The selection of a frequency converter depends on the type of production machinery, speed range, static speed accuracy, and starting torque requirements. The most suitable frequency converter is one with the appropriate control method. "Suitable" means both easy to use and economical, meeting the basic conditions and requirements of the process and production.
I. The motor and frequency converter itself that need to be controlled
1. Number of poles of the motor: Generally, the number of poles of the motor should not exceed 4; otherwise, the capacity of the frequency converter should be increased appropriately.
2. Torque characteristics, critical torque, and acceleration torque: Under the same motor power conditions, the inverter specifications can be selected at a reduced rating compared to the high overload torque mode.
3. Electromagnetic compatibility: To reduce interference from the main power supply, a reactor can be added to the intermediate circuit or the inverter input circuit, or a pre-isolation transformer can be installed. Generally, when the distance between the motor and the inverter exceeds 50m, a reactor, filter, or shielded cable should be connected in series between them.
II. Selection of Inverter Power
System efficiency equals the product of inverter efficiency and motor efficiency. Only when both operate at high efficiencies will the system efficiency be high. From an efficiency perspective, the following points should be considered when selecting inverter power:
1. It is most suitable when the power value of the frequency converter is comparable to that of the motor, so that the frequency converter can operate at a high efficiency value.
2. When the power rating of the frequency converter is different from that of the motor, the power rating of the frequency converter should be as close as possible to the power rating of the motor, but should be slightly greater than the power rating of the motor.
3. When the motor is frequently started and braked, or is under heavy load and operates frequently, a larger inverter can be selected to ensure long-term and safe operation of the inverter.
4. After testing, the actual power of the motor is indeed sufficient. It is advisable to select a frequency converter with a power rating lower than that of the motor. However, attention should be paid to whether the instantaneous peak current will cause the overcurrent protection to trip.
5. When the power of the frequency converter and the motor are different, the energy-saving program settings must be adjusted accordingly to achieve a higher energy-saving effect.
III. Selection of Inverter Enclosure Structure
The enclosure structure of the frequency converter must be adapted to environmental conditions, meaning factors such as temperature, humidity, dust, acidity/alkalinity, and corrosive gases must be considered. The following are some common structural types available for users to choose from:
1. Open-type IPOO inverters have no enclosure and are suitable for installation in control boxes or on panels, racks, or racks in electrical rooms. This type is particularly suitable when multiple inverters are used together, but it requires higher environmental conditions.
2. The enclosed IP20 type is suitable for general applications and can be used in environments with small amounts of dust or moderate temperature and humidity.
3. The sealed IP45 type is suitable for harsh industrial environments;
4. The sealed IP65 type is suitable for harsh environmental conditions, such as the presence of water, dust, and certain corrosive gases.
IV. Determining the Capacity of the Frequency Converter
Choosing the right capacity is itself a form of energy conservation and emission reduction. Based on existing data and experience, there are three relatively simple methods:
1. Determine the actual power of the motor. First, measure the actual power of the motor to select the capacity of the frequency converter.
2. Formula method. When one frequency converter is used for multiple motors, the following should be met: the impact of the starting current of at least one motor should be considered to avoid overcurrent tripping of the frequency converter.
3. Motor rated current method frequency converter.
The process of selecting the inverter capacity is essentially an optimal matching process between the inverter and the motor. The most common and safest approach is to ensure the inverter capacity is greater than or equal to the motor's rated power. However, in practice, the difference between the motor's actual power and its rated power must be considered. Often, the selected capacity is too large, while the actual required capacity is too small. Therefore, selecting the inverter based on the motor's actual power is reasonable, avoiding the selection of an oversized inverter and increased investment. For light loads, the inverter current should generally be selected based on 1.1N (where N is the motor's rated current), or based on the maximum motor power that matches the inverter's rated output power as specified by the manufacturer in the product specifications.
