What is low voltage inverters, Application of low voltage inverters, and how to select low voltage inverters?

low voltage inverters
low voltage inverters

What is low voltage inverters, Application of low voltage inverters, and how to select low voltage inverters?

Low-voltage VFD, with adjustable output frequency AC motor drives with voltage levels below 690V, are classified as low-voltage VFD.

Ccontrol Method

As the technology of low-voltage inverters continues to mature, the application of low-voltage inverters determines its different classifications. From the technical point of view alone, the control method of the low voltage frequency converter also shows its technical school to a certain extent. Here we analyze the following types of control:

Sinusoidal Pulse Width Modulation (SPWM) Its characteristic is that the control circuit has a simple structure, low cost, good mechanical characteristics and hardness, and can meet the smooth speed regulation requirements of general transmission. It has been widely used in various fields of the industry. However, this kind of control method is in the low frequency, because the output voltage is lower, the torque is influenced by the voltage drop of the resistance of the stator more prominently, make the output maximum torque reduce. In addition, the mechanical characteristics of the DC motor is not as hard as any of them. The dynamic torque capability and static speed regulation performance are still not satisfactory. The system performance is not high, the control curve changes with the load, the torque response is slow, and the motor rotation The moment utilization rate is not high, and the performance is degraded due to the existence of the stator resistance and the dead time effect of the inverter at low speed, and the stability becomes worse. Therefore, people have also developed vector control frequency control. However, this kind of control method is also one of the control methods commonly used by inverters. It is also one of the most used control methods for domestic brands.

Voltage space vector (SVPWM) It is premised on the overall effect of three-phase waveform generation. For the purpose of approximating the ideal circular rotating magnetic field trajectory of the air gap of the motor, a three-phase modulation waveform is generated at a time, and the control is performed in such a way that the inward polygon approximates the circle. . After being used in practice, it has been improved, that is, the introduction of frequency compensation can eliminate the error of speed control; through estimating the amplitude of magnetic flux through feedback, eliminating the influence of stator resistance at low speed; and closing the output voltage and current to improve the dynamic accuracy and stability. However, there are many control circuits and no adjustment of torque is introduced. Therefore, the system performance has not been fundamentally improved. Since many domestic inverters still have a certain gap with foreign brands in vector control, the SVPWM control mode is more common in domestic inverter vector control modes.

The vector control frequency conversion speed regulation method is that the asynchronous motor in the three-phase coordinate system stator current Ia, Ib, Ic, through the three-phase-two-phase transformation, equivalent to two-phase static coordinate system under the alternating current Ia1Ib1, and then through According to the rotational orientation of the rotor field, the DC current Im1, It1 (Im1 corresponds to the excitation current of the DC motor; It1 corresponds to the armature current proportional to the torque) in the synchronous rotating coordinate system, and then imitates the DC motor The control method obtains the control quantity of the direct current motor and realizes the control of the asynchronous motor through the corresponding coordinate inverse transformation. The essence of this is that the AC motor is equivalent to a DC motor, which independently controls the speed and magnetic components. By controlling the rotor flux linkage, and then decomposing the stator current to obtain two components of torque and magnetic field, the orthogonal or decoupled control is achieved through coordinate transformation. Using vector control can make the motor at low speed, such as (without speed sensor) 1Hz (for 4-pole motors, its speed is about 30r/min), the output torque can reach the motor torque output at 50Hz (the most approximate 150% of rated torque). For the conventional V/F control, the voltage drop of the motor is relatively increased as the motor speed decreases, which results in the motor being unable to obtain sufficient rotational force due to insufficient excitation. In order to compensate for this deficiency, the inverter needs to increase the voltage to compensate for the voltage drop caused by the reduced motor speed. This function is the torque boost. The torque boost function increases the output voltage of the inverter. However, even if a lot of output voltage is increased, the motor torque cannot be increased corresponding to its current. Because the motor current contains the motor generated torque component and other components (such as excitation components). Vector control assigns the motor's current value to determine the value of the motor current component and other current components (such as the excitation component) that produce the torque. The vector control can be optimized and compensated by the response to the voltage drop at the motor end, allowing the motor to produce large torque without increasing the current. This function is also effective for improving the temperature rise of the motor at low speed. The vector control method has also become an important advantage for foreign brands to occupy the high-end market.

