Editing Power electronics (section)

=== Three-phase voltage source inverter ===

[[File:Three-Phase Voltage Source Inverter Circuit Schematic.jpg|thumb|left|'''FIGURE 5:''' Three-phase voltage source inverter circuit schematic]]
[[File:Three-Phase Square-Wave Operation a) Switch State S1 b) Switch State S3 c) S1 Output d) S3 Output.jpg|thumb|left|'''FIGURE 6:''' Three-phase square-wave operation a) Switch state S1 b) Switch state S3 c) S1 output d) S3 output]]

Single-phase VSIs are used primarily for low power range applications, while three-phase VSIs cover both medium and high power range applications.<ref name=Rashid3 /> Figure 5 shows the circuit schematic for a three-phase VSI.

Switches in any of the three legs of the inverter cannot be switched off simultaneously due to this resulting in the voltages being dependent on the respective line current's polarity. States 7 and 8 produce zero AC line voltages, which result in AC line currents freewheeling through either the upper or the lower components. However, the line voltages for states 1 through 6 produce an AC line voltage consisting of the discrete values of Vi, 0 or −Vi.<ref name=Rashid3 />

For three-phase SPWM, three modulating signals that are 120 degrees out of phase with one another are used in order to produce out-of-phase load voltages. In order to preserve the PWM features with a single carrier signal, the normalized carrier frequency, mf, needs to be a multiple of three. This keeps the magnitude of the phase voltages identical, but out of phase with each other by 120 degrees.<ref name=Rashid3 /> The maximum achievable phase voltage amplitude in the linear region, ma less than or equal to one, is '''{{math|v<sub>phase</sub> {{=}} v<sub>i</sub>{{\}}2}}'''. The maximum achievable line voltage amplitude is  '''{{math|V<sub>ab1</sub> {{=}} v<sub>ab</sub>{{*}}{{radic|3}}{{\}}2}}'''

The only way to control the load voltage is by changing the input DC voltage.