5/4/2008 4:19 PM | |
Posts: 76 Rating: (16) |
dear all , we will continue the posting of our series. As mentioned before we use inverter part in the drive to reach different frequencies to change our speed to desired one. we are going to mention the basic principle of (pwm) ,some names of other techniques very fast .then we will log to one of the most well known and widely applied method which is ( S V M) which our dear friend Tambe has mentioned, but we will focus on how does it work ?.ok let's begin The fig.1 shows circuit model of a single-phase inverter with a center-tapped grounded DC bus, and Fig 2 illustrates principle of pulse width modulation. it simply compares to signals Vcarrier(the tri) to Vref (here the sine).when at any instant the ref is higher than the carrier a pulse for switch to close is given in what is known firing signals. When Vcontrol > Vtri, VA0 = Vdc/2 When Vcontrol < Vtri, VA0 = −Vdc/2 the output is shaped like pulses for firing ,so we can see it's no became like ac but surely not as we desire . Also, the inverter output voltage has the following features: PWM frequency is the same as the frequency of Vtri Amplitude is controlled by the peak value of Vcontrol Fundamental frequency is controlled by the frequency of Vcontrol that is the main concept for inverters .All other ways depends on that but will differ between each other in the reference signal. so we may hear -stepped modulation - trapezoidal - harmonic injection -modified sinusoidal -stair case - and delta or hysteresis modulation. now we will see the SPACE VECTOR MODULATION . Principle of Space Vector PWM The circuit model of a typical three-phase voltage source PWM inverter is shown in Fig. 4. S1 to S6 are the six power switches that shape the output, which are controlled by the switching variables a, a′, b, b′, c and c′. When an upper transistor is switched on, i.e., when a, b or c is 1, the corresponding lower transistor is switched off, i.e., the corresponding a′, b′ or c′ is 0. Therefore, the on and off states of the upper transistors S1, S3 and S5 can be used to determine the output voltage. Fig. 4 Three-phase voltage source PWM Inverter. Also, the relationship between the switching variable vector [a, b, c]t and the phase voltage vector [Va Vb Vc]t can be expressed in fig.5 The relationship between the switching variable vector [a, b, c]t and the line-to-line voltage vector [Vab Vbc Vca]t is given as in fig .6 As illustrated in Fig. 7, there are eight possible combinations of on and off patterns for the three upper power switches. The on and off states of the lower power devices are opposite to the upper one and so are easily determined once the states of the upper power transistors are determined. According to previous equations , the eight switching vectors, output line to neutral voltage (phase voltage), and output line-to-line voltages in terms of DC-link Vdc, are given in Table1 and Fig. 5 shows the eight inverter voltage vectors (V0 to V7). THE Space Vector PWM (SVPWM) refers to a special switching sequence of the upper three power transistors of a three-phase power inverter. It has been shown to generate less harmonic distortion in the output voltages and or currents applied to the phases of an AC motor and to provide more efficient use of supply voltage compared with sinusoidal modulation technique as shown in Fig. 8..the circle is for SVM and hexagon for normal inverter To implement the space vector PWM, the voltage equations in the abc reference frame can be transformed into the stationary dq reference frame that consists of the horizontal (d) and vertical (q) axes as depicted in Fig. 9. the relation between these two reference frames is fdq0 = Ksfabc As a result, six non-zero vectors and two zero vectors are possible. Six nonzero vectors (V1 - V6) shape the axes of a hexagonal as depicted in Fig. 8, and feed electric power to the load. The angle between any adjacent two non-zero vectors is 60 degrees. Meanwhile, two zero vectors (V0 and V7) are at the origin and apply zero voltage to the load. The eight vectors are called the basic space vectors and are denoted by V0, V1, V2, V3, V4, V5, V6, and V7. The same transformation can be applied to the desired output voltage to get the desired reference voltage vector Vref in the d-q plane. The objective of space vector PWM technique is to approximate the reference voltage vector Vref using the eight switching patterns.hence we can control the flux in our machine One simple method of approximation is to generate the average output of the inverter in a small period, T to be the same as that of Vref in the same period. now if i can control the magnitude and angle of Vref we can reach every position on our circle ,but what does that circle mean ?? if the outbput voltage is pure sine wave (unlike the pulses of Normanl pwm ) the reference voltage Vref will draw that circle(known as quasi circular locus method) , but if output was steps as previously shown it will draw the hexagon. now we will end this thread , the next two threads will be how to control Vref to get the (quasi circular locus method) . after that we will how to simulate it and if there is enough time and the members are not bored we will show how to make its hardware . FIG.s are attached hope that was clear and hope you may like it . references : power electronics devices ( M H Rashid ) SVM Technique with DSP (Abdul Rahman Abbas) SVM Jin Woo REGARDS Attachmentseries.zip (647 Downloads) |
This contribution was helpful to1 thankful Users |
5/5/2008 9:51 AM | |
Joined: 2/7/2008 Last visit: 10/3/2008 Posts: 43 Rating: (1) |
Thank you very much, your post are very clarifying. |
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