Inverters derived from voltage source (VSI) are ideally suited for supplying power to a resistive and inductive load and the applications that require constant torque operation of AC motors. Current Source Inverters (CSI) are ideally suited for supplying power to largely capacitive loads and AC voltage buses. Inverters can be further classified on the basis of the mode of control of the output variable: voltage or current.
Inverter control in practice concerns three aspects: fundamental frequency, amplitude, harmonic profile.
Fundamental frequency is either equal to the switching frequency of the inverter (square-wave and PWM inverters) or its simple fraction (1/3, 1/5, and so on) such as in Multiple PWM or sine PWM inverters.
Power electronic applications require the amplitude control of output voltage or output current, called Voltage-Mode control and Current-Mode control respectively. An example of Voltage-Mode control is the adjustable speed drives of induction and synchronous motors. An example of Current-Mode
control is a constant torque drive for induction motors. Output amplitude in both modes can be controlled by varying the DC source voltage or by varying the pulse-width (pulse-width modulation, PWM). The inverters based on the control of DC source voltage are also termed DC-Link Inverters. These inverters use an AC to controlled DC converter on the input side. The inverter is square-wave controlled. This arrangement is expensive but exhibits a faster response time than PWM inverters. The control circuitry is also simpler. Harmonic profiling is an extra feature added over the amplitude control. The objective of harmonic profiling is to bolster the amplitude of the fundamental component of the output voltage and the elimination or reduction of high order harmonics. One of the techniques involves the notching or adding step pulses to a base pulse. The other technique involves multiple pulses per cycle of the fundamental, each pulse-width modulated to achieve desired wave-shaping of the output waveform (Sine-PWM).
The DC-AC inverter can be thought of as a three-position switch as shown in Fig.3-7(a). The load is connected to the pole of the switch and the first two positions are connected to dc voltage or dc current sources of opposite polarities. The third position is connected to the zero terminal of the source. The pole of the switch stays in the first and second positions for equal amounts of time in a switching period. The output waveform for zero amount of time on the third position is a square wave. The output waveform for nonzero time is pulse wave. The three-position switch may be synthesized by two unidirectional electronic switches as shown in Fig.3-7(b). The three-position switch may be synthesized by two unidirectional electronic switches as shown in Fig.3-7(b). The third position of the switch is simulated by turning both switches off in VSI topologies and on in CSI topologies.