Main Parameters of Frequency Inverter Explained

The frequency inverter controls the power control equipment of the AC motor by changing the frequency of the motor's operating power supply. It is connected upstream of a motor to generate an AC voltage that can be adjusted to meet customer requirements. The inverter relies on the switching of the internal IGBT to adjust the voltage and frequency of the output power supply, and provides the required power supply voltage according to the actual needs of the motor, thereby achieving the purpose of energy saving and speed regulation. In addition, it also has many protection functions, such as overcurrent, overvoltage, overload protection, etc. With the continuous improvement of industrial automation, frequency converters have also been widely used.

Main Parameter When Use Frequency Inverter

The main function of the frequency converter is to adjust the power of the motor and realize variable speed operation of the motor to achieve the purpose of saving power. So it is particularly important to set related parameters reasonably when use it. The main parameters are introduced below:

1. Control method

That is, speed control, torque control, PID control or other methods. After they are adopted, static or dynamic identification is generally performed based on the control accuracy.

2. Minimum operating frequency

It means the minimum speed at which the motor runs. When the motor runs at low speed, its heat dissipation performance is very poor. If the motor runs at low speed for a long time, it will cause the motor to burn out. Moreover, at low speed, the current in the cable will also increase, which will also cause the cable to heat up.

3. Maximum operating frequency

The maximum frequency of the general inverter is 60Hz, and some special working conditions require it to reach 400Hz. High frequency will make the motor run at high speed. Here is a problem, for ordinary motors, its bearings cannot run at over-rated speeds for a long time.

4. Carrier frequency

The higher the carrier frequency is set, the greater the high-order harmonic component will be, which is closely related to factors such as the length of the cable, motor heating, cable heating, and inverter heating.

5. Frequency hopping

At a certain frequency point, resonance may occur, especially when the entire device is relatively high; when controlling the compressor, the surge point of the compressor must be avoided.

6. Acceleration time

The acceleration time is the time required for the output frequency to rise from 0 to the maximum frequency, and the deceleration time is the time required for the output frequency to drop from the maximum frequency to 0. Usually the rise and fall of the frequency setting signal is used to determine the acceleration and deceleration time. When the motor accelerates, the rising rate of the frequency setting must be limited to prevent overcurrent; and when decelerates, the falling rate must be limited to prevent overvoltage.

Acceleration time setting requirements: limit the acceleration current below the overcurrent capacity of the inverter to prevent the inverter to trip. The acceleration time can be calculated based on the load, but during debugging, it is often necessary to set a longer acceleration time based on the load and experience, and then observe whether there are overcurrent and overvoltage alarms when turn on or off the motor; finally shorten the acceleration time gradually based on the principle that no alarm will occur during operation, and repeat the operation several times to determine the optimal acceleration time.

7. Torque improvement

Also called torque compensation, it is a method of increasing the low frequency range to compensate for the torque reduction at low speeds caused by the resistance of the motor stator winding. When set to automatic, the voltage during acceleration can be automatically increased to compensate for the starting torque, allowing the motor to accelerate smoothly. If manual compensation is used, a better curve can be obtained through experiments based on the load characteristics. For variable torque loads, if the selection is improper, the output voltage at low speed will be too high, which will waste electric energy. There will even be a phenomenon where the current is large when the motor is started with a load, but the speed cannot increase.

8. Electronic thermal overload protection

This function is set up to protect the motor from overheating. It uses the CPU in the inverter to calculate the temperature rise of the motor based on the operating current value and frequency, thereby providing overheating protection.

Electronic thermal protection setting value (%) = [motor rated current (A)/inverter rated output current (A)] ×100%.

9. Frequency limitation

It is the upper and lower amplitude limits of the inverter output frequency. Frequency limit is a protective function to prevent misoperation or failure of the external frequency setting signal source, causing the output frequency to be too high or too low, thereby preventing damage to the equipment. Just set it according to the actual situation in the application. This function can also be used as a speed limiter. For example, if there is a belt conveyor, since there is not too much material to be transported, in order to reduce the wear of the machine and the belt, it can be driven by a frequency converter and the upper limit frequency of the inverter can be set to a certain value, so that this conveyor can run at a fixed, lower working speed.

