

095

096

0.97

0.38

0.99

Sec

(a)

accuracy.

A

IV SIMULATION RESULTS

The SVPWM inverter which feeding a small three-phase induction motor is simulated using the Matlab software package. The simulation is performed under the following conditions: Vd,= 366V, switching frequency= 5kHz, and three-phase induction motor (370W, 380V, 0.94A, 50Hz, 2750 rpm, R,=35Q, R,=19Q, LS=1.396H, Lr=1.396H, Lm=1.365H, J=0.003kg.m2, and P=1). Fig. 5 shows the motor line voltage at inverter frequency f0 =25 Hz and modulation index M =0.85. Also, the motor current at different loading conditions is shown in Figures 6 and 7. The motor current has low harmonic distortion.

0 .2-

I O

SOOO00

10 0 b00

lS000

20000

(b) Current ofthe motor at

Figure 6

fL=25 Hz, M=0.85, and T1= 0.7 Nm

(a) current waveform (b) current spectrum (rms)

3

Hz

V CONCLUSION

v

In this paper, a drive system consists of SVPWM inverter and a small three-phase induction motor has been introduced. The SVPWM inverter is used to offer 15% increase in the output voltage and low output harmonic distortions compared with the conventional sinusoidal PWM inverter. The voltage/frequency control is used for open loop speed control of induction motor with a reasonable degree of accuracy. The advantages of the proposed drive system are confirmed by the simulation results.

i,

1

:-.

-1

l.l

...

...

-2

..j

0.92

0.93

0.94

0.95

0.91

0.97

0.98

0.99 Sec

(a)

REFERENCES [1] H. W. van der broeck, H. C. Skudelny, "Analysis and realization of a pulse width modulator based on voltage space vectors," IEEE Trans. Industry Applications, Vol. 24, pp. 142-150, January/February 1988. [2] S. R. Bows, Y. S. Lai, "The relationship between space-vector modulation and regular-sampled PWM," IEEE Trans. Industrial Electronics, Vol. 44, pp. 670-679, October 1997. [3] Y. Tzou, H. Hsu, "FPGA realization of space-vector PWM control IC for three-phase PWM inverter," IEEE Trans. Power Electronics, Vol. 12, pp. 953-963, November 1997. [4] F. Zare, G. Ledwich, "Space vector modulation technique with reduced switching losses," Proc. Int. European Conf.: Power Electronics and Applications (EPE'99), Switzerland, pp. 1-7, September 1999. [5] Analog Devices, "Space vector modulation," Technical Report, September 1988. [6] Texas Instruments, "Field orientated control of three-phase AC motors," Application Report, February 1998. [7] M. Morimoto, K. Sumito, S. Sato, K. Oshitani, M. Ishida, S. Okuma, "High efficiency, unity power factor VVVF drive system of an induction motor," IEEE Trans. Power Electronics, Vol. 6, pp. 498503, July 1991. [8] A. Maamoun, A. M. Soliman, A. M. Kheireldin, "Near unity power

A 1.0

Psooo

20000

Hz

(b) Figure 7 Current of the motor at fL=25 Hz, M=0.85, and Tl=l .18 Nm (a) current waveform (b) current spectrum (rms)

factor single-phase to three-phase converter feeding an induction motor," Proc. IASTED Int. Conf: Artificial Intelligence and Applications, Innsbruck, Austria, pp. 131-135, February 2006.

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