The frequency converter is an electronic system that allows for smooth regulation of the rotational speed of three-phase AC motors by changing the frequency of the supply voltage. Many years have passed since the first converters based on thyristors. During this time, the converters underwent design changes, but the main principle of operation remained the same.
Design changes to frequency converters are dictated primarily by the continuous increase in automation in industry. Production processes are constantly improved and efforts are made to achieve the highest possible efficiency, reliability and quality of devices. Progressive miniaturization, development of semiconductor systems and the increasing use of frequency converters mean that they can be found in virtually every industry. We are all familiar with the concept of a repeater. We can also operate it more or less, but the basic criterion determining the correct operation of the drive system is the appropriate selection of the frequency converter.
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Step no. 1 – what is the nature of the load on the electric motor?
It is very important to determine what the torque characteristics (torque - force generated on the motor shaft) look like as a function of speed for a given machine? Generally, there are two types of such loads: variable and constant-moment. It is necessary to consider which of the two most common load characteristics is in a given case. Variable torque loads are those in which the torque characteristics change as a function of speed. Such loads include pump and fan drives, for which the torque characteristic is squared with the speed (M~1/f 2 ). When the speed of centrifugal pumps and fans increases, the power drawn from the network increases by as much as the third power. Therefore, the greatest energy savings can be achieved by regulating the speed of the pump or fan.
During normal operation of pumps and fans, the speed is adjusted within the range of 50-90% of the rated speed. The load increases as the square of the speed and ranges from 30% to 80%. For this reason, pumps and fans are usually underloaded and converters can be selected according to the so-called dual rating. An example from the ANIRO offer: IS7 series converter with a power of 0.75KW can be used for a pump or fan with a power of 1.5KW - selection of a type series lower than indicated by the motor nameplate. For variable torque loads, an overload of no more than 120% In for a period of 60 seconds is usually required (underload).
Constant-moment loads are those for which the moment value remains constant over time. If the load on the motor is constant, the motor must be able to produce more torque than the loading torque. The excess torque is used to ensure proper acceleration of the motor shaft. For such a load, the converter should be able to generate 60% excess torque relative to the load, which allows for easy control during sudden load changes. The overload capacity of the drive for such loads is usually 150% In for a period of 60 seconds. Constant-torque loads include: long belt conveyors, chippers, rolling mills, mills, mixers, crushers, etc. Constant-torque loads are more demanding and often, unlike variable-torque loads, a drive of a higher type is selected. Unlike what the engine nameplate indicates.
Once the load characteristics are known, proceed to step number two - the motor nameplate.
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Step no. 2 – electrical quantities describing the engine?
A critically important factor in drive selection is the electrical characteristics of the motor being driven. Always compare the drive data with the data on the motor nameplate. The converter should be selected based on the motor's rated current (not power) or the apparent power consumed by the motor (not active power). What parameters should you pay attention to? First of all, we check the rated current of the motor and its supply voltage. Motors are usually manufactured in such a way that the user can choose from two power supply standards, depending on the internal connections of the stator (star and delta connection). Thus, motors with a power of approximately 4 KW are manufactured in such a way that when connected in a triangle, they operate at a voltage of 3x230VAC, and when connected in a star, they operate at a voltage of 3x400VAC. At this point the user must decide how he wants to connect the engine. This determines what type of drive you should use (single-phase or three-phase). After this process, we know the nature of the load and we have data on the motor voltage and its rated current.
Step no. 3 – What is/supposed to be the dynamics of the system (start and stop)?
We need to answer the question: does our drive application require very high dynamics? Do you experience frequent and sudden controlled engine braking? We also need to check whether a sudden change in speed is required under heavy load? Working with high inertia? If so, ours frequency converter should be equipped with appropriate braking or regenerative systems. In applications where braking is performed very frequently, it is advisable to use a converter with the possibility of regeneration, i.e. returning electrical energy back to the grid. During braking, the motor becomes a generator and the energy returns to the frequency converter. It is necessary to collect this energy using a braking module and a braking resistor or to transmit the energy to the electrical network. At this stage, it should be determined whether the converter should be equipped with a braking module and a resistor or whether a regenerative module is required. Usually in applications where a quick and controlled stop is required or when working with demanding inertia (fan with large blades), there is a need to use an appropriate braking support system.
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Step No. 4 – Installation site, environmental conditions?
After conducting an extensive interview regarding our application and engine, it was time to consider the installation location. This is quite important for at least several reasons: what IP rating should the device have? Is it exposed to moisture? Pollination? Vibrations? What is the ambient temperature? Is the installation location an industrial or residential environment?
In the food industry, an IP 66 rating is required due to frequent washing of equipment after production. If the device is to be installed in a control cabinet that already has the appropriate IP rating, there is no need to overpay for the increased protection rating of the housing. A simple, typical IP20 is enough. Depending on the installation location, our converter should also be equipped with an appropriate EMC filter class. For a residential environment, the restrictions are greater and the filter should have class C2. For industrial environments – class C3. It is best to purchase converters with a built-in EMC filter and a choke in the DC circuit. This guarantees lower interference generation, lower current distortion and better power factor.
Step 5 – Necessary accessories, filters, chokes, option cards, communication?
Finally, you should ask yourself about the necessary additional accessories, such as: input/output chokes , numerous expansion option cards (additional inputs/outputs, safe stop input, PTC, emergency power supply, PLC card), or communication cards (profibus, modbus, ethernet, profinet, ethercat).
Correct analysis of the drive system, engine and installation site guarantees the selection of the correct frequency converter. Described above five steps when selecting a drive allows for failure-free and long-lasting operation of our drive system. Please remember that a poorly or incorrectly selected frequency converter may do more harm than good.
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