Frequency converters and inverters – definitions, properties and differences
Frequency converters and inverters – definitions, properties and differencesDate of publication: 05-12-2022 Update date: 10-04-2026 🕒 5 min read
The primary task of a frequency converter is to convert current supplied from the grid into single- or multi-phase alternating current demonstrating the required parameters. This process consists in converting voltage supplied by the grid into direct current, and then to alternating voltage adapted to specific requirements. This conversion offers the possibility to adjust the voltage and frequency as required. It is important, as every industrial plant contains machines that often perform very precise tasks. In order for a process to be conducted in line with pre-set parameters, motors should be controlled in real time. To this end, frequency converters are a perfect solution making it possible to vary the torque and adjust the speed to the currently required values, while keeping the current intensity constant.
Frequency converter designs and types
Frequency converter types can be divided in line with the following criteria:
- Power supply type:
- Intended use:
- 1-phase motors;
- 3-phase motors;
- Control method:
- Scalar control – the simplest method to control frequency converters. It consists in constantly maintaining a suitable voltage (U) to frequency (f) ratio.
Such frequency converters are most often used in applications that do not require particularly precise rotational speed control and current or electromagnetic torque control. - Vector control – this solution ensures high-precision control of the motor rotational speed by varying its torque. Due to the high control precision and the resulting higher prices, such devices are used in more demanding applications.
- Square characteristics control – this type of a control system increases the squared output frequency, as the output voltage increases. This solution is therefore very similar to scalar control, but here the listed parameters increase proportionally in a linear, not quadratic, manner.
- Linear characteristics control
- Scalar control – the simplest method to control frequency converters. It consists in constantly maintaining a suitable voltage (U) to frequency (f) ratio.
Applications of frequency converters
Frequency converters are most widely used to adjust the operating parameters of equipment, whether for a wide range of household appliances or large industrial applications. Although we are often unaware of their presence (as they frequently operate in the background, protected by enclosures or concealed in control cabinets), they are responsible for the correct operation of various appliances, including such common ones as washing machines or fans.
Frequency converters are of particular importance in large manufacturing and industrial plants, where they control complex machinery, e.g. in the mining and power generation industries or in glass and steel works. Examples of applications requiring frequency converters include turbines, compressors, turbofans and pumps. All such components come with rotors whose rotational speed is closely related to their efficiency. Belt conveyors, often associated with the mining industry, are also a good example. As the drive motor speed increases, the material feeding speed grows, too. In the case of a coal mine which transports excavated material directly to a power station, the fuel transfer efficiency is closely dependent on the direct demand for power generation. Thus, a suitable frequency converter ensures the most convenient way to control such conveyors.
High-tech facilities continuously service and manufacture equipment that is necessary for the company’s operations. These include bench drills, machine tools or lathes. Without the use of frequency converters allowing the speed to be adjusted to a current activity, their operation would be imprecise, and the parts manufactured would not be suitable for complex industrial applications.
Inverters – description and functions
Inverters belong to a particularly popular group of frequency converters these days, as they are used to convert electricity generated by photovoltaic installations and to adjust voltage values. The significant demand for such systems has led to a dynamic development of such current transformers and expansion of the range of products offered in this field. In addition to converting direct current into alternating current, highly developed frequency converter systems facilitate the ongoing monitoring of the plant status and power generation, as well as enhance protection of the entire electric system against failure. Inverters monitor current voltage and frequency and, if the maximum voltage on either phase is exceeded (253 V), they disconnect the system from the grid.
Similarly to the equipment discussed earlier, inverters can be divided into 1-phase and 3-phase devices. In addition, there are also on-grid solutions working directly with the power grid, and off-grid solutions, operating fully independently from the grid (e.g. for energy storage purposes). Frequency converters can be additionally equipped with transformers.
Frequency converter selection guidelines
Taking into account the broad range of frequency converter applications, proper selection of a device is not an easy task. This is because the drive system control must be fault-free, operate with the required precision and be available at a price that is economically justified in relation to a specific application. The most important inverter features and capabilities should be specified in an operating manual in order to facilitate the selection of the best solution.
As far as inverters for PV installations are concerned, the choice is closely related to the installed power and required system performance. In order to maximise the electricity generation yield, it is recommended that a frequency converter should be oversized by up to 20% above the installed power. This will compensate for the losses resulting from excessive system heating. In addition, the frequency converter must comply with strict safety-related requirements, especially in rooftop applications for residential or public utility buildings.
During operation, frequency converters generate heat, so they should not be exposed to high temperatures. This will protect them against overheating.
Basic parameters of frequency converters
- Power supply voltage [V] ;
- Rated and maximum motor power [W];
- Rated and maximum motor speed [RPM];
- Output frequency [Hz];
- Maximum input current [A];
As expectations regarding the inverter accuracy grow, the number of parameters a user should pay attention to increases. These include, among others, setpoint ranges or functional parameters, such as the way a machine starts and stops operating or, for example, automatic restart functions. In this article, we have only provided hints regarding parameters that we recommend as worth considering, however, the final selection should always be based on the needs of the specific application.
Frequency converters – summary
There are numerous indications confirming the fact that frequency converters are essential for the operation of both household appliances and large industrial plants. They are also key elements of photovoltaic installations which are hugely popular and largely beneficial both in terms of financial savings and environmental protection. Technological developments ensure that these units offer ever-increasing performance and user protection levels.
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