Let's look at the purpose of the frequency switch for a three-phase electric motor together. Frequency converter for working with an asynchronous motor How to choose a frequency converter for an electric motor

One of the main disadvantages of asynchronous motors is the difficulty of adjusting the rotation speed. It can be changed in three ways: changing the number of pole pairs, changing the slip and changing the frequency. Recently, to regulate the rotation speed of an asynchronous squirrel-cage motor, the current frequency is changed using frequency converters for the electric motor.

Recently, high-frequency devices have become widely used in production; many inexperienced beginners who encounter them in practice often have the question of what a frequency converter is and what it is needed for. The advantages of a frequency drive for an electric motor are:

• reduction of engine power consumption;
• improved performance: smooth start-up and rotation speed adjustment;
• eliminating possible overloads.

Smooth starting is ensured by the converter due to its reduction of the starting current, which without a frequency converter exceeds the rated current by 5–7 times.

The main parts in the converter device are the inverter and capacitors. The inverter is usually made of diode bridges. Its task is to rectify the input voltage, which can take a value of 220V or 380V depending on the number of phases, but maintain ripple. Then the capacitors smooth out and filter the rectified voltage.

Then the direct current is sent to the microcircuits and output bridge IGBT switches. Typically, a bridge IGBT switch consists of six transistors connected in a bridge circuit. Protection against reverse polarity voltage breakdown is provided by diodes. In earlier circuits, thyristors were used instead of transistors, the significant disadvantages of which were some slowness in operation and interference.

Thanks to these devices, a pulse width sequence occurs with the required frequency. At the output of the frequency converter, the voltage pulses have a rectangular shape. And after they pass through the stator winding, due to its inductance, they take on a sinusoidal form.

To understand why an inverter is needed, you need to understand that current can be constant and alternating. And if frequency converters are used when working with alternating current, then to control a DC electric motor, a DC electric drive is required. It is called an inverter and its purpose in the circuit is to control the excitation current. And it can also maintain the rotor rotation speed within the required limits and perform its braking, regardless of load changes.

When choosing a frequency generator, the lowest cost is determined by a set of minimal functions. The increase in value is proportional to their increase.

Initially converters are classified by power. No less important parameters are the overload capacity and type of execution.

The power of the frequency generator must be no less maximum power installations. For prompt repair or replacement in the event of a breakdown of the frequency drive for the electric motor, it is advisable that the service center be located in close proximity.

When choosing a converter, an important factor is its voltage. If you select a frequency generator of a certain voltage, and in the network it turns out to be lower, then it will turn off. If the network voltage remains within the permissible voltage for a long time, this will lead to its damage and the impossibility of further operation. Taking these risks into account, it is necessary to select frequency generators with a large permissible voltage range.

There are two types of converter control: vector and scalar.

With scalar control, constancy is maintained between the value of voltage and frequency at the output. This is the simplest type of frequency generators, and, as a result, cheaper.

With vector control, due to the reduction in static error, control is carried out more accurately. But the cost of an asynchronous frequency converter with this type of control is higher in comparison with scalar control.

The current frequency regulation zone must be within the required limits. For ranges with frequency adjustment greater than 10 times, it is better to choose vector control.

The number of inputs must be optimal, because if their number is too large, the price of the device for changing the frequency will be unreasonably high, and some difficulties may arise when setting it up.

It is necessary to take into account the overload capabilities of the frequency converter in terms of current and power. The current of the frequency converter should be slightly greater than the rated current of the motor. In the event of shock loads, a reserve for peak current is required, which must be at least 10% of the shock current.

Frequency calculation for an electric motor

To ensure that the frequency converter can operate reliably and maintain the specified values, it is necessary to calculate its main parameters:

• type of execution;
• power.

The converter current is calculated using the formula:

where P is the rated power of the engine, kW;

U – voltage, V

сosφ – power factor value

The correct choice of device power for changing frequency affects the efficiency of the installation. If the power of the frequency converter is underestimated, the equipment performance will be low. Long-term overloads during operation can lead to damage to the frequency converter.

