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.
With the rapid development of industrial automation, frequency converters (VDCs), used for variable frequency drive (VFD) adjustments, have been widely adopted. As a crucial component for variable frequency speed control and energy saving, the VDC primarily controls AC motors by altering the frequency of the power supply. Its advantages not only improve production processes but also significantly contribute to energy conservation for businesses. So, how does one choose the right VDC?
First, what aspects should be considered when choosing a frequency converter?
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.
How exactly do you determine and select a frequency converter?
1. The motor to be controlled and the frequency converter itself
Number of motor poles. Generally, it's best to keep the number of motor poles to no more than 4; otherwise, the inverter capacity will need to be increased accordingly. Torque characteristics, critical torque, and acceleration torque. For motors of the same power, the inverter specifications can be downrated for high overload torque conditions. Electromagnetic compatibility. To reduce mains power supply interference, reactors can be added to the intermediate circuit or 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.
2. 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:
The optimal power rating for the frequency converter is when it is comparable to that of the motor, so that the frequency converter can operate at a high efficiency.
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.
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.
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.
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.
3. 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:
The open-type IPOO inverter has no enclosure and is suitable for installation in the control box or on the panel, rack, or rack in the electrical room. It is especially suitable when multiple inverters are used together, but the environmental conditions are more demanding. The enclosed IP20 type is suitable for general use and can be used in environments with a small amount of dust or temperature and humidity. The sealed IP45 type is suitable for harsh industrial environments. The closed IP65 type is suitable for harsh environments with water, dust, and certain corrosive gases.
4. 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:
The actual power of the motor is determined first. This is used to select the capacity of the frequency converter.
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.
Motor rated current method for frequency converters. The process of selecting the frequency converter capacity is actually an optimal matching process between the frequency converter and the motor. The most common and safest approach is to make the frequency converter capacity greater than or equal to the rated power of the motor. However, in actual matching, it is necessary to consider how much the actual power of the motor differs from the rated power. Usually, the selected capacity is too large, while the actual required capacity is too small. Therefore, it is reasonable to select the frequency converter according to the actual power of the motor to avoid selecting an oversized frequency converter and increasing investment.
For light load applications, the inverter current should generally be selected based on 1.1N (where N is the rated current of the motor), or based on the maximum motor power that matches the rated output power of the inverter as specified by the manufacturer in the product specifications.
5. Main power supply
Power supply voltage and fluctuations. Special attention should be paid to ensuring compatibility with the inverter's undervoltage protection setting, as the mains voltage is more likely to be low in actual use.
Fluctuations in the main power supply frequency and harmonic interference can increase the heat loss of the inverter system, leading to increased noise and reduced output.
The power consumption of both the frequency converter and the motor during operation should be taken into account when designing the main power supply for the system.
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