Space Vector Modulation Pdf
All eight combinations and the resulting motor voltage and current directions are shown below. Visibility Others can see my Clipboard. Further, at the bottom of the above figure two schematics are shown. Six different examples are shown, each with a different angle in different sectors. This is quite simply a factor used to scale the voltage.
The equation to determine how much we are using each basic vector can be written as. Here, in the four different sections of the halv period.
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This will reveal the motor winding voltage and if the third harmonics will have any effect on them. The above example shows a transistor combination where the leftmost leg W has its upper transistor closed and bottom transistor open. Note that the third harmonic black is common to all three phases. The zero-vectors come in addition and will increase for lower voltage levels. It can also be seen that every pattern starts and ends with a zero vector, and that between every transition, only one switch is being changed.
How these three values are intersected with the triangular wave will directly translate to which switches we want to close and how long we are keeping them closed. This greatly reduces harmonics in the motor which cause overheating and torque fluctuations. The actual synthesizing can be achieved using a triangular wave as trigger. This low voltage might come to some as a slight disappointment. Next we will look at what the motor thinks of this.
The simplified version of a two level inverter. Clipping is a handy way to collect important slides you want to go back to later.
This is what an induction machine is experiencing when connected directly to the grid. The two other legs V and U has its upper transistor open and lower transistor closed.
Space Vector Modulation(SVM) Technique for PWM Inverter
Note that this applies to induction motors. The progress bar above each configuration indicates how long it is on compared to the total on-time for both basic vectors. As just shown, we are faced with a challenge with too low output voltage compared to what we really want. The only thing that change is the width i.
The supply and diode rectifier will thus not be shown for the rest of the article. The figure to the right illustrates all three phase voltages with third harmonic injection.
Rapid alternations between these to vectors along with the zero vectors can emulate the reference voltage. When your grid voltage is V, you would want to buy a V motor to go with it. To the right there is an example of a reference vector which the inverter has to synthesize using the basic vectors available. The third and fundamental harmonic exposed, revealing the increased amplitude.
The available voltage at the inverter terminals are not readily apparent by looking at the schematics alone due to the averaging of different vectors and the use of a modulation index. The two-level inverter is the most used topology today because it is simple and cheap. Press any image to enlarge.
Between the start of the three axes and the red line, there are no active voltage vectors. Also, note that over-modulation is no longer linear. This is controlled by a scaling factor called modulation index.
The observant reader might notice a circle located inside the hexagon. Further, it is assumed that each inverter leg has one of two transistors closed at any given time. The U-phase is normally basis for all angles. For example, if the angle is exactly between to basic vectors i.
An ordinary three phased system, here shown in both vector form and in sinusoidal form. SlideShare Explore Search You. Three output terminals and a star-connected ungrounded induction machine equivalent. Snubbers and anti-parallel diodes are not shown.
Well, the trick is that it's all hidden inside. What is the phase-to-neutral voltage across each winding? This figure contains a lot of information, and to better explain different parts of it, I have clipped out four different regions of it that will be explained below.
Let us take a breathing pause there. With the above information an updated figure of the six basic vectors can be presented. The thin line green line inside the space vector circle are visual aides to illustrate that these two basic vectors do indeed form the desired space vector as a vector sum. The percentage indication above indicate their on-time relative to the other. The above schematic is the well-known and well-used inverter topology.
The main focus in this part are the dashed lines protruding downwards from the triangular wave. We have now successfully circumvented physical limitations in terms of maximum voltage output and packed it all neatly with the use of third harmonic injection. When the desired amplitude is low, the zero vectors will have an increased on-time and opposite when the amplitude is high.
The reference vector is then synthesized using a combination of the two adjacent active switching vectors and one or both of the zero vectors. The scenario of both transistors in one leg being open is not interesting and will not be necessary. The by far most important part of the figure is the area below the lowest axis W. This voltage level is what we have to play with to synthesize an output voltage.
The frequency of this waveform is many times higher than the frequency of the space vector. As can be seen, the space vector's magnitude is always constant. By getting the on-times correctly, physiology of the nervous system pdf this will emulate a voltage vector at the same angle as the input voltage space vector. These represent the state of the switches for the two basic vectors which are in use in the current section.
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