If the power of the frequency converter is too high and voltage surges or overloads occur, the motor protection will not work, which will lead to its damage. U

The power of the frequency generator must be 15% greater than the rated power of the corresponding engine.

Frequency converters for motors with a power of about 3 kW are the most common due to their compactness, relatively low price, ease of installation and maintenance

There is no point in manually assembling frequency converters for motors with a power of 3 kW or more - they will be quite expensive in price and will not always provide the necessary accuracy in operation.

For 3 kW motors, frequency converters are used:

• in ventilation systems to control fan rotation speed;
• for simultaneous operation of the receiving and supply conveyors;
• for supplying raw materials with control of its volume;
• for controlling several pumps;
• to control the operation of a submersible pump;
• for adjusting the feed speed of raw materials in crushers.

Frequency converters for higher-power motors differ in the maximum output frequency, the presence of an electromagnetic compatibility (EMC) filter, and the type of control mode.

For example, a variable speed drive for a 15 kW electric motor has a lower maximum output frequency than a converter for a 3 kW motor. There is no EMC filter for this engine. The control mode is scalar only.

/ VFD-F / VFD-L / VFD-M / VFD-S / VFD-V / VFD-VE / VFD-VL /

How to choose a frequency converter (frequency drive)

When choosing a model frequency converter should be based on the specific task that the electric drive must solve:

• type and power of the connected electric motor,
• accuracy and range of speed control,
• accuracy of maintaining torque on the motor shaft.

You can also take into account the design features of the converter, such as:

• sizes,
• form,
• possibility of remote control panel, etc.

When working with standard asynchronous motor The converter should be selected with the appropriate power. If you want to high starting torque or short acceleration/deceleration times, choose a converter one step higher than the standard one.

When selecting a converter for operation with special engines(braked motors, submersible motors, retractable motors, synchronous motors, high speed, etc.) should be guided primarily by rated current of the converter, which must be greater than the rated current of the motor, as well as features of setting the converter parameters. In this case, it is advisable to consult with the supplier’s specialists.

For increase accuracy of maintaining torque and speed implemented on the motor shaft in the most advanced converters vector control, allowing you to work with total motor torque in the zero frequency range, maintain speed at variable load without feedback sensors, accurately control the torque on the motor shaft.

1. Apply frequency method in cases where the dependence of the motor load torque is known and the load practically does not change at the same frequency value, as well as the lower limit of frequency regulation is not lower than 5...10 Hz at a torque independent of frequency. When working on centrifugal pump or fan (these are typical loads with torque depending on the rotation speed) frequency control range is from 5 to 50 Hz and above. When working with two or more engines.

2. Frequency with speed feedback- for precision control (it is necessary to use an incremental encoder) with a known dependence of the torque on the rotation speed.

3. Vector– for cases when during operation the load can change at the same frequency, i.e. there is no clear relationship between the load torque and the rotation speed, and also in cases where it is necessary to obtain an extended frequency control range at rated torques, for example, 0...50 Hz for a torque of 100% or even for a short time 150-200% of Mnom. The vector method works normally if the engine's nameplate values ​​are entered correctly and its autotest has successfully passed. The vector method is implemented through complex real-time calculations performed by the converter processor based on information about the output current, frequency and voltage. The processor also uses information about passport characteristics engines that are entered by the user. The response time of the converter to a change in the output current (load torque) is 50...200 ms. The vector method allows you to minimize the reactive current of the motor when reducing the load by adequately reducing the voltage on the motor. If the load on the motor shaft increases, the converter adequately increases the voltage on the motor.

4. Vector with speed feedback– for precision regulation (it is necessary to use an incremental encoder) of speed, when during operation the load can change at the same frequency, i.e. there is no clear relationship between load torque and rotation speed, and also in cases where a maximum frequency control range is required at torques close to the nominal one.

