422 HANDBOOK OF ELECTRICAL ENGINEERING
Figure 15.10 Waveform of the load current in a single-phase quazi-square-wave inverter.and the centre-tapped inductor are used to provide forced commutation where the ‘off’ state is not
controllable. Figure 15.10 shows the voltage applied to the load.
The method described above can be modified to operate as a three-phase inverter. Single and
three-phase inverters operating in this manner form the basis for many types of uninterruptible power
supplies (UPSs), and variable speed drives for AC motors.
15.4.2 Three-phase power inversion
High-power inverters were initially developed for the long-distance transmission of power from a
three-phase source to a remote three-phase sink using a DC overhead transmission line or cable. Early
DC power transmission used mercury arc thyratrons (gas-filled values or tubes), which functioned
in a manner very similar to the early types of thyristors. The ‘on’ state of the valves was control-
lable, but the ‘off’ state was determined by natural commutation made available by the sinusoidal
voltages of the sink power system, see Reference 13. A brief description of three-phase invert-
ers follows.
There are two basic types of high-power inverters that are used to supply AC induction or
synchronous motors, see References 2 and 9.
- Voltage source inverter.
- Current source inverter.
The voltage source inverter was the first to be developed for the control of induction motors.
It consists of a supply rectifier, a DC link inductor, a DC link capacitor and an inverter for the
motor. The inductor provides some smoothing of the DC current and short-circuit current limiting for
the supply rectifier elements. The capacitor is relatively large and stores sufficient charge to provide
current into the inverter. It also provides smoothing of the DC current. Figure 14.3 shows the basic
configuration.