Direct Torque Control (DTC) Method This technique has largely solved the above-mentioned deficiencies of vector control and has been rapidly developed with novel control concepts, concise and clear system architecture, and excellent dynamic and static performance. At present, this technology has been successfully applied to high-power AC drives for traction of electric locomotives. Direct torque control directly analyzes the mathematical model of the AC motor in the stator coordinate system and controls the flux linkage and torque of the motor. It does not require that the AC motor be equivalent to a DC motor, thus eliminating many of the complex calculations in the vector rotation transformation; it does not need to mimic the control of a DC motor, nor does it need to simplify the mathematical model of the AC motor for decoupling. ABB's ACS800 series uses this type of control.
Matrix-type AC-ACV control method VVVF frequency conversion, vector control frequency conversion, direct torque control frequency conversion is a kind of AC-DC-AC frequency conversion. The common disadvantages are the low input power factor, large harmonic currents, large energy storage capacitors required in the DC circuit, and the inability to feed back the energy back to the grid, which means that four-quadrant operation is not possible. For this reason, matrix-type AC-AC frequency conversion came into being. Because the matrix-type AC-AC frequency conversion eliminates the intermediate DC link, it eliminates the need for bulky, expensive electrolytic capacitors. It can achieve a power factor of l, input current is sinusoidal and can operate in four quadrants, and the power density of the system is large. This technology is not yet mature. Its essence is not to indirectly control the current and flux linkage, but to directly use the torque as the controlled variable. Matrix-AC-AC frequency conversion has a fast torque response (<2ms), high speed accuracy (±2%, no PG feedback), high torque accuracy (<+3%); also has a higher start Torque and high torque accuracy, especially at low speed (including 0 speed), can output 150% to 200% torque.

 

Application field of low voltage inverters

In terms of applications, inverters have been applied in various industries, such as metallurgy, chemical industry, papermaking, machinery and other industries. Specific applications are more extensive, small applications such as blowers, conveyors, feeders, mixers, grinders, crushers, cutters, calenders, extruders, valves, compressors, cooling treads, plastics from all walks of life Machinery, elevators, various textile industries, etc.; large-scale applications such as paper mill paper machine, injection molding machine mold factory, metallurgical plant rolling mill, and chemical industry fans, pumps, cranes, oil pipelines.

The use of frequency converters in the chemical, stone, and textile industries is highly variable. Various grades of products can find corresponding users in these industries. Therefore, chemical, stone, and textile industries are the most complex industries for inverter brands. Almost all brands are Chemical, stone and textile industries all have markets. The common applications of low-voltage inverters are as follows:

1.fans

With the continuous development of industry, the application of fans has become more and more extensive, especially in the chemical, stone, textile industry can be applied to many aspects of the fan. In the design of wind turbines, there is a general over-air volume problem. When the air volume needs to be adjusted, throttling is generally adopted, that is, the damper is adjusted. This is an unnecessary loss. After the inverter is used, it can be easily adjusted according to actual needs. Unnecessary losses have been removed while power factors have been increased, and their combined power saving rates are all above 30%.

2.speed control machinery

Speed-adjusting machinery such as: extruder, material feed adjustment, conveyor belt, etc. basically use slip adjustment motor, three-phase commutator motor or DC motor speed regulation, speed regulation motor speed regulation performance is not good, stable speed Poor performance, high failure rate, and low efficiency. The DC motors and commutator motors have complex structures, high failure rates, and high maintenance costs. They generate sparks during operation and are extremely unsafe. These motors have poor stability of speed control, such as the use of asynchronous motors and variable frequency speed controllers to replace these types of motors, which can greatly improve efficiency, save electricity, have a small amount of maintenance, are safe, reliable, and have a short return on investment. favorable.

