Regulation by a frequency drive allows using a special converter to flexibly change the operating modes of the electric motor: start, stop, accelerate, brake, change the speed of rotation.
A change in 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 principle of operation of the drive frequency converter
The main disadvantage of induction motors is the difficulty of controlling speed in traditional ways: by 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 advent of IGBTs and microprocessor control systems allowed foreign manufacturers to create affordable devices. The most advanced 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 pole pairs.
The method provides smooth speed control. In this case, the sliding speed of the engine does not increase.
To obtain high energy performance of the engine - efficiency, power factor and overload capacity, together with the frequency they change the supply voltage according to certain dependencies:
- constant load moment - U 1 / ƒ 1 = const;
- fan nature of the load moment - U 1 / ƒ 1 2 = const;
- load moment inversely proportional to speed - U 1 / √ ƒ 1 = const.
These functions are implemented using a converter that simultaneously changes the frequency and voltage at the motor stator. Electricity is saved due to regulation with the help of the necessary technological parameter: pump pressure, fan capacity, machine feed speed, etc. At the same time, the parameters change smoothly.
Methods of frequency control of asynchronous and synchronous motors
In a variable frequency drive based on squirrel-cage induction motors, two control methods are used - scalar and vector. In the first case, the amplitude and frequency of the supply voltage simultaneously change.
This is necessary to maintain engine performance, most often a constant ratio of its maximum moment to the moment of resistance on the shaft. As a result, the efficiency and power factor in the entire rotation range remain unchanged.
Vector control consists in simultaneously changing the amplitude and phase of the current on the stator.
The frequency drive of a synchronous type motor only works at low loads, with the growth of which is higher than the permissible values, synchronism can be broken.
Advantages of the frequency drive
Frequency control has a range of advantages over other methods.
- Automation of engine operation and production processes.
- Soft start eliminating typical errors that occur during engine acceleration. Improving the reliability of the frequency drive and equipment by reducing overloads.
- Improving operating efficiency and overall drive performance.
- Creating a constant frequency of rotation of the electric motor regardless of the nature of the load, which is important during transients. The use of feedback makes it possible to maintain a constant engine speed under various disturbing influences, in particular at variable loads.
- The converters are easily integrated into existing technical systems without significant alteration and stopping of technological processes. The range of capacities is large, but with their increase, prices increase significantly.
- The ability to abandon variators, gearboxes, throttles and other regulatory equipment or to expand the range of their application. This provides significant energy savings.
- Elimination of the harmful effects of transients on process equipment, such as water hammer or increased fluid pressure in pipelines while reducing its consumption at night.
disadvantages
Like all inverters, chastotniki are sources of interference. They need to install filters.
The cost of brands is high. It increases significantly with increasing power devices.
Fluid Frequency Adjustment
At facilities where water and other liquids are pumped, the flow rate is controlled mainly by valves and valves. Currently, a promising area is the use of a frequency drive of a pump or fan, which drives their blades.
The use of a frequency converter as an alternative to the throttle provides an energy-saving effect of up to 75%. The valve, restraining the flow of fluid, does not perform useful work. At the same time, losses of energy and substances for its transportation increase.
The frequency drive makes it possible to maintain a constant pressure with the consumer when the fluid flow rate changes. A signal is sent from the pressure sensor to the actuator, which changes the engine speed and thereby regulates its speed, maintaining a given flow rate.
Pumping units are controlled by changing their performance. The power consumption of the pump is cubic depending on the capacity or speed of rotation of the wheel. If the speed is reduced by 2 times, the pump performance will drop by 8 times. The presence of a daily schedule of water consumption allows you to determine the energy savings for this period, if you control the frequency drive. Due to it, it is possible to automate the pumping station and thereby optimize the water pressure in the networks.
The operation of ventilation and air conditioning
Maximum airflow in ventilation systems is not always needed. Functional conditions may require reduced performance. Traditionally throttling is used for this, when the wheel speed remains constant. It is more convenient to change the air flow rate due to a variable frequency drive when seasonal and climatic conditions change, heat, moisture, vapors and harmful gases are released.
Energy saving in ventilation and air conditioning systems is achieved no lower than that of pumping stations, since the consumed power of rotation of the shaft is cubic depending on the speed.
Frequency converter device
A modern frequency drive is arranged according to a double converter circuit. It consists of a rectifier and a pulse inverter with a control system.
After rectifying the mains voltage, the signal is smoothed out 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 electric motor. The unit converts the rectified signal into a three-phase signal of the desired 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 signal amplitude
The value of the output voltage is regulated by two methods:
- Amplitude - a change in the magnitude of the voltage.
- Pulse width modulation is a method of converting a pulse signal, in which its duration changes, and the frequency remains unchanged. Here, the power depends on the pulse width.
The second method is most often used in connection with the development of microprocessor technology. Modern inverters are made on the basis of lockable GTO thyristors or IGBT transistors.
Features and applications of converters
Frequency drive has many features.
- Frequency control of a three-phase supply voltage from zero to 400 Hz.
- Acceleration or braking of the electric motor from 0.01 sec. up to 50 minutes according to a given law from time to time (usually linear). During acceleration, not only a decrease, but also an increase of up to 150% of dynamic and starting moments is possible.
- Reverse engine with preset braking and acceleration to the desired speed in the other direction.
- The inverters use customizable electronic protection against short circuits, overloads, earth leaks and breaks in the motor power lines.
- The digital displays of the converters display data on their parameters: frequency, supply voltage, speed, current, etc.
- In the converters, the volt-frequency characteristics are adjusted depending on what kind of load on the motors is required. The functions of control systems based on them are provided by built-in controllers.
- For low frequencies, it is important to use vector control, which allows you to work with the full torque of the engine, maintain a constant speed when changing loads, control the torque on the shaft. The variable frequency drive works well with the correct entry of motor data and after successful testing. Known products of companies HYUNDAI, Sanyu, etc.
Scopes of converters are as follows:
- pumps in hot and cold water and heat supply systems;
- slurry, sand and slurry pumps of concentration plants;
- transportation systems: conveyors, live rolls, and other means;
- mixers, mills, crushers, extruders, batchers, feeders;
- centrifuges;
- elevators;
- metallurgical equipment;
- drilling equipment;
- electric drives of machines;
- excavator and crane equipment, manipulator mechanisms.
Frequency converter manufacturers reviews
The domestic manufacturer has already begun to manufacture products suitable for users in terms of quality and price. The advantage is the ability to quickly get the right device, as well as detailed advice on setting up.
The company "Effective Systems" produces serial production and pilot batches of equipment. Products are used for domestic use, in small business and in industry. The manufacturer "Vesper" produces seven series of converters, among which there are multifunctional, suitable for most industrial mechanisms.
The leader in the production of chastotniks is the Danish company Danfoss. Her 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 designs from which you can assemble the necessary devices without unnecessary parts, which saves on components. Converters of the international concern ABB, which are used in industry and in everyday life, are also known.
Judging by the reviews, to solve simple typical problems, you can use cheap domestic converters, and for complex ones you need a brand with much more settings.
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, breaks in the supply network. Such electric drives perform three main functions related to acceleration, braking and engine speed. This allows you to increase the efficiency of equipment in many areas of technology.