Single-phase current source inverter

A current source inverter (CSI) is fed from a dc source of constant current. A large inductor $L_d$ in series with the dc supply keeps the input current $I_d$ nearly constant, so the inverter switches only steer this current. The output current is a square wave of $\pm I_d$, and the output voltage is decided by the load.

Circuit (single-phase, capacitor commutated, parallel type)

        Ld   Id
 +-----mmm-----+---------+
 |             |         |
 |            T1        T3
 Vdc           |         |
 |             A---+-C---B
 |             |   |     |
 |             +-[Load]--+
 |            T4        T2
 |             |         |
 +-------------+---------+

The commutating capacitor C is connected across the load.

Operation

1. T1, T2 ON: $I_d$ flows A → B through the load (and charges C with A positive): $i_0 = +I_d$.
2. T3, T4 fired at $T/2$: C (charged A positive) is placed across T1 and T2, reverse biasing them, so they turn off. Current $I_d$ now flows B → A: $i_0 = -I_d$. C discharges and recharges with B positive, ready for the next commutation.
3. T1, T2 fired again at $T$: C reverse biases T3, T4, and the cycle repeats.

The thyristors must withstand reverse voltage (no anti-parallel diodes are used).

Waveforms (resistive load)

 i0  +Id |________
         |        |
       0 |--------+--------- t
         |        |________
     -Id   T1,T2     T3,T4
 v0      |  ___
         | /   '--.
       0 |/--------\--------- t
         |          \   .--'
         |           '-'
         0       T/2        T

• Load current: square wave of amplitude $I_d$.
• Load voltage: exponential (the current divides between R and C after each reversal; C charges towards $I_dR$ with time constant $RC$).
• Fundamental output current: $I_{1,rms} = \dfrac{4I_d}{\sqrt2\,\pi} = 0.9I_d$.

Features of CSI

Uses: large induction motor drives, induction heating, synchronous motor starting.