How to choose the right frequency converter

Selection of inverter based on energy (electrical compatibility with the motor as an electrical load)

1. When operating one inverter with one motor
2. When operating one inverter with several motors

General notes on the selection and operation of the converter

1. Notes on selection
2. Notes on setting parameters

How to choose the right engine

1. Standard asynchronous motor
2. Special engines

Mechanisms for converting mechanical energy

Engine torque

VFD-M (A) - compact multifunctional;

VFD-M (B) - compact multifunctional;

VFD-EL - new compact, built-in Class B RF filter;

VFD-E - new multifunctional, with built-in PLC;

VFD-E (P) - modification with a separated cooling system;

VFD-E (T) - modification with a built-in brake key;

VFD-F - for pumps and fans;

VFD-G - for compressors and extruders;

VFD-B - general purpose;

VFD-VE - maximum functions and capabilities;

Today it is quite easy to buy a frequency converter. Often, we do this using search engines or calling already verified suppliers. It must be remembered that the correct choice of equipment is one of the most important tasks for any business facility! Taking into account all the important criteria and characteristics, you purchase a drive that will work with maximum efficiency.

1. Frequency converter power. The choice must be made taking into account the rated value of the drive motor, taking into account the overload capacity. To do this, it is necessary to know the type of overload of the controlled mechanism: the magnitude of the overload, its duration and frequency of occurrence.
2. Mains voltage. Most often we use a low-voltage three-phase power supply network of 380 V. But there are cases when electrical equipment is used at 660, 690 V, 3 kV, 6 kV and 10 kV.
3. Frequency regulation. Almost any frequency generator can be installed in cases where the speed is reduced to 50% of the nominal speed. But if it is necessary to ensure a reliable operating process at frequencies close to zero, then a special electric motor is needed with the ability to operate at such parameters. Here, it is also important to note the method of cooling the engine. In these cases, temperature protection of the electric motor is important.
4. Motor control method. Work process control is possible both through a local and remote control. Also, data transfer via various protocols that will allow the implementation of an automated control system should be taken into account here.
5. Functionality. The frequency converter must have the set of functions that is necessary to combine the optimal price and perform the assigned tasks. Here, the orientation for the operation of the frequency generator is important: control of standard components (pumps, fans) or special ones (cranes, rollers, multi-motor systems).
6. Design. The design of the frequency converter must correspond to the operating conditions. In these cases, versions for work in aggressive environments, wet, dusty, etc. are possible.

When choosing a frequency converter for the needs of an enterprise, you can know the exact model of the required equipment and order it via the Internet without any difficulty. But we, as a reliable manufacturer of frequency converters, recommend turning to professionals for the selection of frequency converters. A wide range of Triol frequency converters allows you to select a model of the required power with a wide range of functionality. The company's warehouse has standard drives, and equipment is also manufactured to suit your individual requirements. Experts in the field of electrical equipment from Triol Corporation will help you select, deliver, install and further service frequency converters.

A frequency converter is used in conjunction with an asynchronous motor, automatically converting the frequency of alternating current to the required parameters. Thus, the device controls the speed and torque of electric motors in a continuous process. Using an electrical device, you can not only fully automate production processes, but also achieve significant energy savings - up to 50%.

Modern frequency converters

The electrical equipment market is represented by frequency converters with a wide range of applications. The devices can be either low power or high voltage units. Modern equipment provides continuous process control in systems with asynchronous and synchronous motors.

Frequency control devices are widely used in almost all industries and transport. The bulk of all electricity produced in the world is used for work electric motors, and the function of controlling their operation is assigned to frequency converters.

Modern frequencies are used as controls in the following systems and equipment:

· conveyor mechanisms;

· lifting equipment (cranes, elevators);

· pumps and water purification systems;

· industrial machines;

· fans.

The correct choice of device according to predetermined criteria will ensure continuous and stable operation of the drive and reduce energy costs.

Types of frequency converters

Depending on the operating conditions, the frequency generator must have the appropriate technical characteristics and the appropriate level of protection. So, in the simplest case, a device with a degree of protection of IP 20 has a standard housing that reliably protects from moisture and dust. The chemical and mining industries require the use of devices with degrees of protection IP 54 and IP 65. The modular architecture of frequency converters allows you to customize the device to individual conditions and take advantage of additional options.

