Problems and countermeasures in the application of frequency converters - Solutions - Huaqiang Electronic Network

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With the improvement of inverter technology, the application of AC motor frequency converter speed regulation is more and more extensive. The use of frequency converter speed regulation can improve the control precision, production efficiency and product quality of production machinery, which is conducive to the automation of production process. The drag system has excellent control performance and significant energy savings in many production situations.

1, the application of the inverter

The electricity consumption of electric motors in China accounts for 60%~70% of the national power generation. The annual power consumption of wind turbines and water pump equipment accounts for 1/3 of the national electricity consumption. The main reason for this situation is: the traditional speed control method for fans, pumps and other equipment is to adjust the air supply and water supply by adjusting the baffle and valve opening of the inlet or outlet. The input power is large, and a large amount of energy is consumed. The baffle and the valve are in the process of intercepting. Since most of the fans and pumps are the torque load of the house, the shaft power and the speed are established. Therefore, when the speed of the fan and the pump decreases, the power consumed is also greatly reduced. Therefore, the energy saving potential is very large. The most effective energy saving measure is to use the frequency conversion. The governor is used to regulate the flow rate, and the power consumption rate of the inverter is 20%~50%, and the benefit is remarkable.

Many machines require motor speed regulation due to process requirements. In the past, due to the difficulty of AC motor speed regulation, DC speed regulation was adopted in the occasions with high speed regulation performance, while DC winter structure is complicated, large in size and difficult to maintain. Therefore, with the maturity of frequency conversion speed regulation technology, AC speed regulation is gradually replacing DC. Speed ​​regulation often requires quantitative and direct torque control to meet various process requirements.

By using the frequency converter to drive the motor, the starting current is small, soft start and stepless speed regulation can be realized, and the acceleration and deceleration control can be conveniently performed. The motor obtains high performance and greatly saves electric energy, so the frequency converter is obtained in industrial production and life. More and more widely used applications.

2, existing problems and countermeasures

With the expansion of the application range of the frequency converter, there are more and more problems in operation, mainly including: high-order harmonics, noise and vibration, load matching, and heat generation. This paper analyzes the above problems and proposes corresponding measures.

3. Harmonic problems and countermeasures

The main circuit form of the general-purpose inverter is generally composed of three parts: rectification, inverter and filtering. The rectification part is a three-phase bridge type uncontrollable rectifier, the middle filter part adopts a large capacitance as a filter, the inverter part is an IGBT three-bridge inverter, and the input is a PWM waveform. The output voltage contains harmonics other than the fundamental wave. The lower harmonics usually have a greater influence on the motor load, causing torque ripple; and the higher harmonics increase the leakage current of the inverter output cable, making the motor The output is insufficient, so the high and low harmonics of the inverter output must be suppressed. The following methods can be used to suppress harmonics.

(1) Increase the internal impedance of the inverter power supply

Usually, the internal impedance of the power supply device can be used to buffer the reactive power of the DC filter capacitor of the inverter. The larger the internal impedance is, the smaller the harmonic content is. This internal impedance is the short-circuit impedance of the transformer. Therefore, when selecting the inverter power supply, it is best to choose a transformer with a short circuit impedance.

(2) Installing a reactor

A suitable reactor is connected in series with the output end of the inverter, or a harmonic filter is installed. The composition of the filter is LC type, which absorbs harmonics and increases the power supply or load impedance for suppression purposes.

(3) Multiple operation using transformer

The general-purpose inverter is a six-pulse rectifier, so the harmonics generated are large. If multi-phase operation of the transformer is used, the phase angles are mutually different by 30°. For example, the combination of Y-△ and △-△ constitutes a 12-pulse effect, which can reduce the low-order harmonic current and suppress the harmonics well.

(4) Set special harmonics

A dedicated filter is set to detect the frequency converter and phase, and generate a current with the same amplitude and opposite phase as the harmonic current, which is passed to the inverter, so that the harmonic current can be effectively absorbed.

4. Noise and vibration problems and countermeasures

Using frequency converter speed regulation will generate noise and vibration, which is the influence of high harmonic components in the output waveform of the inverter. As the operating frequency changes, both the fundamental component and the higher harmonic component vary over a wide range, which is likely to cause resonance with various parts of the motor.

