Frequency converters are gaining importance in many application areas of Industry 4.0. Their great merit is that they optimize the start-up and speed behavior of three-phase motors. Without frequency inverters, these types of motors could not be controlled continuously, but could only be operated at a constant speed that depends on the frequency of the supply voltage. The article explains what frequency converters are, how they work and what advantages they bring to industry.
A frequency converter (FC) is an electronic device that can be used to change and control the speed of electric motors. Unlike AC voltage and fixed speed, the frequency converter produces an AC voltage that can be varied in frequency and amplitude (the level of the output voltage).
Since the devices are capable of regulating output frequency and output voltage, they are connected upstream of the three-phase motors. This does not change the torque of the motor. However, the speed range is increased. In addition, a frequency converter can be used to achieve a direct change in the direction of rotation.
The reason for using frequency converters is as follows: When electrical machines are operated directly on the AC grid, they only have a fixed speed available – depending on the frequency fed into the public grid. However, if the application requires a variable AC voltage, frequency converters are used. These generate a variable AC voltage from a constant AC voltage, i.e. a controllable speed.
A distinction is made between voltage-controlled converters, which are suitable for applications in the lower megawatt and kilowatt range, and current-controlled devices, which are sufficient for the upper megawatt range. In industry, the electronic devices are used, for example, for pump drives and fans, cranes, and conveyor and assembly belts.
Difference between frequency converter and frequency inverter: Although the two terms sound similar, they refer to different things. For example, the mechanical speed of the frequency converter does not deviate from that of the electric motor due to the fixed coupling. Frequency converters, on the other hand, can switch between several speeds – from zero to the nominal speed of the mains frequency.
The first series-produced frequency converter came from the Danfoss company and was launched on the market in 1968. Since then, there have been continuous developments in this field of electrical engineering – which today have led to the use of fully electronic frequency converters with LCD display.
It is true that a lot has happened in the field of research and development in recent decades. There are already frequency converters that contribute to environmental protection by saving energy. However, the design of the frequency inverters has hardly changed. Thus, the devices usually consist of a rectifier, an inverter, an intermediate circuit and a control circuit. The rectifier is used to generate a DC voltage from the incoming AC voltage, which is then stabilized by the DC link. Subsequently, the inverter generates an AC voltage whose output frequency corresponds to that of the connected motor.
The control circuit in turn fulfills the task of monitoring the components and the critical parameters. In the so-called control card, all information required for the respective calculations as well as specifications are collected.
There are frequency converters with and without a DC link. While most devices have a DC link, there are also those that manage without one. These are so-called direct converters or matrix converters, which require a continuous power supply – for example by means of three-phase alternating current – and generate a variable voltage from the alternating voltage on the basis of fixed parameters. These devices are mostly used in the high-power or megawatt range, where high dynamic performance is required. Since they are regenerative, they are suitable for bridge operation to implement switching operations without large losses. This type of frequency converter is used in elevators, for example.
Frequency converters do not always work in the same way, but adapt to the type of electrical machine they support. Basically, there are two types of frequency inverters: the current-guided and the voltage-guided inverters.
While the former are suitable for applications in the megawatt range and the ratio of current to frequency remains constant, the latter are suitable for all applications and the ratio of voltage to frequency remains constant. An example: If one uses a voltage-controlled frequency converter and a motor with a voltage of 230 volts and a frequency of 50 hertz is to be operated at 25 hertz, it halves the voltage to 115 volts.
Built-in controls help to control and adjust voltage or current-frequency ratio. The result is the speed needed for the application.
Depending on the requirements, frequency converters are available in various designs and numerous optional functions. Frequency converters can be mounted on the wall or in a control cabinet (centralized and protected) or in the field (decentralized), depending on the application. Where the device is placed is decisive for the purchase.
In addition, when selecting the right frequency converter, you should pay attention to how demanding the respective application is. For simple applications, single frequency converters are usually sufficient. More demanding applications may require the purchase of an application inverter with more functions or a servo multi-axis amplifier.
