What are the classifications of motor control inverters?

The rectifier circuit is reversed, one end is connected to direct current (DC), and the other end can lead to alternating current (AC). This is an inverter, a device that converts direct current into alternating current.

Most commercial, industrial, and residential loads require AC power, but AC power cannot be stored in batteries, which is important for backup power. Today, this drawback can be overcome with DC power.

The polarity of DC power does not change over time like AC power, so DC power can be stored in batteries and supercapacitors. So we can convert AC power into DC power first, and then store it in the battery, so that whenever AC power is needed to run AC appliances, DC power will be converted back to AC power to run AC appliances.

Based on the applied input source, connection method, output voltage waveform, etc., inverters are classified into the following 17 main categories.

Classification of inverters

1. Classification by input source

The input of the inverter can be a voltage source or a current source, so it is divided into voltage source inverter (VSI) and current source inverter (CSI).

Voltage Source Inverter (VSI)

When the input to the inverter is a constant DC voltage source, the inverter is called a voltage source inverter.

The input of the voltage source inverter has a rigid DC voltage source with zero impedance. In practice, the impedance of the DC voltage source is negligible. Assuming that the VSI is powered by an ideal voltage source (a very low impedance source), the AC output voltage is completely determined by the state of the switching devices in the inverter and the DC power supply applied.

Current Source Inverter (CSI)

When the input of the inverter is a constant DC current source, the inverter is called a current source inverter.

Rigid current is supplied to the CSI from a DC power source, where the DC power source has high impedance. Usually, a large inductor or closed-loop control current is used to supply the rigid current. The resulting current wave is rigid and is not affected by the load. The AC output current is completely determined by the switching devices in the inverter and the state of the DC applied power source.

2. Classification by output phase

According to the output voltage and current phase, inverters are mainly divided into two categories: single-phase inverters and three-phase inverters.

Single-phase inverter

The single-phase inverter converts the DC input into a single-phase output. The output voltage/current of the single-phase inverter has only one phase, and its nominal frequency is a nominal voltage of 50Hz or 60Hz.

Nominal voltage is defined as the voltage level at which the electrical system operates. There are different nominal voltages, namely 120V, 220V, 440V, 690V, 3.3KV, 6.6KV, 11kV, 33kV, 66kV, 132kV, 220kV, 400kV and 765kV. Low nominal voltages can be achieved directly by an inverter using an internal transformer or a buck-boost circuit, whereas for high nominal voltages, an external step-up transformer is used.

Single-phase inverters are used for low loads. Single-phase losses are more and single-phase efficiency is lower than three-phase inverters. Therefore, three-phase inverters are the first choice for high loads.

Three-phase inverter

A three-phase inverter converts direct current into a three-phase power supply. The three-phase power supply provides three evenly separated alternating currents. All three waves produced at the output have the same amplitude and frequency, but vary slightly due to the load, and each wave is 120 degrees phase shifted from each other.

Basically, a single three-phase inverter is 3 single-phase inverters where the phases of each inverter are 120 degrees apart and each single-phase inverter is connected to one of the three load terminals.

3. Classification by commutation technology

According to the commutation technology, there are two main types: line commutation and forced commutation inverters. In addition, there are auxiliary commutation inverters and complementary commutation inverters, but since they are not commonly used, we will briefly discuss the two main types here.

Line reversal

In these types of inverters, the line voltage of the AC circuit is made available through the device; when the current in the SCR experiences a zero characteristic, the device is turned off. This commutation process is called line commutation, and the inverter working on this principle is called line commutated inverter.

Forced commutation

In this type of commutation, the supply does not have a zero point. That is why some external source is required to rectify the device. This commutation process is called forced commutation and the inverter based on this process is called forced commutated inverter.

Comparison and difference between two commutation inverters

4. Classification by connection method

According to the connection method of thyristors in the circuit, it can be divided into series inverter, parallel inverter and bridge inverter. Among them, bridge inverter is further divided into half-bridge, full-bridge and three-phase bridge.

