Frequency drive: description and reviews

Adjustment by frequency drive allows using a special converter to flexibly change the operating modes of the electric motor: start, stop, accelerate, decelerate, change the speed of rotation.

Changing the frequency of the supply voltage leads to a change in the angular velocity of the stator magnetic field. When the frequency decreases, the engine speed decreases, and the slip increases.

The operating principle of the drive frequency converter

The main disadvantage of asynchronous motors is the difficulty of speed control by traditional methods: changing the supply voltage and introducing additional resistances into the winding circuit. More perfect is the frequency drive of the electric motor. Until recently, converters were expensive, but the emergence of IGBT-transistors and microprocessor control systems allowed foreign manufacturers to create affordable devices. The most perfect now are static frequency converters.

The angular velocity of the stator magnetic field ω ₀ varies in proportion to the frequency ƒ ₁ in accordance with the formula:

Ω ₀ = 2π × ƒ ₁ / p,

Where p is the number of pairs of poles.

The method provides a smooth speed control. The speed of the motor slip does not increase.

To obtain high engine power parameters - EFFICIENCY, power factor and overload capacity, together with the frequency change the supply voltage for certain dependencies:

• Constant load moment - U ₁ / ƒ ₁ = const;
• Fan character of the load moment - U ₁ / ƒ ₁ ² = const;
• The load moment inversely proportional to the velocity is U ₁ / √ ƒ ₁ = const.

These functions are implemented using a converter that simultaneously changes the frequency and voltage on the stator of the motor. Electricity is saved through regulation with the help of the required process variable: pump pressure, fan output, machine feed rate, etc. The parameters change smoothly.

Methods of frequency control of asynchronous and synchronous electric motors

In a frequency-controlled drive based on asynchronous motors with a squirrel-cage rotor, two methods of control are used: scalar and vector. In the first case, the amplitude and frequency of the supply voltage change simultaneously.

This is necessary to maintain the performance of the engine, most often - a constant ratio of its maximum torque to the moment of resistance on the shaft. As a result, the efficiency and power factor remain constant over the entire rotation range.

Vector control is the simultaneous change in the amplitude and phase of the current on the stator.

The frequency drive of the synchronous motor works only for small loads, the growth of which is higher than the allowable values, synchronism may be violated.

Advantages of the frequency drive

Frequency control has a whole range of advantages over other methods.

1. Automation of the engine and production processes.
2. Smooth start, eliminating the typical errors that occur during the acceleration of the engine. Increase the reliability of the drive frequency and equipment due to reduced overload.
3. Increase the economy of operation and drive performance in general.
4. Creating a constant speed of the motor regardless of the nature of the load, which is important for transient processes. The use of feedback makes it possible to maintain a constant engine speed under various disturbances, in particular under variable loads.
5. Converters are easily integrated into existing technical systems without significant alteration and shutdown of technological processes. The range of capacities is great, but with their increase prices are significantly increasing.
6. Ability to abandon variators, reducers, inductors and other control equipment, or expand the range of their application. This results in significant energy savings.
7. Elimination of the harmful effect of transients on process equipment, such as hydraulic shocks or increased pressure of liquid in pipelines with a decrease in its consumption at night.

disadvantages

Like all inverters, the frequencies are sources of interference. They need to install filters.

The cost of brands is high. It increases significantly with increasing capacity of the apparatus.

Frequency control for the transport of liquids

At sites where water and other liquids are pumped, flow control is mostly done with valves and valves. Currently, a promising direction is the use of the frequency drive of a pump or a fan that drives their blades.

The use of a frequency converter as an alternative to a throttle provides an energy-saving effect of up to 75%. The valve, restraining the flow of liquid, does not perform useful work. At the same time, energy and substance losses on transportation are increasing.

The frequency drive makes it possible to maintain a constant pressure at the consumer when the flow rate of the fluid changes. A pressure signal is applied from the pressure sensor to the drive, which changes the engine speed and thereby regulates its speed, maintaining the set flow rate.

Managing pumping units is done by changing their productivity. The power consumption at the pump is in cubic dependence on the output or wheel speed. If the speed is reduced by a factor of 2, the pump output will drop 8 times. The availability of a daily water consumption schedule allows you to determine the energy savings for this period, if you manage the frequency drive. Due to this, it is possible to automate the pumping station and thereby optimize the water pressure in the networks.