The use of frequency converters is associated with higher speeds and thus greater power. Since the devices improve numerous processes and thus help to save energy, they are used in a wide range of areas – for example, building automation and other automation technology. But frequency converters are also becoming increasingly important as stand-alone solutions.
Frequency converters are used to control the speed of motors in a wide range of industrial sectors – for example, in building management and in water and wastewater technology. There they are used, for example, for pumps, ventilation systems, electric trains or servo drives. The application is possible in the low as well as in the medium and direct voltage range. In addition, the devices can be installed centrally (e.g. by wall mounting or control cabinet installation) or decentrally (e.g. as motor starters).
In which industries are frequency converters primarily used? In addition to the automotive industry, the main areas of application are heating, air conditioning and ventilation technology as well as water and wastewater management and the manufacture of industrial spare parts. However, converter technology is also used in the chemical and logistics industries, oil and gas production, as well as in the wind power, hydropower and photovoltaic sectors.
Electric motors consume a large proportion of the energy generated worldwide. Overall, the share is 48 percent; if we consider only industrial applications, the figure is as high as 65 to 75 percent. The potential for saving energy with frequency converters is correspondingly high.
Saving energy by means of frequency converters can be implemented quickly and easily. Frequency converters are already contributing to the energy efficiency of newly developed motors and can save up to ten percent of the energy normally used. Variable speed control during motor operation reduces power consumption by up to 30 percent.
Pumping and ventilation systems in particular can benefit from a large savings potential. For this reason, the German Federal Ministry of Education and Research is funding research into and development of powerful frequency converters for energy-efficient circulating pumps in heating systems. The aim of the project is to bring the converters to a size that allows them to be installed in the motor housing.
Such mini frequency converters bring numerous advantages and would be much smaller than existing converters. They could be used not only in circulating pumps but also in other electric drives. In order to realize an appropriate size, attempts are being made to accommodate control electronics and line semiconductors in one printed circuit board. Cables and external interfaces would be unnecessary in this case, which would also reduce costs.
Frequency converters also offer a wide range of possibilities for the expansion of renewable energies. The reason: In the context of the energy turnaround, optimization of the power grid is becoming necessary, because electricity must be able to flow in both directions in the so-called smart grid (intelligent power grid). This requires converters that ensure the stability and efficiency of the grid.
There are already pilot projects testing and further developing such new circuit concepts and other conditions for the power grid of the future. The concepts focus on powerful frequency converters, which on the one hand have a comparatively low number of stages and on the other hand are connected to a multilevel converter. As a result, the multilevel converter develops an extremely high power density. In addition, it has many output voltage stages.
Frequency converters are used to change and control the speed of an electric motor. No matter what the load is, the speed can be set and maintained to within 0.5 percent using a frequency converter. This stepless adjustment of speed ranges from near zero to the rated speed for the application and has several advantages. Because you get a more extensive speed range, but the torque of the motor remains unaffected, system efficiency increases and energy is saved.
The user thus has the option of setting the speed and drive power according to the respective requirements. In addition, the direction of rotation of the downstream motor changes. The fact that the drive technology can be adapted in this way is an advantage in itself. In addition, the motors or machines have to withstand less mechanical stress when a frequency converter is used. The machines also run more quietly, which also improves the working environment.
The product standard for electric power drive systems with adjustable speed is DIN EN IEC 61800-5-1. In addition, there are more and more standards, regulations and country-specific rules that are demanded by the users themselves – for example, plant and network operators.
The reason for this is that operators have special requirements with regard to power quality. This is because active power converters are increasingly being used in power grids, and these load the grid with harmonics. These arise when the electrical voltage in the network is distorted by various influences.
When three-phase motors are directly connected to AC mains, the available speed cannot be changed. The speed depends on the number of poles of the motor and the frequency of the power supply system. However, if you need a variable speed and thus a variable AC voltage for a specific application, you can use a modern frequency converter. A frequency converter is therefore a variable frequency drive and a type of motor control. Through it, an electric motor is driven, with the frequency and voltage varying In addition, frequency converters provide a control that is used to ramp the motor up and down at will during the starting and stopping process.