10. Bias frequency

It is also called deviation frequency or frequency deviation setting. When the frequency is set by an external analog signal (voltage or current), this function can be used to adjust the output frequency when the setting signal is the lowest. For some inverters, when the frequency setting signal is 0, the deviation value can be in the range of 0~fmax, and some inverters can also set the bias polarity. For example, when the frequency setting signal is 0 during debugging, the inverter outputs frequency xHz, then setting the offset frequency to -xHz will make the inverter output frequency to 0Hz.

11. Setting signal gain by frequency

This function is only effective when the frequency is set with an external analog signal. It is used to compensate for the inconsistency between the external setting signal voltage and the internal voltage of the inverter (+10v); at the same time, it facilitates the selection of the analog setting signal voltage. During setting, when the analog input signal is at the maximum (such as 10v, 5v or 20mA), find the frequency percentage that can output the f/V pattern and set it as a parameter; when the external setting signal is 0~5v, if the inverter output frequency is 0~50Hz, then the gain signal is set to 200%.

12. Torque limit

There are two types: driving torque limit and braking torque limit. It calculates the torque through the CPU based on the inverter output voltage and current values, which can significantly improve the impact load recovery characteristics during acceleration, deceleration and constant speed operation. The torque limit function enables automatic acceleration and deceleration control. Assuming that the acceleration and deceleration time is less than the load inertia time, it can also ensure that the motor automatically accelerates and decelerates according to the torque setting value.

The driving torque function provides powerful starting torque. During steady-state operation, the torque function will control the motor slip and limit the motor torque to the maximum set value. When the load torque suddenly increases, even the acceleration time is set too short, it will not cause the inverter to trip, at the same time, the motor torque will not exceed the maximum set value. In short, a large driving torque is beneficial to starting, so it is more appropriate to set it to 80% to 100%.

The smaller the braking torque setting value, the greater the working force, which is suitable for situations of rapid acceleration and deceleration. If the braking torque setting value is set too high, an overvoltage alarm will occur. If the braking torque is set to 0%, the total amount of regeneration added to the main capacitor can be close to 0, so that when the motor decelerates, it can decelerate to stop trip without using a braking resistor. However, on some loads, such as when the braking torque is set to 0%, a brief idling phenomenon will occur during deceleration, causing the inverter to start repeatedly and the current to fluctuate significantly. In severe cases, the inverter will trip.

13. Acceleration and deceleration mode selection

Also called acceleration and deceleration curve selection. Generally, frequency converters have three types of curves: linear, nonlinear and S. Usually, linear curves are chosen; nonlinear curves are suitable for variable torque loads, such as fans; S curves are suitable for constant torque loads, and their acceleration and deceleration change are relatively slow. When setting, the corresponding curve can be selected according to the load torque characteristics.

14. Torque vector control

Vector control is based on the theoretical belief that asynchronous motors and DC motors have the same torque generation mechanism. The vector control method is to decompose the stator current into a prescribed magnetic field current and a torque current, control them respectively, and at the same time output the combined stator current to the motor. Therefore, the same control performance as that of a DC motor can be obtained in principle. Using the torque vector control function, the motor can output maximum torque under various operating conditions, especially in the low-speed operating area.

Now frequency inverters almost all use non-feedback vector control. Since the frequency converter can perform slip compensation according to the load current size and phase, the motor has very hard mechanical properties and can meet the requirements for most occasions. There is no need to add any additional components to the circuit. To set this function, you can choose between valid and invalid according to the actual situation. The related function is slip compensation control, which is used to compensate for the speed deviation caused by load fluctuations, and the slip frequency corresponding to the load current can be added.

15. Energy saving control

Fans and water pumps are torque-reducing loads, that is, as the speed decreases, the load torque decreases in proportion to the square of the speed. Inverters with energy-saving control functions are designed with a dedicated V/f mode, which can improve the efficiency of the motor, which can automatically reduce the inverter output voltage according to the load current to achieve energy saving.

It should be noted that the two parameters nine and ten are very advanced, but some users cannot enable these two parameters at all during equipment modification. That is, the frequency converter trips frequently after enabling it, but everything goes normal after disabling it. The reasons are:

(1) The original motor parameters are too different from the motor parameters required by the inverter.

(2) Insufficient understanding of parameter setting functions. For example, the energy-saving control function can only be used in V/f control mode and cannot be used in vector control mode.

(3) The vector control mode is enabled, but the manual setting and automatic reading of the motor parameters are not done, or the reading method is improper.

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