Space vector modulation

Principle

A three-phase inverter as shown to the right converts a DC supply, via a series of switches, to three output legs which could be connected to a three-phase motor.

The switches must be controlled so that at no time are both switches in the same leg turned on or else the DC supply would be shorted. This requirement may be met by the complementary operation of the switches within a leg. i.e. if A⁺ is on then A⁻ is off and vice versa. This leads to eight possible switching vectors for the inverter, V₀ through V₇ with six active switching vectors and two zero vectors.

Note that looking down the columns for the active switching vectors V_1-6, the output voltages vary as a pulsed sinusoid, with each leg offset by 120 degrees of phase angle.

To implement space vector modulation, a reference signal V_ref is sampled with a frequency f_s (T_s = 1/f_s). The reference signal may be generated from three separate phase references using the α β γ transform. The reference vector is then synthesized using a combination of the two adjacent active switching vectors and one or both of the zero vectors. Various strategies of selecting the order of the vectors and which zero vector(s) to use exist. Strategy selection will affect the harmonic content and the switching losses.

More complicated SVM strategies for the unbalanced operation of four-leg three-phase inverters do exist. In these strategies the switching vectors define a 3D shape (a hexagonal prism in α β γ coordinates or a dodecahedron in abc coordinates) rather than a 2D hexagon..