For asynchronous electric motors

Asynchronous power units occupy a leading position in terms of their use in industry and everyday life. In view of design features These drives have their drawbacks, which is what the speed control device was actually designed to eliminate. A properly selected frequency controller can reduce the starting current by almost 80% and achieve smooth control of the rotor rotation process.

For fans

The frequency converter in ventilation systems is of primary importance. Thanks to it, the change in speed and rotation frequency of the fan is carried out softly and continuously. Stable and automatic adjustment of equipment operation is configured based on preset parameters, which usually include air temperature and humidity, concentration of third-party substances, etc. There is an option to configure automatic on/off of the system or its individual components.

Frequency converters for pump (equipment)

The main working element of modern pumps is an electric motor, the operation of which is regulated by a number of mechanical devices. In the recent past, such mechanisms were shut-off and control valves (valves, gate valves, closures). In modern pumping systems, fluid flow is controlled using frequency converters. Today, frequency converters can work in tandem with a pump in the same way as electric motors, which in turn can extend the life of pumping equipment several times.

Frequency converter capabilities

The functionality of modern frequency generators has been significantly expanded and makes it possible to automate the operation of electric drives even in the most difficult conditions.

Operation at unstable voltage

Not all Electricity of the net can provide connected equipment with stable power. Ideally, modern converters correctly perform their functions in the supply voltage range of 380-460 V, the permissible deviation is 10%. The frequency drive models presented on the page allow you to maintain the functionality of the electric motor through automatic restart after a short-term power outage (drain) with a smooth change in the speed and torque of the motor.

Operating at resonant frequencies

The natural resonant frequency of some mechanisms can cause unacceptable vibrations, which often cause failure of the control system. Thanks to the function of eliminating unacceptable frequencies, the operation of the frequency generator becomes safe, and the mechanism itself is protected from possible damage.

Network exchange

For joint operation of the electric motor and the system automatic control Various data transfer protocols are used. The most widely used communication protocol is Modbus with the RS-485 interface, however, depending on the equipment used, the question of using one or another protocol is specified for each specific case.

The optimal choice of a frequency converter comes down to matching its functionality with the technical characteristics of the electric motor. The website of the ENERGOPUSK company contains a huge range of electronic control devices, where you can stop at optimal choice electrical device based on the economic feasibility of purchase and operation.

Frequency converter power

Power is one of the most basic parameters of an electric drive. When choosing a frequency generator, first of all, you should determine its load capacity. In accordance with the available rated power of the engine, a variable speed drive designed for the same power is selected. And such a choice will be correct provided that the load on the shaft does not change dynamically and the current does not significantly exceed the nominal set value, both for a given motor and a frequency distribution device. Therefore, it would be more correct to make a choice according to maximum value current consumed from the emergency, taking into account the overload capacity of the latter. Typically, the overload capacity is specified as a percentage of the rated current together with the maximum permissible time of the overload before activation of direct protection. Thus, for the right choice you need to know the nature of the overloads of your particular mechanism, in particular: what is the level of overloads, what is their duration and how often they appear.

Mains voltage for frequency converter

The issue of supply voltage is also important. The most common case is power supply from a three-phase industrial network of 380V, but options are possible when the drive is designed to operate from a single-phase network of 220-240V. As a rule, the latter is limited to a number of powers up to 3.7 kW. There are also options for high-voltage drives that make it possible to control more powerful motors, with powers already measured in MW, at relatively lower current values.

Each of the options is applicable for various types of solutions, and depends both on the power supply capabilities and on a number of possibilities determined by the use of the corresponding drive.

Frequency converter regulation range

If the speed does not fall below 10% of the nominal, then almost any frequency converter will do, but if you need to reduce the speed further, while ensuring the rated torque on the shaft, you need to make sure that the motor frequency converter is able to provide operation at frequencies close to zero. In addition, another issue that needs to be addressed is related to the speed control range - electric motor cooling. Typically, an asynchronous electric motor (with self-ventilation) is cooled by a fan mounted on its shaft, so when the speed decreases, the cooling efficiency drops sharply.

