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 singlephase inverter with a centertapped 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 threephase 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 Threephase 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 linetoline 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 linetoline voltages in terms of DClink 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 threephase 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 nonzero 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 nonzero 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 dq 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 (189 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. 
5/6/2008 5:57 AM  
Posts: 148 Rating: (10) 
Dear Abdelal, May I suggest you to bring all this information(different threads on informations) under one roof by just extending your original thread on the subject and just adding the reply’s. All such threads can be mark as highest tip and keep on the top. I fear that All separate topic will get lost in such big forum. Hope moderator take the similar actions on these tips. 
5/6/2008 8:29 AM  
Posts: 148 Rating: (10) 
Dear Abdelal , Before touching to Space vector I would like to add some information on the subject. As we are aware that today’s Motor control techniques revolve around three type.
And all these control techniques are achieved by means of PWM. Again PWM is achieved from the some of following methods. 1.Sinusoidal PWMIn tis type a sinusoidal weighted signal is generated using a carrier frequency.Please refer diagram 1 in the first thread. This is simplest method and need not require complex calculations and model theory. only one look up table for sine wave is require and all three waveforms are genereted by keeping 120 degree phase shift theory. Whereas it gives two disadvantages High Harmonics magnitude on PWM switching frequency.(and on its multiple) Output voltage is less than 90 percent of DC link voltage. 2. Six Step PWMIn this type of inverters usually six different switching state are used. Advantages are like No complex calculation , It gives higher output voltage than the DC link voltage. Where as High Cost, Very high harmonics at lower frequency, poor response at low speed, unstable system at lower frequency these are the Disadvantages, Hence it is been used only in High voltage Motors with Specific requirements. 3.Space Vector ModulationThis one is lately developed when it is proved that Modulation is not required throughout the Pulse time. This is widely used . A single rotating Vector is developed from the three individual rotating vectors. Hence the modulator output is controlled by all three vector whereas still easy to develop. The modulation is done only at the edges of the signal hence there is significant reduction in noise, heat in the motor. It gives better performance at all frequency and that to with minimum Harmonics. But with A less output voltage than th DC link. We can achive high o/p voltage than DC link ,if we use space vector with over modulation. In our Masterdrive and Micromasters we are using Space Vector Modulation technique. When it comes to Decide whether it is to be operated for Vector control or V/f Mode an additional block is inserted in main flow chart. Regards, Vikram tambe After Writing such a big page I realized that I have not take permission from you to write in this tip. Hope you don’t Mind.. …

5/6/2008 1:11 PM  
Posts: 76 Rating: (16) 
dear tambe, I appreciate your participatation in the thread , with that we can enrich the thread , and your post is great about issuses i didn't mention in my main thread. I always appreciate your opinions and all members' opinions Regards 
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