On the left, the voltage source, which distributes positive voltage upwards and a negative downwards. Just as the switches are set here, the left side of the meter is positive and the right negative. The pointer deflection is corresponding. Below exactly the reverse position of the switches, which has a polarity reversal on both sides of the meter and again the corresponding display

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It is important that both switches are actuated absolutely simultaneously, otherwise there will be a short circuit. What do we achieve through this? Provided correspondingly fast switching is thereby produced a transformation from DC to AC. This is the electric car e.g. necessary to drive the electric motor with help of the battery. Conversely, we need it when charging the battery from the AC electricity grid.

However, we wanted to feed it as e.g. if the inverter of a solar system does, then we would have to generate 50 Hz, so to switch 100 times a second. Too fast for a mechanical or purely electrical operation. In addition, in the manner described above, unfortunately, we only generate a square wave signal that does not match a the reality of a AC electricity grid.

That's the course we need, nice sinusoidal. Below you can see that a power transistor is needed here, four times to replace the mechanical switches. With power transistors, switching frequencies of 10,000 Hz and more are possible. So there are still about 100 switching operations left within a sine wave. And should this transistor become too warm, it can be provided on the back with cooling plate and thermal grease.

Of course, they still need to be addressed accordingly. We use this control for a so-called pulse width modulation. The meanwhile appearance in the car quite often. Imagine that you have designed the ventilator for the air stream to the interior for the fourth stage and the other to be realized by interposing various resistors.

Depending on the power of the engine, they could become so hot that they had to be deposited in a ventilation duct. Being hot indicates a waste of energy and we should no longer afford that. Today, in the weaker stages of the engine, the electronics are activated different very short breaks cause a weakening of the engine power.

Here you can see how the switch-on time for the output of such a PWD circuit is extended to the greatest amplitude and then becomes shorter again. Of course, such a rough division would lead to a stepped sinusoidal oscillation. So the control with the described technology is much finer. And of course not only the amplitude but also the frequency can be changed, exactly what you need to control the motor of the electric drive.

So that would be the reasonably complete circuit diagram, from the direct current of the high-voltage battery to the three phases for the electric motor. They are called bridges, in each of which two switches are located and half bridges with one switch. Each bridge would thus be perpendicular to generating each L1, L2 and L3. The fast switches now allow pulse width modulation.

The control of the switches must be exactly aligned, as well as the top of the hand-operated. Thus, the bridges follow each other by 120° offset. At the same time, the two switches of the half bridges are in each case actuated simultaneously, but in opposite positions.

One speaks of comparators controlling the switches. These need the course of a sinusoidal, if they are to emulate these by pulse width modulation. The likewise important clocking takes place by fast oscillations (picture above), which look recorded like triangles. It is so taken measure at each oscillation on the sinusoidal and this passed as pulse and waiting length to the respective switch.

Since then is not the actual engine control included. Presumably, the sine wave for this must be changed. Put quite roughly, their frequency should be changed to influence the engine speed and the amplitude, if required more torque by the engine at the same speed. However, the control of the engine is anyway more complicated, depending on whether it is at a constant distance leading (synchronous motor) or repeatedly overhauling electric field.