Energy feedback unit suppliers remind you that the application history of frequency converters in China has been more than 30 years. With the continuous progress of technology, the application scope of frequency converters has also begun to involve multiple fields, and the market size is expanding year by year. At present, there are more than 140 domestic and foreign brands of frequency converters, and newly established frequency converter manufacturers and distributors are also spread throughout the country. Although there is still a certain gap in performance between domestic and imported frequency converters, with the rapid development of science and technology in China, this gap is not insurmountable. At the same time, benefiting from the integrity of the domestic industrial chain, there is great potential for the production efficiency and manufacturing cost of domestic frequency converters.
The variable frequency drive system is composed of a frequency converter, which has achieved or exceeded the performance of a DC speed control system. The frequency converter has the advantages of small size, low noise, low cost, and easy maintenance of asynchronous motors, greatly simplifying the production process and reducing initial investment costs. Generally speaking, the reasonable use of frequency converters can improve labor productivity, product quality, and equipment automation, while saving energy and reducing production costs.
1、 Classification, working principle and structure of low-voltage frequency converters
1. Classification of low-voltage frequency converters
There are different standards for classifying frequency converters. Variable frequency drives can be divided into general frequency drives and special frequency drives. According to the working principle, frequency converters can be divided into AC-AC frequency converters and AC-DC-AC frequency converters, among which AC-DC-AC frequency converters can also be divided into current type and voltage type frequency converters according to the working mode of the main circuit. In addition, from the perspective of the development direction of frequency converter technology, it can be divided into VVVF frequency converters, vector frequency converters, direct torque control frequency converters, and so on.
2. Low voltage working principle frequency converter
Generally speaking, frequency converters adopt a crossover direct crossover working mode. Relatively speaking, low-voltage frequency converters are widely used due to their mature technology, low cost, and easy maintenance. The working principle of a frequency converter is simply to convert AC power into adjustable frequency electrical equipment. According to the formula of synchronous speed N=60f/p for AC motors (where N is the synchronous speed of the motor, f is the power frequency, and p is the number of poles of the motor), the speed of the AC motor can be changed by changing the frequency. The frequency converter is developed based on this principle.
3. Low voltage frequency converter structure
The main circuit composition of the frequency converter:
Voltage type: The voltage power is converted from DC to AC frequency converter, and the circuit filter is capacitor.
Current type: The current power supply changes from DC to AC frequency converter, and the circuit filter is inductor.
The frequency converter mainly consists of the following four parts:
(1) Rectifiers: Currently, diode converters are widely used, which can convert power frequency into DC power and can also form reversible converters. Because its power direction is reversible, it can regenerate and operate.
(2) Flat wave circuit: The DC voltage rectified by the rectifier has pulsating voltage, which is 6 times the frequency of the power supply. To suppress voltage fluctuations, capacitors and inductors are needed to absorb pulsating voltage (i.e. current). When the capacity of the device is small, if there is excess capacity, a smoothing circuit can be directly used.
(3) Inverter: The inverter converts DC power into AC power, thereby obtaining three-phase output within a fixed time.
(4) Control circuit: Provide signal control circuit for the main circuit of asynchronous motor power supply. Including voltage frequency operation circuit, main circuit current and voltage detection circuit, motor speed detection circuit, operational circuit driving circuit that can amplify control signals, motor and inverter protection circuit.
2、 Selection of Low Voltage Inverter Types
1. Overview of Low Voltage Inverter Type Selection
Currently, most users choose based on the instructions or selection manual provided by the inverter manufacturer. Generally, the frequency converter manufacturer provides the rated current of the frequency converter, which can match the rated power and capacity of the motor. The parameters of the available motors are all provided by the manufacturer based on the manufacturer or national standard motors, and cannot truly reflect the carrying capacity of the frequency converter. Therefore, when choosing a frequency converter, the principle of the motor's rated current not exceeding the frequency converter's rated current should be taken as a reference. In addition, when choosing a frequency converter, one should also understand the process conditions and relevant parameters of the motor, and pay attention to the type and working characteristics of the motor.
(1) Selection of rated current for frequency converter. According to the design specifications, in order to ensure the safe and reliable operation of the frequency converter, the rated current of the frequency converter must be greater than the rated current of the load (motor), especially for motors with frequently changing load characteristics. According to experience, the rated current of a frequency converter is more than 1.05 times the rated current of a motor.
(2) Selection of rated voltage for frequency converters. The rated voltage of the frequency converter is selected based on the input side bus voltage of the frequency converter. In principle, the rated voltage of the frequency converter should be consistent with the input voltage. If the input voltage is too high, the frequency converter [3] will be damaged.
