What are the classifications of motor control inverters?

The function of the bi-peak inverter changes with the load. If there is a problem with the grid or when the power of renewable energy is sufficient to meet the load, its function changes to a stand-alone inverter (it becomes a stand-alone inverter). In this case, the transfer switch disconnects the inverter from the grid.

Once the renewable energy source starts to generate additional energy, the operation mode changes from stand-alone mode to grid-connected mode. The inverter synchronizes its phase and frequency with the inverter and starts injecting additional energy into the grid.

6. Classification by output waveform

An ideal inverter is one that converts a DC signal into a pure sinusoidal AC output. The problem with actual inverters is that their output signals are not pure sinusoidal. Inverters are divided into three categories based on the output waveform:

Square wave inverter

These are the simplest inverters that convert DC to AC, but the output waveform is not the required pure sine wave. These inverters have a square wave at the output. In other words, these inverters convert the DC input to AC in the form of a square wave. At the same time, square wave inverters are also cheaper.

The simplest structure of these inverters can be an H-bridge inverter. A simpler version can be achieved using a SPDT (Single Push Double Throw) switch before the transformer as shown. This transformer will also help achieve any desired output voltage level.

The working operation of the given model is extremely simple. Simply opening and closing the switch will simultaneously change the current at the output. In other words, switching the SPDT at the desired frequency will produce an AC square wave at the output of a typical inverter (i.e. a center-tapped transformer). The harmonic distortion of a typical sine wave is about 45%, which can be further reduced by using filters that will filter out some of the harmonics.

Quasi-sine wave inverter

Quasi-sine wave inverters or simply modified sine wave inverters with stepped sine waves. In other words, the output signal of these inverters increases step by step with positive polarity. After reaching the positive peak, the output signal starts to decrease step by step until it reaches the negative peak, as shown in the figure.

The structure of the quasi-sine wave inverter is much simpler than that of the pure sine wave inverter, but it is more complicated than that of the pure square wave inverter.

Although the final output waveform of these inverters is not a pure sine wave, the harmonic distortion of the output is still reduced to 24%. Filtering will further reduce the distortion, but the amount of distortion is still large. For this reason, these inverters are not the first choice for driving many loads including electronic circuits.

Quasi-sine waves may permanently damage electronic devices with timers in the circuit. If connected to a quasi-sine wave inverter, all electrical appliances with motors will not work as efficiently as when connected to a pure sine wave inverter. In addition, the rapid transition of the waveform may cause noise. Due to these problems, the application of quasi-sine wave inverters is limited.

Pure sine wave inverter

A pure sine wave inverter converts DC into almost pure sinusoidal AC. The output waveform of a pure sine wave inverter is still not an ideal sine wave, but it is much smoother than square wave and quasi-sine wave inverters.

The output waveform of a pure sine wave inverter has extremely low harmonics. Harmonics are sine waves that have odd multiples of the fundamental frequency with varying amplitudes. Harmonics are highly undesirable as they can cause serious problems with a variety of electrical appliances. These harmonics can be further reduced by using various PWM techniques and then passing the output signal through a low pass filter.

The construction and operation of a pure sine wave inverter is much more complicated than that of square wave and modified square wave inverters.

These inverters are preferred over the previous two types because most electrical devices require pure sine waves to operate better. As mentioned earlier, square wave or quasi-sine wave inverters can damage electrical appliances, especially those with motors. Therefore, for practical purposes, pure sine inverters are used.

7. Classification by output level quantity

The number of output levels of any inverter can be at least two or more. According to the number of output levels, inverters are divided into two categories: two-level inverters and multi-level inverters.

Two-level inverter

A two-level inverter has two output levels. The output voltage alternates between positive and negative and alternates at a fundamental frequency (50Hz or 60Hz).

Some so called "two level inverters" have three levels in their output waveform. The reason a three level inverter is put in this category is because 1 of the levels is zero voltage. Actually zero is the third level but it is still classified as a two level inverter.

The two-level inverter circuit consists of a source and some switches that control the current or voltage. Due to the switching losses and device ratings, the two-level inverter is limited in high-frequency operation in high-voltage applications. However, the rating of the switch can be increased by series and parallel combinations. The set of switches that provide the positive half cycle in the two-level inverter is called the positive group of switches, while the other set of switches that provide the negative half cycle is called the negative group.

Two-level inverters are not preferred for the following reasons. The inverter needs to operate with the minimum number of switches and the minimum power supply to convert power in small voltage steps. Smaller voltage steps will provide high-quality waveforms. In addition, it can also reduce voltage (dv/dt) stress on the load and electromagnetic compatibility issues. Therefore, multi-level inverters are a more practical choice.

Multi-Level Inverter (MLI)

The multilevel inverter converts the DC signal into a multilevel step waveform. The output waveform of the multilevel inverter is not directly positive and negative alternating, but multi-level alternating. Since the smoothness of the waveform is proportional to the number of voltage levels. Therefore, the multilevel inverter produces a smoother waveform. As mentioned earlier, this characteristic makes it useful for practical applications.

Comparison and difference between two-level inverter and multi-level inverter

Conclusion:

The space is limited, but in fact, there are many classifications of inverters. For example, multi-level inverters can be divided into flying capacitor inverters (FCMI), diode clamped inverters (DCMI), and cascaded H-bridge inverters.

From the perspective of practical application, three-phase inverters are suitable for high-load applications, pure sine inverters can better protect electrical appliances, and multi-level inverters are a more practical choice.