Test: Current Source Inverters

In VSIs the input voltage is maintained at a constant value and the amplitude of the output voltage does not depend on the load conditions. However, the waveform of the load current as well as its magnitude depends upon the nature of the load impedance.

In CSIs, the amplitude of the output current is independent on the load impedance, as the input current (to the CSI) is kept constant.

In order that the current input to the CSI must remain ripple free an constant, L-filter is used before the CSI (in series with the energy source).

Considering the SCRs as ideal voltage sources, when T1, T2 are on the load current is = I. When T3, T4 conduct, the load current is negative with respect to the previous value and = -I.

The line voltage is the difference of two phase voltages, the 3rd harmonic gets cancelled out, though it is present in the phase voltages.

In VSIs the input voltage is maintained at a constant value and the amplitude of the output voltage does not depend on the load conditions. However, the waveform of the load current as well as its magnitude depends upon the nature of the load impedance.

In CSIs, the amplitude of the output current is independent on the load impedance, as the input current (to the CSI) is kept constant. However, the magnitude of output voltage and its waveform depends upon the nature of the load impedance.

CSI converts the input dc current to an ac current at its output terminals.

The load current waveform is a square wave with frequency = 1/T. As the load consists of a capacitor, dv/dt must be constant over every half cycle. This slope is positive from zero to T/2 and negative from T/2 to T.

All the CSIs need capacitors for their commutation if force commutation is required. Force commutation is essential for lagging power factors.