Some electronic devices for changing frequency are equipped with a thermal control function using feedback through a temperature sensor installed on the engine itself. There are other options for solving this issue, but without using this device.

The need for braking mode of the frequency converter

Coasting braking (inertial braking) is similar to disconnecting the motor from the power supply, but the process can take a long time. Especially if these are highly inertial mechanisms. Using the frequency distribution of the electric pulse, it is possible to stop or brake with a transition to a lower operating speed in a shorter period of time. Several options are possible:

• supply electricity to the network (regenerative braking mode);
• stop by applying a lower frequency voltage to the stator windings or DC voltage, then the excess stored kinetic energy will be released in the form of heat through radiators that convert electricity and the engine itself (DC braking mode);
• stop or brake using a brake chopper and a set of brake resistors

The feasibility of using a particular method is considered mainly from the point of view of economic benefits. So, regeneration into the network is more profitable in terms of energy savings, a drive using a braking resistor is cheaper technical solution, engine braking does not require additional costs at all, but in turn is possible only at low power.

Frequency converters as a way to control an electric motor

Some mechanisms can be controlled from a master signal under conditions of a smooth change in speed, and in some cases operation at fixed speeds is required. Moreover, in both cases it is possible to control both from the emergency control panel and from the terminals of the control circuits of the electronic device, smoothly lowering or increasing the current, buttons, switches and potentiometers.

When implementing the latter option, you must ensure that there are a sufficient number of required inputs. In the case of using an external control device (controller, logical relay, etc.), it is necessary to ensure that the technical parameters are consistent. Typically these are current or voltage signals with the ranges 0%u202620mA, 4%u202620mA and 0%u202610V respectively. If the electric drive is controlled via a network, then the presence of an appropriate interface and support for the appropriate data transfer protocol are required.

The motor can be controlled automatically; this requires a PID controller and the ability to organize feedback from the sensor of the controlled parameter

Indication of electric drive parameters

Basically, any frequency changer has a panel with a display and the necessary controls for commissioning and control. During operation, the same display can be used to display any parameters.

Displays may differ in the number of lines, and therefore in the information content, and in the type of display itself (seven-segment indicator or liquid crystal). If it is impossible to observe the parameters on the display of the electric drive itself during operation, using analog and discrete (relay, transistor) outputs, you can display the necessary information on the remote control.

In addition to displaying parameters (states “operation”, “emergency”, “braking mode”, load current value, engine speed, frequency and voltage of the supply network, etc.), some devices have the ability to generate control signals using the same analog and discrete outputs, thus most implement more complex control systems.

Protection functions

In addition to the control functions on electronic device Frequency changes are usually assigned protection functions. As a rule, the main set is:

• current limitation at start-up, during continuous operation, during stop and short circuit;
• overvoltage and undervoltage protection;
• engine temperature control;
• Radiator overheating protection;
• output IGBT protection.

Mounting and installation of frequency converter

An important point is the choice of the intended installation location of the frequency converter, and hence the conditions of its operation:

• current limitation at start, during continuous operation, during stop and short circuit
• Operating temperature range
• humidity
• altitude
• vibrations
• degree of protection (IP)

Compactness in some cases is a decisive factor at the selection stage. What are the dimensions of the installed drive and installation method? Is it possible to move the radiators of the power section of the emergency control unit to the rear part, providing sufficient ventilation with the smaller dimensions of the cabinet?

Conditions information environment It is an integral part technical characteristics, when choosing a frequency converter, and failure to comply with them during installation can lead to its failure. During the installation process, many questions arise, but these are some of the first ones you encounter.

Functionality

Modern electric drives have many functional capabilities. We list the most common ones in order of importance.

Working with unstable power supply.

This is a relevant parameter especially when used in Russia. Hence the question: “what is the permissible range of the supply voltage?” A good supply voltage range for modern frequency generators is 380-460 V with a deviation of ±10%. It is necessary to clarify what the actions of the frequency converter are when there is a drawdown or complete shutdown food for a short or very short time?

