VFD control allows using a special converter to flexibly change the operating modes of the electric motor: start, stop, accelerate, brake, change the rotation speed.
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 motor speed decreases and the slip increases.
The principle of operation of the drive frequency converter
The main disadvantage of asynchronous motors is the complexity of speed control 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 IGBT transistors and microprocessor control systems allowed foreign manufacturers to create affordable devices. Mostperfect now are static frequency converters.
Angular velocity of the stator magnetic field ω0 changes in proportion to the frequency ƒ1 according to the formula:
ω0=2π׃1/p, where p is the number of pairs of poles.
The method provides smooth speed control. In this case, the sliding speed of the motor does not increase.
To get high energy performance of the engine - efficiency, power factor and overload capacity, together with the frequency, change the supply voltage according to certain dependencies:
- constant load torque – U1/ ƒ1=const;
- fan character of load moment - U1/ ƒ12=const;
- Load torque inversely proportional to speed - U1/√ ƒ1=const.
These functions are implemented using a converter that simultaneously changes the frequency and voltage on the motor stator. Electricity is saved due to regulation using the necessary technological parameter: pump pressure, fan performance, machine feed speed, etc. In this case, 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 control methods are used - scalar and vector. In the first case, they simultaneously changeamplitude and frequency of the supply voltage.
This is necessary to maintain the performance of the motor, most often a constant ratio of its maximum torque to the moment of resistance on the shaft. As a result, efficiency and power factor remain unchanged over the entire rotation range.
Vector control consists in the simultaneous change of the amplitude and phase of the current on the stator.
The frequency drive of the synchronous type motor works only at small loads, with the growth of which above the permissible values, synchronism may be broken.
Advantages of frequency drive
Frequency control has a whole range of advantages over other methods.
- Automation of the engine and production processes.
- Soft start that eliminates typical errors that occur during engine acceleration. Improving the reliability of the frequency drive and equipment by reducing overloads.
- Improve overall drive economy and performance.
- Creation of a constant speed 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 motor speed under various disturbing influences, in particular under variable loads.
- Converters are easily integrated into existing technical systems without significant alteration and stop of technological processes. The power range is large, but with their increaseprices are rising substantially.
- The ability to abandon variators, gearboxes, throttles and other control equipment or expand the range of their application. This results in 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.
Flaws
Like all inverters, chastotniki are sources of interference. They need to install filters.
The cost of brands is high. It increases significantly with an increase in the power of the devices.
Frequency adjustment for transporting liquids
In facilities where water and other liquids are pumped, flow control is mostly done with the help of gate valves and valves. Currently, a promising direction is the use of a frequency drive of a pump or a fan that sets their blades in motion.
The use of a frequency converter as an alternative to the throttle valve gives an energy saving effect of up to 75%. The valve, holding back the flow of fluid, does not perform useful work. At the same time, the loss of energy and matter for its transportation increases.
The frequency drive makes it possible to maintain a constant pressure at the consumer when the fluid flow changes. From the pressure sensor, a signal is sent to the drive, which changes the engine speed and thereby regulates itrevolutions, maintaining the set flow rate.
Pumping units are controlled by changing their performance. The power consumption of the pump is in the cubic dependence on the performance 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.
Operation of ventilation and air conditioning systems
Maximum air flow in ventilation systems is not always needed. Operating conditions may require a reduction in 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 the frequency-controlled drive, when seasonal and climatic conditions change, the release of heat, moisture, vapors and harmful gases.
Energy savings in ventilation and air conditioning systems are achieved no lower than in pumping stations, since the power consumption of shaft rotation is in the cubic dependence on revolutions.
Frequency converter device
Modern frequency drive is arranged according to the double converter scheme. It consists of a rectifier and a pulse inverter with a control system.
Afterrectifying 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 electric motor. The unit converts the rectified signal into a three-phase signal of the required frequency and amplitude. The power IGBTs in the output stages have a high switching frequency and provide a crisp, distortion-free square wave. Due to the filtering properties of the motor windings, the shape of the current curve at their output remains sinusoidal.
Methods of signal amplitude regulation
The output voltage is regulated by two methods:
- Amplitude - change in voltage value.
- Pulse width modulation is a method of converting a pulsed signal, in which its duration changes, but the frequency remains unchanged. Here the power depends on the pulse width.
The second method is used most often in connection with the development of microprocessor technology. Modern inverters are made using either GTO or IGBT turn-off transistors.
Capability and application of converters
The frequency drive has many possibilities.
- Regulate the frequency of the three-phase supply voltage from zero to 400 Hz.
- Acceleration or deceleration of the electric motor from 0.01 sec. up to 50 min. according to a given law of time (usually linear). During acceleration, not only a decrease, but also an increase of up to 150% of dynamic and starting torques is possible.
- Reversing the engine with the given modes of braking and acceleration to the desiredspeed in the other direction.
- Inverters feature configurable electronic protection against short circuits, overloads, earth leakage, and open motor power lines.
- The digital displays of the converters display data on their parameters: frequency, supply voltage, speed, current, etc.
- V/f characteristics are tuned in the converters depending on what motor loads are 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 motor, maintain a constant speed when loads change, and control the torque on the shaft. The variable frequency drive works well with the correct input of the motor passport data and after successful testing. Products of HYUNDAI, Sanyu and others are known.
The areas of application of converters are as follows:
- pumps in hot and cold water and heat supply systems;
- slurry, sand and slurry pumps of concentrators;
- transportation systems: conveyors, roller tables and other means;
- mixers, mills, crushers, extruders, dispensers, feeders;
- centrifuges;
- elevators;
- metallurgical equipment;
- drilling equipment;
- electric drives of machine tools;
- excavator and crane equipment, manipulator mechanisms.
Frequency converter manufacturers, reviews
The domestic manufacturer has already begun to produce 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.
Company "Effective systems" produces serial products and pilot batches of equipment. Products are used for domestic use, in small business and in industry. The Vesper manufacturer produces seven series of converters, among which there are multifunctional converters suitable for most industrial mechanisms.
The Danish company Danfoss is the leader in the production of chastotnikov. 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 designs from which you can assemble the necessary devices without unnecessary parts, which saves on components. Also known are the converters of the international concern ABB, used in industry and in everyday life.
Judging by the reviews, cheap domestic converters can be used to solve simple typical problems, while complex ones 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 actuators perform three main functions,related to acceleration, deceleration and motor speed. This improves the efficiency of equipment in many areas of technology.
