A Current Source Inverter (CSI) is a type of DC-AC Inverter that converts DC input current into AC current at a given frequency. The frequency of the output AC current depends on the frequency of the switching devices such as thyristors, transistors, etc. It is also known as a current-fed inverter (CFI) and the input current of this inverter remains constant. In an ideal CSI, the output current is independent of the load. However, the output voltage is dependent on the nature of the load. Current source inverters are ideal for various applications such as speed control drives for AC motors, synchronous motor starting, plasma generators, induction heaters, UPS units, IM motors, lagging Var compensators, and power factor correction units

A current source inverter consists of a DC current source ( which can be a battery connected to an inductor, or other switching devices such as thyristors, IGBTs, MOSFETs, etc), filters, and an AC load. It is available in single-phase and three-phase configurations. 

Before explaining the working of a CSI, it is important to understand the difference between a current source and a variable voltage source. 

An ideal variable voltage source looks like this:

A current source is derived from a voltage source by connecting a large inductor in series with the voltage source. The resulting current source looks like this:

Due to the presence of a large inductor (ideally infinite impedance), the current source provides a constant current. The output current is varied by varying the source voltage. 

Single Phase Current Source Inverter

The circuit diagram of a single-phase CSI is shown below:

Single-phase CSI

The thyristors are turned on/off by triggering their gates. There are four thyristors, T1-T4, present in the circuit. The thyristors are turned on and off diagonally in pairs(T1, T3, and T2, T4) alternatively. The thyristors are assumed to be ideal with zero turn-on/off delay. When T1 and T3 are on, the current Is flows through the circuit and the load current, i_o, equals Is with positive polarity. The equivalent circuit looks like this:

When T1 and T3 are turned on

Similarly, when T2 and T4 are turned on, the direction of the current is reversed and the value of the load current i₀ equals Is with negative polarity.  The equivalent circuit looks like this:

When T2 and T4 are turned off

The output waveform of an ideal single-phase CSI is shown below:

The amplitude of the output square wave waveform is equal to the value of the input DC current ‘Is’. The two pairs of thyristors turn on alternatively and have a reverse direction of flow of current. It can be seen from the waveform that each pair is alternatively triggered at n*T/2 (half the time period of the waveform). Between 0 < t < T/2, the T1-T3 thyristor pair is triggered while between T/2 < t < T, the T2-T4 thyristor pair is triggered.

Three-Phase Current Source Inverters

Three-phase CSI

The thyristors are again switched on in pairs diagonally. Each output line is positive for 120°, followed by 60° with no current, 120° negative, and another 60° with no conduction. The three line-current waveforms are symmetric, and phase shifted by 120° with respect to each other.

The output waveforms are controlled so as make sure that the sum of positive currents is equal to the positive DC link current and the sum of negative currents is equal to the negative DC link current. To reduce voltage spikes at the machine terminals (from V = L dI/dt) it is common to place line-line capacitors at the output of a CSI drive. The output current, i_0, then becomes the function of the input voltage and is represented by the following equation:

A CSI is used whenever a constant current output is required by any application. This configuration offers many advantages like easy operation, no need for flyback diodes, easier commutation, can handle large voltage spikes, offers four quadrant operations without extra power usage, and better short circuit prevention. They are also better at handling reactive loads and it’s easier to control peak current flowing through thyristors in CSIs. 

However, there are certain disadvantages associated with the CSI such as low load and high frequency, it has stability problems, and sluggish performance. The commutation of the thyristor in the circuit shows dependency on the load current which limits the operating frequency. It also requires a large reactor/inductor which increases the size and harmonic distortion of the circuit. The dynamic response of a CSI is slower than that of a VSI hence, it can not be used for high-speed operations. 

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