Operation of ventilation and air conditioning systems

Maximum air flow in ventilation systems is not always necessary. Operating conditions may require performance degradation. Traditionally, throttling is used for this, when the wheel speed remains constant. It is more convenient to change the airflow due to the frequency-controlled drive, when seasonal and climatic conditions, heat, moisture, vapors and harmful gases change.

Electricity savings in ventilation and air conditioning systems are achieved no lower than for pumping stations, since the power consumption of shaft rotation is in cubic dependence on the revolutions.

Frequency converter device

The modern frequency drive is designed according to the scheme of a double converter. It consists of a rectifier and a pulse inverter with a control system.

After rectifying the mains voltage, the signal is smoothed by a filter and fed to an inverter with six transistor switches, where each of them is connected to the stator windings of an asynchronous motor. The unit converts the rectified signal into three-phase required frequency and amplitude. Power IGBT-transistors on the output stages have a high switching frequency and provide a clear rectangular signal without distortion. Due to the filtering properties of the motor windings, the shape of the current curve at their output remains sinusoidal.

Methods for adjusting the signal amplitude

The output voltage is regulated by two methods:

1. Amplitude - change in the value of voltage.
2. Pulse width modulation is a method of converting a pulse signal, at which its duration changes, and the frequency remains unchanged. Here the power depends on the width of the pulse.

The second method is most often used in connection with the development of microprocessor technology. Modern inverters are manufactured on the basis of lockable GTO-thyristors or IGBT-transistors.

Possibilities and application of converters

The frequency drive has many possibilities.

1. Frequency regulation of three-phase supply voltage from zero to 400 Hz.
2. Acceleration or braking of the motor from 0.01 sec. Up to 50 min. According to a given law from time (usually linear). When overclocking, it is possible not only to decrease, but also to increase up to 150% of the dynamic and starting moments.
3. Reverse the engine with the specified modes of braking and acceleration to the desired speed in the other direction.
4. The converters use a configurable electronic protection against short circuits, overloads, earth leaks and motor power line interruptions.
5. Digital displays of the converters show data on their parameters: frequency, voltage, speed, current, etc.
6. In the converters, the volt-frequency characteristics are adjusted depending on what loads are required for the motors. Functions of control systems based on them are provided by built-in controllers.
7. For low frequencies it is important to use vector control, which allows to work with the full moment of the engine, maintain a constant speed with changing loads, monitor the moment on the shaft. The frequency-controlled drive works well with the correct introduction of the engine's passport data and after the successful testing. The products of the companies HYUNDAI, Sanyu and others are known.

Areas of application of the converters are as follows:

• Pumps in hot and cold water and heat supply systems;
• Slurry, sand and pulp pumps of concentrating plants;
• Transportation systems: conveyors, roller tables and other means;
• Mixers, mills, crushers, extruders, batchers, feeders;
• Centrifuges;
• Elevators;
• Metallurgical equipment;
• drilling equipment;
• Electric drives of machine tools;
• Excavator and crane equipment, manipulator mechanisms.

Manufacturers of frequency converters, reviews

The domestic manufacturer has already started to manufacture products that are suitable for users in terms of quality and price. The advantage is the ability to quickly get the right machine, as well as detailed advice on customization.

The company "Effective Systems" produces serial products and experimental batches of equipment. Products are used for domestic use, in small businesses and in industry. Manufacturer "Vesper" produces seven series of converters, among which there are multifunction, suitable for most industrial mechanisms.

The leader in the production of chastotniki is the Danish company Danfoss. Its products are used in ventilation, air conditioning, water supply and heating systems. The Finnish company Vacon, which is part of the Danish company, produces modular structures from which it is possible to build the necessary devices without unnecessary details, which saves on components. Transformers of the international concern ABB, used in industry and in everyday life, are also known.

Judging by the feedback, cheap domestic converters can be used to solve simple typical problems, and for complex ones a brand with much more settings is needed.

Conclusion

The frequency drive controls the electric motor by changing the frequency and amplitude of the supply voltage, while protecting it from malfunctions: overloads, short circuits, interruptions in the supply network. Such electric drives perform three main functions related to acceleration, braking and engine speed. This allows you to improve the efficiency of equipment in many areas of technology.