2. Precautions for selecting low-voltage frequency converters
(1) Match the load type with the frequency converter.
The load of the petrochemical industry mainly includes pumps and fans. Pumps are divided into water pumps, oil pumps, additive pumps, metering pumps, lifting pumps, mixing pumps, and washing pumps. Among them, lifting pumps, mixing pumps, and washing pumps are mostly heavy-duty, while the rest are conventional loads. Fans are divided into air-cooled fans, boiler induced draft fans, axial fans, air compressors, etc. When the air cooling fan and boiler induced draft fan are started, they are both heavy loads, generally considered heavy loads, and the rest are conventional loads. When choosing a frequency converter, the selection should be based on the load properties. If the load type is unclear or may change under different process conditions, it is recommended to choose a frequency converter based on heavy load to avoid selecting mismatches.
(2) Environmental conditions affect the frequency converter.
Usually, frequency converters require higher ambient temperatures and humidity. When the ambient temperature is below 30 degrees Celsius, the relative humidity is below 80%, and the altitude is below 100 meters, the frequency converter operates safely at rated current; If the ambient temperature exceeds 40 ℃, the actual capacity and current of the frequency converter will gradually decrease with the increase of ambient temperature. If the relative humidity of the environment exceeds 90%, condensation may occur, causing short circuits in the internal components of the frequency converter. If the height exceeds 100 meters, the output power of the frequency converter will decrease. In addition, frequency converters should be avoided from use in dusty environments.
(3) Selection of optional components for frequency converters.
Improper selection of optional components for frequency converters can lead to a high failure rate, mainly concentrated in the selection of filters and reactors.
3、 Practical application of low-voltage frequency converter
1. Primary connection of low-voltage frequency converter
Due to the significant impact of the installation positions of contactors, filters, and reactors in the primary circuit on the frequency converter, the following will focus on analyzing these three devices.
(1) Contactor
There are two main connection methods for contactors: installation on the rear side of the inverter body and installation on the front side of the inverter body. The contactor is installed on the back of the inverter body, and the advantage is that the inverter does not have frequent impacts when the motor is frequently started. The disadvantage is that the charging time of the frequency converter is long and there is power loss. The contactor is installed on the front side of the inverter body and has the advantage of completely cutting off power when the motor is in standby mode without losing power. The disadvantage is that frequent starting of the motor will cause frequent charging shocks to the frequency converter, affecting the service life of the frequency converter components.
In summary, if the motor starts infrequently, the contactor can be installed on the front and rear sides of the inverter body, but it is more suitable to be installed on the rear side of the inverter body. If the motor starts frequently, it is recommended to install the contactor on the rear side of the inverter body.
(2) Filter
The input filter is mainly used to filter the power grid, suppress the harmonic effects of the power grid on the frequency converter, and suppress the harmonic generated by the rectification of the frequency converter from returning to the power grid; The output filter mainly optimizes the frequency converter, filters out harmonics, and makes the output waveform more sinusoidal.
(3) Reactor
The input reactor can suppress harmonics on the grid side and protect the rectifier bridge; When the output cable of the frequency converter exceeds the specified length (generally allowing a cable length of 250m), an output reactor should be selected.
2. Installation environment for low-voltage frequency converter
Experiments have shown that the failure rate of frequency converters increases significantly in harsh environments, especially when sensitive to temperature, humidity, and dust. Therefore, when choosing an installation environment, it is necessary to choose an environment with controllable temperature, humidity, and low dust.
(1) Environmental temperature
In practical use, it has been found that frequency converters are suitable for working in environments with temperatures less than or equal to 35 degrees Celsius, otherwise the higher the temperature, the lower the load capacity of the frequency converter.
(2) Environmental humidity
When the surrounding humidity is high, the inverter is prone to condensation inside, which can easily cause short circuit accidents. Therefore, we must control the environmental humidity for the frequency converter.
(3) Dust environment
Frequency converters should be used in dusty environments as much as possible, as dust accumulation can cause short circuits and damage to the electronic components of the frequency converter.
4、 Common faults and solutions of low-voltage frequency converters
1. Unable to start
Reason: It is caused by excessive rotational inertia or torque of the load.
Solution: Increase the starting frequency and torque appropriately, and check the protection settings.
2. Overvoltage tripping
Reason: Caused by high power supply voltage or short downhill time.
Solution: Check if the running status is normal.
3. Overload
Reason: The overload capacity of the low-voltage inverter is relatively poor or the motor parameter settings are unreasonable.
Solution: Check the internal current detection circuit and the parameter settings of the frequency converter.