What is an inverter power supply?

The principle of inverter power supply

The reverse process of converting direct current into alternating current by using a thyristor circuit is defined as inversion. For example, when an electric locomotive using thyristors is running downhill, the DC motor is used as a generator for braking, and the potential energy of the locomotive is converted into electrical energy and sent back to the AC power grid. Another example is that if a running DC motor is to be braked quickly, the motor can also be operated as a generator, and the kinetic energy of the motor can be converted into electrical energy and sent back to the power grid.

The circuit that converts direct current into alternating current is called an inverter circuit. In certain situations, the same set of thyristor converter circuits can be used for both rectification and inversion.

When the converter works in the inverter state, if the AC side of the converter is connected to the AC power supply, the DC power is inverted into AC power of the same frequency and then sent back to the grid, it is called active inversion. If the AC side of the converter is not connected to the grid, but directly connected to the load, that is, the DC power is inverted into AC power of a certain frequency or adjustable frequency to supply the load, it is called passive inversion. AC variable frequency speed regulation works on this principle. In addition to being used in DC reversible speed regulation systems, active inversion is also used in cascade speed regulation of AC winding rotor asynchronous motors and high-voltage DC transmission.

Relative concept of inverter power supply

The method of converting AC into DC is rectification; and the method of converting DC into AC is inversion.

Rectification, the full-wave rectifier circuit uses the unidirectional conduction characteristics of the diode and uses 4 diodes to connect a bridge rectifier circuit (see the figure below), so that the input end is an AC current with a sinusoidal waveform and an alternating current direction, while the output end waveform current changes to the same direction, and then the waveform is filtered out by the filter circuit to obtain DC power.

Bridge rectifier circuit

Application of Pulse Width Modulation Technology in Inverter Power Supply

The basic square wave inverter circuit is simple, but the harmonic content of the output voltage waveform is too large, that is, the THD (current harmonic distortion rate) is too large; the harmonic content of the output voltage waveform of the phase-shifted multiple superposition inverter is small, that is, the THD is small, but the circuit is more complicated. The PWM pulse width modulation inverter has both computer circuits and output voltage waveforms, so it has been widely used.

The so-called PWM pulse width modulation technology (Pulse Width Modulation, PWM) uses a reference wave (usually a sine wave, sometimes a trapezoidal wave or a sine wave or square wave injected with zero-sequence harmonics, etc.) as the modulating wave, and uses a triangular wave (sometimes a sawtooth wave) with N times the frequency of the modulating wave as the carrier wave for waveform comparison. A set of rectangular pulse sequences with equal amplitude and width proportional to the modulating wave are generated in the part where the modulating wave is greater than the carrier wave to be equivalent to the modulating wave, and the analog quantity is replaced by the switching quantity. By controlling the on/off of the inverter power switch tube, the DC power is converted into AC power. This technology is called pulse width control inverter technology. Since the up and down scale of the carrier triangular wave (or sawtooth wave) changes linearly, this technology is called pulse width control inverter technology. Since the upper and lower widths of the carrier triangle wave (or sawtooth wave) change linearly, this modulation method is also linear. When the modulating wave is a sine wave, the pulse width of the output rectangular pulse sequence changes according to the sinusoidal law. This modulation technology is usually called sinusoidal pulse width modulation (Sinusoida PWM) technology.