Is it possible to maintain operability with a proportional change in speed, engine torque, automatic restart after power restoration, pick up the speed of a running engine when restarting after a power failure, etc. If the available functionality ensures an acceptable mode of operation of the mechanism while maintaining its operational state, then we can assume that the issue of unstable power supply has been removed for you, otherwise you should either resolve the issue with the power supply or think about choosing other equipment.

Elimination of operation at resonant frequencies.

Some mechanisms have their own resonant frequencies during operation at which unacceptable vibrations are observed, which can lead to equipment failure. In such cases, the function of eliminating unacceptable frequencies in the converter will protect the mechanism from its premature failure.

Network exchange.

Usually it is necessary either to include the drive in an automatic control system, or to provide for the prospect of such use of frequency changing systems electric current in future. To do this, you need to understand the communication standard and protocol.

Currently, there is a wide variety of them, which makes it possible to make work in the self-propelled gun mode most optimal. They may differ in distance, number of connected objects and noise immunity.

The most common option %u2013 is the RS-485 interface and the Modbus data transfer protocol, but to coordinate operation as part of an automatic control system, this issue should be clarified in more detail with the supplier or manufacturer.

Automatic setup.

Today, the choice of electric drives is quite large, but there are still the simplest models in which no adjustment is made to the parameters of the motor, or rather its windings. Later models require a number of additional reference data to be entered.

Frequency converters have the ability to carry out a so-called identification start (auto-tuning mode), in which, even before starting, or already with a rotating motor, the parameters of the windings are determined automatically. If a precision control system is supposed to be implemented on the selected drive, then this issue is especially relevant.

Emergency management principle .

The most common variable frequency drive based on squirrel cage induction motors uses scalar and vector control.

Scalar control is based on the principle of a constant ratio of the output voltage of a frequency converter to its output frequency. That is, when the frequency changes, the voltage amplitude changes in such a way that the ratio of the maximum torque of the electric motor to the current load torque remains unchanged. This ratio is called the overload capacity of the electric motor.

An important advantage of the scalar method is the ability to simultaneously manage a group electric machine. Scalar control is applicable for most practical cases of using a frequency electric drive with a speed control range of up to 1:40 .

Vector control, in turn, can significantly improve the accuracy of maintaining the output frequency, the accuracy of speed control, and the accuracy of maintaining torque. Another distinctive feature of vector control is the ability to control the torque on the motor shaft when it operates at frequencies close to zero. Possibility of using several sets of parameters. The latest generation of converters has functionality select different combinations of settings for several operating modes of the same electromechanical converter or for several with different technical parameters.

The number of functions described above is a small part of their huge variety, already numbering in the hundreds in the latest generation equipment. The necessary ones must be selected based on the requirements that dictate their intended areas of application. It is unlikely that the selection stage of a frequency converter is limited to solving the above issues, but these are the ones that have to be faced at the initial stage.

The choice of a frequency generator as a high-tech equipment is not simple in itself and ultimately comes down to the economic feasibility of acquisition and use. Hence, you should not overestimate the requirements and thereby overpay for unused options, and at the same time refuse the necessary ones, in the hope of making the mechanism, drive and system as a whole operational.

One of the main disadvantages of asynchronous motors is the difficulty of adjusting the rotation speed. It can be changed in three ways: changing the number of pole pairs, changing the slip and changing the frequency. Recently, to regulate the rotation speed of an asynchronous squirrel-cage motor, the current frequency is changed using frequency converters for the electric motor.

Recently, high-frequency devices have become widely used in production; many inexperienced beginners who encounter them in practice often have the question of what a frequency converter is and what it is needed for. The advantages of a frequency drive for an electric motor are:

• reduction of engine power consumption;
• improved performance: smooth start-up and rotation speed adjustment;
• eliminating possible overloads.

Smooth starting is ensured by the converter due to its reduction of the starting current, which without a frequency converter exceeds the rated current by 5–7 times.

The main parts in the converter device are the inverter and capacitors. The inverter is usually made of diode bridges. Its task is to rectify the input voltage, which can take a value of 220V or 380V depending on the number of phases, but maintain ripple. Then the capacitors smooth out and filter the rectified voltage.

