Space Vector Modulation
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.
The basic pattern
construction is repeated for all six sectors. There are other space vector
modulation sequences that can be used to generate the required vector
patterns.
