Multi level converters for AC-motors

High power medium voltage motors fed by traditional inverters, use low voltage inverters and a transformer is used to become a high voltage for the motor. The inverter has to work on a low voltage because the switches of the half bridge have to block the total voltage of the DC-bus. These low voltage inverters have several disadvantages. The currents through the switches is much higher than the phase currents in the motor. The transformer between the inverter and the motor has a large self inductance and results in a slower control of the motor. Also the transformer yields extra losses.


In multi level converters, switches are connected in series so every switch has to block only a part of the DC-bus. This way the voltage of the DC-bus can be taken higher and medium voltage motors can be fed directly by the inverter. The high number of switches gives the possibility to control the voltage vectors for the motor phases more precisely. The desired voltage vector can be better approximated. It is also possible to reduce the switching frequency and to reduce the variations in common mode voltage.


For the moment, the chosen topology is the flying-capacitor. An other much used topology is neutral-point-clamped. These topologies create and control partial voltage levels of the DC-bus voltage. These voltage levels are used to become a better approximation of the desired voltage vector. So the inverter has to control besides the output voltage also the partial bus voltages.


The choice of the desired instantaneous output voltage can be done in different ways. The normal pulse-width-modulation (PWM) technique can be extended for multilevel converters. Instead of 1 triangular carrier signal, N-1 of them are used to compare with the desired voltage (with N the number of levels). By comparing these carrier signals with the desired phase voltage, the demanded switch-state can be derived.


An other method to approximate the desired output voltage vector is with space vector modulation (SVM). This method is shown is underlying figure for a five level inverter. Here, the three phases are represented by a vector in two dimensions, each with a 120 degrees phase shift. A five level inverter can generate five levels at his output, -2, -1, 0, 1 or 2 times a reference voltage. When all possible combinations are plotted, we become a hexagon built out of triangles.


To generate a specific voltage vector, first the triangle which contains the endpoint P of the vector has to be found. The three points of the triangle are determined (the big dots around P). These points can be obtained in several ways. The first point of the triangle can be obtained by the red line (2,0,-1) of the blue line (1,-1,-2). Now the durations of which the vectors of the points of the triangle have to be determined so the mean is the desired vector P.


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