Vol 20, No. 3;Mar 2013
Space vector Modulation for V/f Induction Motor Control Khalaf salloum Gaeid IEEE member Electrical Department, Engineering Faculty Tikrit University Baiji,Salahaddin,Iraq Tel: 00964773057076 E-mail:
[email protected]
Abstract. This paper presents the implementation of a fault tolerant control strategy for an induction motor using the discrete wavelet transform. Space vector modulation to control a three-phase voltagesource inverter was used. The healthy induction motor was operated with closed loop V/f. A switching mechanism based on wavelet indices reconfigures the system to open loop V/f control in the event of open and short circuit faults in the stator winding. A minimum level of motor performance was thus maintained despite the occurrence of faults. Keywords: Space vector modulation, V/f control, Wavelet, Induction Motor faults, Fault Tolerant Control.
1.
Introduction
Inverters convert Direct Current (DC) to Alternating Current (AC) and find use in a wide range of applications, from powering car equipment to being the back-up supply of an entire building. They vary in cost, efficiency and power (J. Doucet et al, 2007). Inverters output a periodic continuous time signal made from rectangular pulses of different widths, which are determined by switches (Saleh et al,2011). Space vector modulation (SVM) is a well established modulation technique to control inverters (Serpa et al, 2007, Dorin et al, 2001). A key application of inverters has been to variable speed AC induction motors. Inverters allow the regulation of the frequency and magnitude of the voltage and current applied to a motor, so as to ensure excellent peformance with reduced noise. The V/f method is the simplest control approach, in which the ratio between the magnitude and frequency of the stator voltage is maintained constant, thus ensuring the same stator field level through the entire operating range.Thus, (maximum) constant torque producing capability is maintained. When transient response is critical, switching power converters also allow easy control of transient voltage and current applied to the motor to achieve faster dynamic response. The constant V/f principle is used in this work. 2.
Space vector Modulation
Figure 1 illustrates the structure of a typical three-phase voltage source power inverter. The three phase voltages are represented by Va, Vb and Vc. The switches are represented by s1 through s6. G1 through G6 are the gate signals. For Induction motor control, when an upper transistor is switched ON, i.e., when G1, G3 or G5 is 1, the corresponding lower transistor is switched OFF, i.e., the corresponding G2, G4 or G6 is 0.
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Vol 20, No. 3;Mar 2013
Fig.1. Inverter with the rectifier stage The relationship between the switching variable vector [G1, G3, G5]t and the line-to-line voltage vector [Vab Vbc Vca]t is given by (1) in the following: Va b 1 − 1 0 G1 = − 1 G3 V V dc 0 1 bc Vca − 1 0 1 G5 (1) Equation (2) determines the phase voltage vector [Va Vb Vc]t, where Vdc is the DC supply voltage, or the bus voltage. Va 2 − 1 − 1 G1 1 Vb = 3 Vdc − 1 2 − 1 G3 Vc − 1 − 1 2 G5 (2) As shown in Fig.1, there are eight possible combinations of ON and OFF patterns for the three upper power transistors that feed the three phase power inverter. Notice that the ON and OFF states of the lower power transistors are opposite to the upper ones and so are completely determined once the states of the upper power transistors are known. The eight combinations and the derived output line-to-line and phase voltages in terms of DC supply voltage Vdc, according to equations (1) and (2), are shown in Table1. Table1. Switching patterns and output voltages of the inverter G1 0 1 1 0 0 0 1 1
G3 0 0 1 1 1 0 0 1
G5 0 0 0 0 1 1 1 1
va 0 2/3 1/3 -1/3 -2/3 -1/3 1/3 0
vb 0 -1/3 1/3 2/3 1/3 -1/3 -2/3 0
vc 0 -1/3 -2/3 -1/3 1/3 2/3 1/3 0
vab 0 1 0 -1 -1 0 1 0
vbc 0 0 1 1 0 -1 -1 0
vca 0 -1 -1 0 1 1 0 0
In the space vector PWM method, the reference voltage vector is approximated using a combination of the eight switching patterns (Zhenyu et al, 1998). Figure 2 shows the space vectors denoted by V0, V60, V120, V180, V240, V300, Org000 and Org111. A straightforward approach of approximating the reference voltage vector is generating an average output for a small time period that equals the average of V0 . Equation (3) illustrates this for time periods T1 and T2, during which switching patterns Ux and Ux±60 form the sector containing Uout.
1 T
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( n +1)T
∫
nT
U out =
1 (T1U x + T2U x ± 60 T
(3)
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Vol 20, No. 3;Mar 2013
n=0,1,2,….,T1+T2
