The simplification from two transistors to one SPDT switch is possible because only one transistor in each bridge leg can be closed at any given time and that one transistor must be closed in each leg in order to have a three-phased current flowing. Further, the scenario of both transistors being closed implies that the DC-link is short circuited and we can all go home and order new parts. The scenario of both transistors in one leg being open is not interesting and will not be necessary. Second, each leg of the inverter bridge can be simplified by replacing the two bridge leg-transistors by one single SPDT-switch which shall indicate that either the top or the bottom transistor is closed. The supply and diode rectifier will thus not be shown for the rest of the article. Normally it varies with load, but not that much during the time frames given in this article. To better understand how space vector PWM is used, the above schematic can be greatly simplified:įirst, the DC-link voltage can be simplified so that it is constant. Not shown: snubber circuit, anti-parallel diodes over the transistors and back-EMF in motor. Three output terminals and a star-connected (ungrounded) induction machine equivalent From left to right the following is shown:Ī three phase supply and a three-phased diode rectifierĪ DC-link capacitor for energy storage and voltage stabilization The above schematic is the well-known and well-used inverter topology. The three phased system is illustrated using two different, but equal forms:Ī vector diagram showing all three phases and their vector sum ( space vector).Īn ordinary instantaneous sine wave representation, also showing the resultant space vector. This is what an induction machine is experiencing when connected directly to the grid. To rotate a motor, a smoothly rotating voltage vector is required - not one that skips 60 degrees per step.Ī grid supplied three phase voltage will look like the animation below. There are only six different voltage angles available. The only voltage level available is the DC-link voltage which can be assumed constant (well, at least for sake of simplicity). transistors) emulate a three-phased sinusoidal waveform where the frequency and amplitude is adjustable. The goal is to use a steady state DC-voltage and by the means of six switches (e.g. Space Vector Pulse Width Modulation (SV-PWM) is a modulation scheme used to apply a given voltage vector to a three-phased electric motor (permanent magnet or induction machine).
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