(1) Noise problems and countermeasures

When the inverter is used to drive the motor, since the output voltage and current contain high-order harmonic components, the higher-order harmonic flux of the air gap increases, so the noise increases. Electromagnetic noise is characterized by an increase in noise near the natural frequency of the rotor because the low-order harmonic component in the output of the inverter resonates with the inherent mechanical frequency of the rotor. The higher harmonic components in the inverter output resonate with the core housing and the like, and the noise near the respective natural frequencies of these components increases.

The noise generated by the drive motor of the inverter, especially the harsh noise, is related to the switching frequency of the PWM control, especially in the low frequency region. The following measures are generally used to stabilize and reduce noise: an AC reactor is connected to the output side of the inverter. If the electromagnetic torque has a margin, U / f can be made smaller. When a special motor is used, the noise level of the lower frequency is more serious (information source: http://) to check the resonance with the natural frequency of the shaft system (including the load).

(2) Vibration problems and countermeasures

When the inverter is working, the magnetic field caused by the higher harmonics in the output waveform generates electromagnetic force for many mechanical components. The frequency of the power can always be close to or coincide with the natural frequency of these mechanical components, causing vibration caused by electromagnetic causes. The higher harmonics that have a large influence on vibration are mainly lower harmonic components, and have a greater influence in the PAM mode and the square wave PWM mode. However, when the sinusoidal PWM method is used, the low-order harmonic components are small and the influence is small.

To reduce or eliminate vibration, an AC reactor can be connected to the output side of the inverter to absorb the higher harmonic current components in the inverter output current. When using the PAM mode or square wave PWM mode inverter, the sine wave PWM mode inverter can be used to reduce the ripple torque. In order to prevent vibration, the mechanical system connected from the motor to the load must make the entire system not harmonic with the electromagnetic force generated by the motor.

5, load matching and countermeasures

Type of load

There are many types of production machinery, different performance and process requirements, and different torque characteristics. Therefore, before applying the inverter, first understand the nature of the load on the motor, that is, the load characteristics, and then select the inverter and the motor. There are three types of loads: constant torque load, fan pump type load and constant power load. Different types of inverters should be selected for different load types.

(1) Constant torque load

The constant torque load is further divided into a friction type load and a potential energy type load.

The starting torque of the friction type load generally requires about 150% of the rated torque, and the braking torque generally requires about 100% of the rated torque, so the frequency converter should choose to have constant torque characteristics, and both the starting and braking torques Larger inverters with large overload times and overload capacities, such as the FR-A540 series.

The position energy can generally achieve a large starting torque and energy feedback function, which can realize the forward and reverse rotation quickly. The inverter should select a frequency converter with four quadrant operation capability, such as FR-A241 series.

(2) Fan pump load

The fan pump type load is a typical square torque load. The load at very low speed is very small and proportional to the square of the speed. The combination of a general-purpose inverter and a standard motor is most suitable. This kind of load does not require high performance of the inverter, only economical and reliable, so choose a frequency converter with U/f=const control mode, such as FR-A540(L). If the inverter output frequency is increased above the power frequency, the power increases sharply, sometimes exceeding the capacity of the motor inverter, causing the motor to overheat or fail to operate. Therefore, for this type of load torque, do not easily increase the frequency above the power frequency.

(3) Constant power load

Constant power load refers to the load whose torque is inversely proportional to the speed, but the power remains constant, such as coilers and machine tools. When using the inverter with constant power characteristics, pay attention to the problem: the output voltage of the inverter is fixed value control within the frequency range above the power frequency, so the torque generated by the motor is constant power, using standard motor and universal There is no problem with the combination of the inverter. In the frequency range below the power frequency, U/f constant value control, the torque generated by the motor and the load torque are opposite, and the combination of the standard motor and the general frequency converter is difficult to adapt, so it is specially designed.

6, fever problems and countermeasures

The heating of the inverter is caused by the internal loss. The main circuit is mainly used, accounting for 98%, and the control circuit accounts for 2%. In order to ensure the normal and reliable operation of the inverter, the inverter must be dissipated. The main methods are:

(1) Heat dissipation by fan: The built-in fan of the inverter can remove the heat dissipation inside the inverter cabinet.

(2) Ambient temperature: The frequency converter is an electronic device, which contains electrolytic capacitors of electronic components, so the temperature has a great influence on its life. The ambient operating temperature of the general-purpose inverter generally requires -10 °C ~ +50 °C. If the operating temperature of the inverter can be reduced, the service life of the inverter is extended and the performance is stable.