Single-phase current source inverter with AC motor

A single-phase current source inverter (CSI) feeding an ac motor is usually the auto-sequential commutated inverter (ASCI). A large dc-link inductor makes the source a constant current $I_d$; the thyristors switch this current into the motor alternately in each direction, so the motor current is a quasi-square wave and the motor voltage is nearly sinusoidal (set by the motor's back EMF).

Circuit

        Ld   Id
 +-----mmm----+------------+
 |            |            |
 |           T1           T3
 |            +----C1------+
 Vdc         D1           D3
 |            A--[Motor]---B
 |           D4           D2
 |            +----C2------+
 |           T4           T2
 |            |            |
 +------------+------------+

The capacitors C1, C2 are for commutation; the series diodes D1–D4 isolate the capacitors from the motor so they do not discharge through it.

Modes of operation (commutation from T1, T2 to T3, T4)

1. Mode 1 — normal conduction: T1, D1, motor, D2, T2 carry $I_d$ ($i_0 = +I_d$). C1 and C2 are charged with the polarity needed to turn off T1, T2.
2. Mode 2 — thyristor turn-off: T3 and T4 are fired. The charged capacitors appear across T1 and T2 and reverse bias them, so they turn off at once. $I_d$ now flows through T3, C1, D1, the motor, D2, C2, T4. The motor current is still $+I_d$, and the capacitors charge linearly at constant current, reversing their voltage. The circuit turn-off time is the time the capacitor voltage takes to fall to zero.
3. Mode 3 — current transfer (overlap): when the capacitor voltage exceeds the motor voltage, D3 and D4 become forward biased. The current shifts from D1, D2 to D3, D4 through a resonance between C and the motor leakage inductance. Motor current falls from $+I_d$ to $-I_d$.
4. Mode 4 — reverse conduction: D1, D2 turn off; T3, D3, motor, D4, T4 carry $-I_d$. The capacitors hold the opposite charge, ready to commutate T3, T4 in the next half cycle.

Waveforms

 i0  +Id |_______
         |       \
       0 |--------\-------/--- t
         |         \_____/
     -Id     (finite slope at
              commutation)
 v0      |   _
         | /  \   spike   _
       0 |/----\--------/--\-- t
         |      \_____/
         ~ sinusoidal + spikes

• Motor current: quasi-square wave $\pm I_d$ with sloped edges during commutation.
• Motor voltage: close to the sinusoidal back EMF, with voltage spikes $L\,di/dt$ at each commutation (leakage inductance).

Features

• Inherent short-circuit and overload protection (current is fixed by the link).
• Regeneration is easy: reversing the dc-link voltage (controlled rectifier) returns energy to the ac supply, so four-quadrant drives are simple.
• No feedback diodes; simple, robust thyristor commutation.
• Drawbacks: torque pulsation at low speed due to square current; voltage spikes; needs a large dc inductor and a controlled rectifier; slower response.
• Used for large induction and synchronous motor drives (fans, pumps, compressors).