Subject : Power Electronics
Unit : DC to AC Converters
Space Vector PWM (SV-PWM) Technique
Space Vector PWM (SV-PWM) Technique:
Fig: The concept of voltage space vectors – three space balanced vectors Fig: The voltage space vectors
- The space vector modulation technique is somewhat similar to the Sine 3^{rd }harmonic PWM technique but the method of implementation is different. Before going into details of this technique, it would be useful to explore the concept of voltage space-vector, in analogy with the concept of flux space-vector as used in three-phase ac machine.
- The stator windings of a three-phase ac machine (with cylindrical rotor), when fed with a three-phase balanced current produce a resultant flux space-vector that rotates at synchronous speed in the space.
- The flux vector due to an individual phase winding is oriented along the axis of that particular winding and its magnitude alternates as the current through it is alternating. The magnitude of the resultant flux due to all three windings is, however, fixed at 1.5 times the peak magnitude due to individual phase windings. The resultant flux is commonly known as the synchronously rotating flux vector.
- Now, in analogy with the fluxes, if a three phase balanced voltage is applied to the windings of a three-phase machine, a rotating voltage space vector may be talked of. The resultant voltage space-vector will be rotating uniformly at the synchronous speed and will have a magnitude equal to 1.5 times the peak magnitude of the phase voltage.
- During each time period of the phase voltages six discrete time instants can be identified, when one of the phase voltages have maximum positive or negative instantaneous magnitude. The resultants of the three space-voltages at these instants have been named V_{1 }to V_{6. }The spatial positions of these resultant voltage space-vectors have been shown.
- At these six discrete instants, these vectors are aligned along the phase axes having maximum instantaneous voltage. The magnitude of these voltage vectors is 1.5 times the peak magnitude of individual phase voltage.
- The instantaneous voltage output from a 3-phase inverter, discussed in earlier lessons, cannot be made to match the three sinusoidal phase voltages at all-time instants. This is so because the inverter outputs are obtained from rectangular pole voltages and contain, apart from the fundamental, harmonic voltages too.
- However, the instantaneous magnitudes of the inverter outputs and the sinusoidal voltages can be made to match at the six discrete instants (talked above) of the output cycle. At these six discrete instants one of the phase voltages is at its positive or negative peak magnitude and the other two have half of the peak magnitude.