 

Selection method of low voltage inverters

The selection of low voltage frequency converters is mainly divided into seven steps:
(1) Determine the equipment in terms of work mode, volume and load type; (2) Determine the equipment in terms of process, function standards and control requirements; (3) Put the system in the I/O interface, communication interface, and build mode Other aspects should be clarified; (4) Summarize all performance standards and requirements; (5) Conduct direct bidding or technical consultation based on conclusions drawn from generalization; (6) Comprehensive comparison of service life, price, performance, and service; (7) Make clear the model, specification, brand, and supplier of the inverter.

As far as the drive control system is in the conventional operation, it is crucial to make a sensible choice for the general low-voltage inverter. First, the ultimate goal of using the general low-voltage inverter should be determined, based on the production machinery in the speed range, On the requirements of speed response, type, starting torque and control accuracy, comprehensively analyze the drive load characteristics of the inverter, and then determine which function constitutes the control system for the general low-voltage inverter, and then determine the optimal control. form. The determined universal low-voltage inverter can not only meet the needs of the production process, but also meet the technical and economic standards. At the same time, we must pay full attention to whether the frequency converter has problems in the production of technical standards, efficacy, harmonics, life, power factor and sales service. At the same time, the general low-voltage inverter continues to operate, the inverter is in the input and output interfaces, communication indicators, It is also important that accessories such as DC braking units and reactors are equipped with accessories.

The selection of general low-voltage frequency converters includes two aspects: type selection and capacity selection of general low-voltage frequency converters. The selection should be followed: first, its main performance can ensure that it can effectively meet the needs of the process, and secondly, the price/performance ratio should be ideal. When selecting a general low-voltage inverter type, the load characteristics should be taken into account. Balance torques such as fans and pumps, and lower load torques at lower speeds, should generally use ordinary or special-purpose general-purpose low-voltage inverters.
The constant-torque type load or the high precision of the static speed requires high-performance general-purpose low-voltage frequency converters with torque control performance in the selection. This general low-voltage frequency converter has relatively low torque speed and hard static machinery. Characteristics, load impact will not affect it, there is the performance of the excavator. In order to be able to perform a constant torque control with a relatively large speed regulation, the volume of a general low-voltage inverter is generally increased. The higher accuracy requirements, better dynamic performance, and faster speed response of the production machinery, such as injection molding machines, papermaking machinery or rolling mills, etc., should be selected directly to the torque control or vector control type of general low-voltage inverter.

In terms of volume, general low-voltage inverters have been marked with parameters such as power, rated current, and rated capacity that can be used with the motor, and can be equipped with the power and rated capacity of the motor to produce a universal low-voltage inverter. The responsible person is usually given by the motor produced by the company or the country, and there is no explicit expression of the performance of the inverter with the load. The rated current is only an important parameter that can express the load performance of the general low-voltage inverter.
Therefore, according to the motor rated current is less than the general low-voltage inverter rated current for the inverter capacity selection is the most important principle, only the motor rated power reference selection.

Moreover, before using a certain frequency changer capacity, should know the electrical concrete setting parameter and the operation flow first, and must consider the electrical machinery operating characteristic and the model, reserves the certain limit. For example, in general, the maximum current used by a motor in a submersible pump is greater than the maximum current limit of an ordinary motor; for a metallurgical enterprise, the maximum current limit of a professional motor used in a submersible pump is not only higher than normal, but also high in a short period of time. The current can also be operated normally under normal current. Generally, a plurality of motors are used together to operate the device. However, during operation, it should be ensured that the total operating current does not exceed the maximum specified limit current of the motor. In addition, if you are using an ordinary motor and you do not have a gear box, you should consider increasing the current storage capacity of the motor to a certain extent to ensure that the motor can operate normally. At the same time, it should be considered that ordinary inverter output is very unstable and may cause damage to the motor in varying degrees. Therefore, when using a frequency converter to supply power to the motor compared to the output current of a common power grid, the current flow of the motor will increase, causing the motor temperature to rise.

Therefore, in the use of frequency inverter power supply, it should consider leaving a certain flow limit to prevent the temperature from rising and destroy the motor.

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