Space Vector Modulation in PWM

Space vector modulation is a means of generating a three phase variable voltage, variable frequency PWM output voltage.

The inverter comprises six solid state switches, two for each phase with one switch on each phase connecting to the positive rail and one switch connecting to the negative rail. By a combination of switching states of these output switches, we can create a sinusoidal output current.
In effect, there are eight states that define six output vectors and two NULL vectors.
S0 = 000 : NULL
S1 = 100 : Vector 1
S2 = 110 : Vector 2
S3 = 010 : Vector 3
S4 = 011 : Vector 4
S5 = 001 : Vector 5
S6 = 101 : Vector 6
S7 = 111 : NULL

Maximum output voltage would be achieved by stepping through the 6 major states in sequence. the output pattern would be S1, S2, S3, S4, S5, S6, S1 etc This would result in maximum voltage and also quite a high level of distortion. The output voltage at any vector angle can be reduced by PWM techniques. If we consider the vector S1, we can reduce the voltage at this angle by switching between S1 and S0. Half voltage would be with 50% time on S0 and 50% time on S1. Hence, we can have a variable voltage ouput waveform that steps round the six vectors by using PWM modulation with the apex vectors and the NULL vectors S0 and S7.
Intermediate vectors can be generated by using PWM techniques between the adjacent apex vectors and the NULL vector. The angle is changed by the ratio between the apex vectors and the voltage is reduced by increasing the NULL time. For example, a 90 degree vector at half voltage would be achieved by 50% time with NULL vector, 25% time with an S2 vector and 25% time with an S3 vector.

If we consider the six non NULL voltage vectors v1 (= S1) to V6, then they describe 6 sectors 1 to 6. Within each sector, we can derive any voltage vector Vs of reduced voltage and an angle within the sector. There are a number of strategies for generating the resultant vectors, each strategy has advantages and disadvantages, affecting THD (Total Harmonic Distortion), switching losses and bearing currents in motors.
Right Aligned Sequence. S0 - S1 - S2 - S7 - S0 - S1 - S2 - S7 etc.
The angle and magnitude of the vector is determined by the ratios of the periods d0, d1 and d2.

Symmetric Sequence. S0 - S1 - S2 - S7 - S2 - S1 - S0 - S1 - S2 - S7 - S2 - S1 - S0 etc.
The angle and magnitude of the vector is determined by the ratios of the periods d0, d1 and d2.