Production machinery comes in many varieties, with varying performance and process requirements, resulting in different torque characteristics. Therefore, before applying a frequency converter , it is essential to first understand the nature of the load driven by the motor, i.e., the load characteristics, and then select the appropriate frequency converter and motor. There are three types of loads: constant torque loads, fan/pump loads, and constant power loads. Different types of loads require different types of frequency converters.
(1) Constant torque loads: These can be divided into friction loads and potential energy loads. Friction loads typically require a starting torque of about 150% of the rated torque and a braking torque of about 100% of the rated torque. Therefore, the inverter should be selected with constant torque characteristics, large starting and braking torques, and large overload capacity. Potential energy loads typically require large starting torque and energy feedback functions, and can quickly achieve forward and reverse rotation. The inverter should be selected with four-quadrant operation capability.
(2) Fan and pump loads: Fan and pump loads are typical square torque loads. The load is very small at low speeds and is proportional to the square of the speed. The combination of a general-purpose frequency converter and a standard motor is most suitable. This type of load does not have high requirements for the performance of the frequency converter, only economy and reliability are required. Therefore, it is sufficient to select a frequency converter with U/f=const control mode.
(3) Constant power load: A constant power load refers to a load in which torque is inversely proportional to speed, but power remains constant, such as a winding machine or machine tool. Regarding the use of frequency converters for clothing with constant power characteristics, the following issues should be noted: In the frequency range above the power frequency, the output voltage of the frequency converter is controlled at a constant value. Therefore, the torque generated by the motor and the load torque tend to be opposite. The combination of a standard motor and a general-purpose frequency converter is difficult to adapt to, thus requiring special design.
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