Then the direct current is sent to the microcircuits and output bridge IGBT switches. Typically, a bridge IGBT switch consists of six transistors connected in a bridge circuit. Protection against reverse polarity voltage breakdown is provided by diodes. In earlier circuits, thyristors were used instead of transistors, the significant disadvantages of which were some slowness in operation and interference.

Thanks to these devices, a pulse width sequence occurs with the required frequency. At the output of the frequency converter, the voltage pulses have a rectangular shape. And after they pass through the stator winding, due to its inductance, they take on a sinusoidal form.

To understand why an inverter is needed, you need to understand that current can be constant and alternating. And if frequency converters are used when working with alternating current, then a direct current electric drive is required to control a direct current electric motor. It is called an inverter and its purpose in the circuit is to control the excitation current. And it can also maintain the rotor rotation speed within the required limits and perform its braking, regardless of load changes.

When choosing a frequency generator, the lowest cost is determined by a set of minimal functions. The increase in value is proportional to their increase.

Initially converters are classified by power. No less important parameters are the overload capacity and type of execution.

The power of the frequency generator must be no less than the maximum power of the installation. For prompt repair or replacement in the event of a breakdown of the frequency drive for the electric motor, it is advisable that the service center be located in close proximity.

When choosing a converter, an important factor is its voltage. If you select a frequency generator of a certain voltage, and in the network it turns out to be lower, then it will turn off. If the network voltage remains within the permissible voltage for a long time, this will lead to its damage and the impossibility of further operation. Taking these risks into account, it is necessary to select frequency generators with a large permissible voltage range.

There are two types of converter control: vector and scalar.

With scalar control, constancy is maintained between the value of voltage and frequency at the output. This is the simplest type of frequency generators, and, as a result, cheaper.

With vector control, due to the reduction in static error, control is carried out more accurately. But the cost of an asynchronous frequency converter with this type of control is higher in comparison with scalar control.

The current frequency regulation zone must be within the required limits. For ranges with frequency adjustment greater than 10 times, it is better to choose vector control.

The number of inputs must be optimal, because if their number is too large, the price of the device for changing the frequency will be unreasonably high, and some difficulties may arise when setting it up.

It is necessary to take into account the overload capabilities of the frequency converter in terms of current and power. The current of the frequency converter should be slightly greater than the rated current of the motor. In the event of shock loads, a reserve for peak current is required, which must be at least 10% of the shock current.

Frequency calculation for an electric motor

To ensure that the frequency converter can operate reliably and maintain the specified values, it is necessary to calculate its main parameters:

• type of execution;
• power.

The converter current is calculated using the formula:

where P is the rated power of the engine, kW;

U – voltage, V

сosφ – power factor value

The correct choice of device power for changing frequency affects the efficiency of the installation. If the power of the frequency converter is underestimated, the equipment performance will be low. Long-term overloads during operation can lead to damage to the frequency converter.

If the power of the frequency converter is too high and voltage surges or overloads occur, the motor protection will not work, which will lead to its damage. U

The power of the frequency generator must be 15% greater than the rated power of the corresponding engine.

Frequency converters for motors with a power of about 3 kW are the most common due to their compactness, relatively low price, ease of installation and maintenance

There is no point in manually assembling frequency converters for motors with a power of 3 kW or more - they will be quite expensive in price and will not always provide the necessary accuracy in operation.

For 3 kW motors, frequency converters are used:

• in ventilation systems to control fan rotation speed;
• for simultaneous operation of the receiving and supply conveyors;
• for supplying raw materials with control of its volume;
• for controlling several pumps;
• to control the operation of a submersible pump;
• for adjusting the feed speed of raw materials in crushers.

Frequency converters for higher-power motors differ in the maximum output frequency, the presence of an electromagnetic compatibility (EMC) filter, and the type of control mode.

For example, a variable speed drive for a 15 kW electric motor has a lower maximum output frequency than a converter for a 3 kW motor. There is no EMC filter for this engine. The control mode is scalar only.