Using thyristor circuit to convert direct current into alternating current, the reverse process corresponding to rectification is defined as inversion. For example, the electric locomotive with thyristor can brake the DC motor as a generator when it goes downhill. The locomotive's position energy can be converted into electric energy and sent back to the AC network. For example, if a running DC motor is to be braked quickly, it can also be used as a generator to convert the kinetic energy of the motor into electricity and send it back to the power grid.

The circuit that converts direct current into alternating current is called inverting circuit. Under certain circumstances, the same set of thyristor converter circuit can be used for both rectification and inversion.

When the converter works in the inverted state, if the AC side of the converter is connected to the AC power supply and the DC power is converted into the same frequency AC back to the power grid, it is called active inversion. If the AC side of the converter is not connected to the power grid, but directly connected to the load, that is to say, converting the DC current into a certain frequency or adjustable frequency AC supply load, it is called passive inversion. AC variable frequency speed regulation is based on this principle. Active inverters are used not only in DC reversible speed regulation system, but also in cascade speed regulation of AC winding rotor asynchronous motor and HVDC transmission.

The basic square wave inverters are simple in circuit, but the harmonic content of output voltage waveform is too large, and THD (current harmonic distortion rate) is too large; the harmonic content of output voltage waveform of phase-shifted multi-overlapping plus inverters is small, that is, THD is small, but the circuit is complex. PWM PWM PWM inverters are widely used because they have not only computer circuits but also output voltage waveforms.

The pulse width modulation technique uses a reference wave (usually a sinusoidal wave, sometimes a step wave or a sinusoidal or square wave injected with a zero sequence sound wave, etc.) as the modulation wave, and a triangular wave (sometimes a sawtooth wave) with N times the modulation frequency as the carrier wave for waveform comparison, where the modulation wave is larger than the loading wave. A set of equivalent modulated waves is generated, and the rectangular pulse sequence of amplitude proportional modulated waves is used to equivalent modulated waves. The analog signals are replaced by the switching signals. Through the on-off control of the switching switches of the converter power supply, the direct current is converted into alternating current. This technology is called pulse control technology. Technology. Because the upper phase of carrier triangular wave (or sawtooth wave) is linear, this technique is called coronary artery control mutation technique. Because the upper and lower phases of the carrier triangular wave (or sawtooth wave) are linear, the modulation mode is also linear. When the modulation wave is sinusoidal, the output of the rectangular coronary pulse sequence is linear. The impact varies regularly, which is also known as orthocoronary modulation (Sinusoidal PWM).