a. Output frequency scale. The output frequency scale that can be controlled by the general inverter, the lowest starting frequency is generally 0.1Hz, and the highest frequency is different according to the functional indicators of the inverter, generally 400Hz, and some models are 650Hz. No matter how high the output frequency is, it depends on the scale of the high-frequency inverter.
b. Set the frequency resolution. Frequency resolution is the smallest frequency value that can be resolved. In the digital general-purpose inverter, if the frequency is set by the external analog signal 0-10V or 4-20mA, the resolution is determined by the internal A/D converter; If the setting is based on a digital signal, the resolution is determined by the number of digits in the input signal. The imitation setting resolution can reach 1/3000, and the panel operation setting resolution can reach 0.01Hz. Some inverters also have functions such as bias adjustment, gain adjustment, and upper and lower limit adjustment for external signals. For occasions that require high control accuracy, it can also be solved by optional parts. Some inverters can choose digital (BCD code, binary code) input and RS-232C/RS-485 serial communication signal input module.
c. Output frequency accuracy. Output frequency accuracy is the degree to which the output frequency changes depending on the operating conditions. Output Frequency Accuracy = Frequency Change Value/Maximum Frequency ×100%, which is generally caused by temperature change or drift. When the imitation is set up, the output frequency accuracy is less than ±0.2%; When the digits are set to keep, the output frequency accuracy is less than ±0.01%.
(3) U/f characteristics of the inverter. The U/f characteristic is the ratio of the output voltage to the frequency of a general-purpose inverter within a frequency-changeable scale. The general general inverter can set the output voltage separately from the fundamental frequency and the highest frequency, and generally give the voltage scale, such as 400V level input, 160-480V.
(4) The torque characteristics of the inverter. When the motor is driven by an inverter, the temperature rise is slightly higher than when the power frequency power supply is used. In addition, when running at low speeds, the cooling effect of the motor decreases, and the promised output torque decreases accordingly. The torque characteristics of the inverter generally include the following.
a. Starting torque: corresponding to the maximum output torque at 0Hz, the percentage of the maximum output torque at 0.5Hz is generally given, such as 0.5Hz, 200%.
b. Torque increase. When the motor is driven by the inverter, there will be under-excitation in the low-frequency area, in order to start the motor smoothly, the under-excitation of the motor should be compensated, so that the torque reduced during low-frequency operation is enhanced, and the torque improvement function is generally settable or self-tuning.
c. Torque constraint. For example, when the torque of the motor reaches the set value, the torque restraint function will automatically adjust the output frequency to avoid overcurrent tripping of the inverter. The torque restraint function is generally programmable and can be selected by the contact input signal.
(5) PID control of inverter. Generally, the type and setting value of the control signal and feedback signal of the PID control function are clarified in the product description, such as keyboard panel setting, voltage input DC 0-+10V, current input DC 4-20mA, multi-stage speed setting, serial communication interface link setting: RS-485, set frequency/maximum frequency × 100%, feedback signal 0-10V, 4-20 mA or 20-4mA, etc.
(6) The speed regulation ratio of the inverter. The speed ratio is the ratio of the upper frequency (e.g., 50 Hz) to the lowest operating frequency that can be reached (e.g., 0.5 Hz). The nominal values corresponding to the lowest frequency, such as torque function, temperature and speed accuracy, speed response, etc., should be able to meet the operating requirements. If the minimum frequency is 0.5Hz and the upper frequency is 50Hz, the speed regulation ratio is 100:1. The speed control ratio indirectly expresses the low-frequency function and speed control accuracy of the general-purpose inverter.
(7) Braking method of inverter. When the motor is controlled by a universal inverter, electric braking is possible. The electrical braking of the general inverter is divided into internal braking and external braking, the internal braking generally has AC braking and DC braking, and the external braking has braking resistance braking and power feedback braking.
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