Series inverter

The series inverter consists of a pair of thyristors and an RLC (resistance, inductance, and capacitance) circuit. One thyristor is connected in parallel with the RLC circuit and one thyristor is connected in series between the DC source and the RLC circuit. This type of inverter is called a series inverter because the load is directly connected in series with the DC source with the help of the thyristors.

Series inverter is also called self-commutated inverter because the thyristors of this inverter are self-commutated by the load. Another name for this inverter is “load-commutated inverter”. This name is given because the LCR is the load that provides the commutation.

Parallel inverters

The shunt inverter consists of two thyristors, a capacitor, center-tapped transformer, and an inductor. The thyristors are used to provide a path for the current to flow, while the inductor is used to make the current source constant. The turn-on and turn-off of these thyristors are controlled by the commutation capacitor connected between them.

It is called a parallel inverter because, in operation, the capacitor is connected in parallel with the load through the transformer.

Half-bridge inverter

A half-bridge inverter requires two electronic switches to operate. The switches can be MOSFETs, IJBTs, BJTs, or thyristors. Half-bridges with thyristor and BJT switches require two additional diodes, except for purely resistive loads, while MOSFETs have a built-in body diode. In short, two switches are sufficient for purely resistive loads, while other loads (inductive and capacitive) require two additional diodes. These diodes are called feedback diodes or freewheeling diodes.

The working principle of the half-bridge inverter is the same for all switches, but the half-bridge with thyristor switches is discussed here. There are two complementary thyristors, which means that one thyristor is turned on at a time. For resistive loads, the circuit works in two modes. The switching frequency will determine the output frequency. At an output frequency of 50HZ, each thyristor is turned on for 20ms at a time.

Full-bridge inverter

The single-phase full-bridge inverter has four controlled switches that control the direction of current flow in the load. The bridge has 4 feedback diodes that feed the energy stored in the load back to the power supply. These feedback diodes only work when all thyristors are off and the load is not a purely resistive load.

For any load, only 2 thyristors are working at a time. Thyristors T1 and T2 will be turned on in one cycle, and T3 and T4 will be turned on in another cycle. In other words, when T1 and T2 are in the ON state, T3 and T4 are in the OFF state, and when T3 and T4 are in the ON state, the other two are in the OFF state. Turning on more than two thyristors at a time will cause a short circuit, generate excessive heat and burn the circuit immediately.

Three-phase bridge inverter

Industrial and other heavy loads require three-phase power. To run these heavy loads from storage devices or other DC power sources, a three-phase inverter is required. A three-phase bridge inverter can be used for this purpose.

The three-phase bridge inverter is another type of bridge inverter, which consists of 6 controlled switches and 6 diodes as shown in the figure.

5. Classification by operation mode

Based on the operation mode, inverters are divided into 3 main categories:

Independent inverter

Standalone inverters are directly connected to the load and are not interrupted by other power sources. Standalone inverters or "off-grid mode inverters" are inverters that power the load on their own without being affected by the grid or other power sources.

These inverters are called off-grid mode inverters as these inverters are not affected by the utility grid. These inverters cannot be connected to the utility grid as they do not have synchronization capability, where synchronization is the process of matching the phase and nominal frequency (50/60hz) of two AC power sources.

Grid-connected inverter

A grid tie or grid tie inverter (GTI) has two main functions. One function of a grid tie inverter is to provide AC power from a storage device (DC source) to the AC loads, while the other function of a grid tie inverter is to provide additional power to the grid.

Grid-tied inverters, also called utility-interactive inverters, grid-interconnected inverters, or grid-fed inverters, synchronize the frequency and phase of the current to the utility grid. They transfer power from a DC source to the utility grid by increasing the voltage level of the inverter voltage.

Dual-peak inverter

Bi-peak inverters can work as both grid-tied inverters and standalone inverters. These inverters can inject extra energy from renewable energy and storage devices into the grid and withdraw power from the grid when insufficient energy is produced by renewable energy. In other words, these inverters can operate as both standalone inverters and grid-tied inverters, depending on the requirements of the load. Bi-peak inverters are multifunctional, including the functions of both standalone inverters and grid-tied inverters.