A frequency converter is essentially a control power supply for an AC motor . Its purpose is to output a certain ratio of voltage and frequency to control the motor speed and torque. Although different brands may have different preset function parameters, and different applications may require different modulation parameters, in principle, its function parameters are designed around several main objectives, such as "controlling the motor, outputting appropriate voltage and frequency, ensuring a certain current and speed limit, and logical connections".
1. Control Mode Selection: Currently, the most common mode for frequency converters is V/F control, suitable for general applications where simple speed adjustments are sufficient. Therefore, a V/F curve needs to be set to meet the requirements of constant torque or loads such as fans and pumps. Manufacturers generally pre-set a constant torque V/F mode. Vector control mode is generally suitable for applications requiring high speed and torque accuracy, such as winding and unwinding equipment, cranes, and synchronization and tension control equipment. Vector control includes sensorless vector and encoder closed-loop vector. Some frequency converters also have a DTC (Direct Torque Control) mode, typically used in applications requiring torque control, such as cranes.
2. Since the design involves voltage and frequency, there are power supply voltage requirements. In China, the standard is generally three-phase 380V, so frequency converters are often designed to operate at 400V. The motor's operating frequency is 50Hz, which corresponds to a speed of 1500 RPM for a commonly used four-pole motor. Some frequency converters allow selection of the number of pole pairs, thus changing the corresponding speed. Frequency includes maximum frequency, starting frequency, minimum frequency, etc. Some equipment also needs to consider resonance issues, so frequency converters have preset frequency hopping functions. The frequency is basically directly proportional to the motor speed. Some frequency converters for spinning equipment also have a oscillation function, allowing the frequency to change according to a certain pattern.
3. Torque issues: Some applications require torque control, which can only be achieved with vector frequency converters. Frequency converters involve speed loop and current loop selection, speed loop output limit parameters, and optimization of PID parameters. Ordinary frequency converters also have torque boosting parameters to compensate for V/F parameters, making motor startup smoother.
4. Start, stop, and speed control functions: The frequency converter will have start source and frequency source parameter selection. For example, it is preset to start the frequency converter and set the frequency value on the panel. In actual use, it can be changed to terminal control mode or some communication method control.
5. In some situations, fast I/O speed regulation is required, so many frequency converters have multi-speed function. Different I/O can be given different frequency values, which can be selected in the parameters. Some also have timing function to cooperate with the time control of multi-speed.
6. Some interlocking functions: Many frequency converters have some simple built-in PLC functions for basic logic control. For example, between two frequency converters, if one starts, the other will also start, and the given speed will be maintained at a certain ratio. In some cases, PID functions are also needed for closed-loop control of process circuits, such as constant pressure water supply.
7. Some protections are necessary, such as overheat protection, overload protection, overcurrent protection, and some even have phase loss protection options. These are preset with parameters that are close to the relevant motor specifications when they leave the factory, but in actual use, different adjustments need to be made according to the equipment conditions.
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