Frequency converter is also known as the variable-frequency drive (VFD), which is a power control device that controls the AC motor by changing the working frequency of the motor by applying variable frequency technology and microelectronic technology.
The frequency converter is mainly composed of rectification (AC to DC), filtering, inverter (DC to AC), braking unit, drive unit, and detection unit micro processing unit. The inverter adjusts the voltage and frequency of the output power supply by the breaking of the internal IGBT, and supplies the required power supply voltage according to the actual needs of the motor, thereby achieving the purpose of energy saving and speed regulation. In addition, the frequency converter has many protection functions. Such as overcurrent, overvoltage, overload protection and so on.
Working principle
The main circuit is a power conversion part that supplies a voltage-regulating and frequency-modulated power supply to an asynchronous motor. The main circuit of the frequency converter can be roughly divided into two types: a voltage type and a current type.
The voltage type is a frequency converter that converts the direct current of the voltage source into an alternating current, and the filtering of the direct current circuit is a capacitor. The current type is a frequency converter that converts the direct current of the current source into an alternating current, and the direct current loop filtering is an inductance.
The frequency converter is usually divided into four parts: rectifier unit, high-capacity capacitor, inverter and controller.
Rectifier unit: converts AC power with fixed operating frequency to DC power.
High-capacity capacitor: Stores the converted power.
Inverter: An electronic switch consisting of a high-power switching transistor array that converts direct current into square waves of different frequencies, widths, and amplitudes.
Controller: Operates according to the set program, controls the amplitude and pulse width of the output square wave, and superimposes the alternating current into an approximate sine wave to drive the AC motor.
Select the type of inverter, according to the type of production machinery, speed range, static speed accuracy, starting torque requirements, it is most appropriate to decide which inverter to use. The so-called right is both easy to use and economical to meet the basic conditions and requirements of the process and production.
Selection of frequency converter
1. The motor to be controlled and the inverter itself
1) The number of poles of the motor. Generally, the number of poles of the motor is not more than (very suitable, otherwise the capacity of the inverter should be appropriately increased.
2) Torque characteristics, critical torque, and acceleration torque. In the case of the same motor power, the inverter specifications can be derated compared to the high overload torque mode.
3) Electromagnetic compatibility. In order to reduce the main power interference, the reactor can be added to the intermediate circuit or the inverter input circuit or the front isolation transformer can be installed. Generally, when the distance between the motor and the inverter is more than 50m, a reactor, a filter or a shielded protective cable should be connected in between.
2. Selection of inverter power
The system efficiency is equal to the product of the inverter efficiency and the motor efficiency. Only when both are working at higher efficiency, the system efficiency is higher. From the efficiency point of view, when selecting the inverter power, pay attention to the following points:
1) The inverter power value is equivalent to the motor power value, so that the inverter can run at high efficiency.
2) When the power classification of the inverter is different from the motor power classification, the power of the inverter should be as close as possible to the power of the motor, but should be slightly larger than the power of the motor.
3) When the motor is frequently started, braked or under heavy load and works more frequently, a larger frequency converter can be selected to operate the inverter for a long time and safely.
4) After testing, the actual power of the motor does have a surplus. You can consider the inverter with power less than the motor power, but pay attention to whether the instantaneous peak current will cause overcurrent protection.
5) When the inverter and motor power are not the same, the energy-saving program settings must be adjusted accordingly to achieve a higher energy-saving effect.
3. Selection of inverter box structure
The cabinet structure of the inverter should be adapted to the environmental conditions, that is, factors such as temperature, humidity, dust, pH, and corrosive gases must be considered. There are several types of structures that are available for users to choose from:
1) The open type IPOO type has no chassis, and it is suitable for being installed in the electric control box or on the screen, disk and rack in the electrical room. Especially when multiple inverters are used intensively, this type is better, but the environmental conditions are better. high;
2) The closed type IP20 is suitable for general use, and may have a small amount of dust or a little temperature and humidity;
3) The sealed IP45 type is suitable for environments with poor industrial site conditions;
4) The sealed IP65 type is suitable for occasions with poor environmental conditions, such as water, dust and certain corrosive gases.
4. Determination of inverter capacity
Reasonable capacity selection is itself a measure of energy saving. Based on available data and experience, there are three simple methods:
1) The actual power of the motor is determined. First measure the actual power of the motor to select the capacity of the inverter.
2) Formula method. When a frequency converter is used for multiple motors, it should be satisfied: at least consider the influence of the starting current of one motor to avoid overcurrent trip of the frequency converter.
3) Motor rated current method inverter. The selection process of the inverter capacity is actually the best matching process between the inverter and the motor. The most common and safer is to make the capacity of the inverter greater than or equal to the rated power of the motor, but the actual matching should consider the motor. The difference between the actual power and the rated power is usually the capacity of the equipment is too large, but the actual required capacity is small. Therefore, it is reasonable to select the inverter according to the actual power of the motor, avoiding the oversized inverter and increasing the investment. . For light loads, the inverter current should generally be selected according to 1.1N (N is the rated current of the motor), or according to the maximum motor power indicated by the manufacturer in the product matching the output power rating of the inverter.
5. Main power supply
1) Power supply voltage and fluctuations. Special attention should be paid to the inverter's low voltage protection setting value, because in actual use, the grid voltage is less likely to be lower.
2) Main power frequency fluctuations and harmonic interference. This interference increases the heat loss of the inverter system, resulting in increased noise and reduced output.
3) The power consumption of the inverter and the motor when it is working. When designing the system's main power supply, both power consumption factors should be taken into account.
