Use of VFD (Variable-frequency Drive) speed control device

Asynchronous motors are the most important power equipment in electric power, chemical and other production enterprises. As a high-energy-consuming device, its output power cannot be changed proportionally with the load, and most of it can only be adjusted by the opening of the baffle or valve, while the energy consumed by the motor does not change much, resulting in a large energy loss. In recent years, with the maturity of VFD (Variable-frequency Drive) production technology and the increasingly wide application range of VFD (Variable-frequency Drive), it has become an important choice for enterprises to use VFD (Variable-frequency Drive) to carry out technical transformation of motor power supply to save energy and reduce consumption. , an important means to improve efficiency.

1. The principle of frequency conversion speed regulation

n=60f(1-s)/p(1)

In the formula, n——the speed of the asynchronous motor;

f——the frequency of the asynchronous motor;

s——motor slip ratio;

p——the number of pole pairs of the motor.

It can be seen from formula (1) that the speed n is proportional to the frequency f, and the speed of the motor can be changed as long as the frequency f is changed. When the frequency f changes within the range of 0 to 50 Hz, the motor speed can be adjusted in a very wide range. Frequency conversion speed regulation is to achieve speed regulation by changing the frequency of the motor power supply.

VFD (Variable-frequency Drive) mainly adopts the AC-DC-AC method. First, the power frequency AC power is converted into DC power through a rectifier, and then the DC power is converted into AC power with controllable frequency and voltage to supply the motor. The VFD (Variable-frequency Drive) circuit generally consists of four parts: rectification, intermediate DC link, inverter and control. The rectification part is a three-phase bridge uncontrolled rectifier, the inverter part is an IGBT three-phase bridge inverter, and the output is a PWM waveform, and the intermediate DC link is filtering, DC energy storage and buffering reactive power.

2. Harmonic suppression

The prominent problem in the use of VFD (Variable-frequency Drive) is harmonic interference. When VFD (Variable-frequency Drive) is working, the harmonic current of the output current will cause interference to the power supply. Although various VFD (Variable-frequency Drive) manufacturers have taken measures to control VFD (Variable-frequency Drive) harmonics and basically meet the requirements of national standards, harmonics are still a major issue in the selection and use of VFD (Variable-frequency Drive) The issues that require the most attention.

The output voltage of VFD (Variable-frequency Drive) contains harmonics other than the fundamental wave. The lower harmonics usually have a greater impact on the motor load, causing torque ripple, and the higher harmonics increase the leakage current of the VFD (Variable-frequency Drive) output cable, making the motor output insufficient, so the VFD (Variable-frequency Drive) Frequency Drive) output high and low harmonics must be suppressed.

Since the rectification part of the VFD (Variable-frequency Drive) uses a diode uncontrollable bridge rectifier circuit, and the middle filter part uses a large capacitor as a filter, the input current of the rectifier is actually the charging current of the capacitor, showing a steep pulse wave. Its harmonic components are relatively large. In order to eliminate harmonics, the following countermeasures are mainly adopted:

a. Increase the internal impedance of the VFD (Variable-frequency Drive) power supply. Normally, the internal impedance of the power supply equipment can buffer the reactive power of the VFD (Variable-frequency Drive) DC filter capacitor. This internal impedance is the short-circuit impedance of the transformer. When the power supply capacity is smaller than the VFD (Variable-frequency Drive) capacity, the internal impedance value is relatively larger, and the harmonic content is smaller; the power supply capacity is larger than the VFD (Variable-frequency Drive) capacity, and the internal impedance value is relatively small. The greater the harmonic content. Therefore, when choosing a VFD (Variable-frequency Drive) power supply transformer, it is best to choose a transformer with a large short-circuit impedance.

b. Installing a reactor Installing a reactor actually increases the internal impedance of the VFD (Variable-frequency Drive) power supply from the outside. Installing reactors or both on the AC side of the VFD (Variable-frequency Drive) or the DC side of the VFD (Variable-frequency Drive) can suppress harmonic currents.

c. Transformer multi-phase operation Usually the rectification part of VFD (Variable-frequency Drive) is a 6-pulse rectifier, so the harmonics generated are relatively large. The multi-phase operation of the application transformer, such as the combination of two transformers of Y-△ and △-△ with a phase angle difference of 30° to form a 12-pulse rectifier, can reduce the harmonic current and play a role in harmonic suppression .

d. Adjusting the VFD (Variable-frequency Drive) carrier ratio and increasing the VFD (Variable-frequency Drive) carrier ratio can effectively suppress low-order harmonics.

e. Application of filter The filter can detect the amplitude and phase of VFD (Variable-frequency Drive) harmonic current, and generate a current with the same amplitude and opposite phase as the harmonic current, so as to effectively absorb and eliminate the harmonic current.

3. Load matching

3.1 square torque load

Fan and pump loads are the most widely used equipment in industrial sites, and VFD (Variable-frequency Drive) is most widely used on such loads. It is a square torque load. In general, the VFD (Variable-frequency Drive) with U/f=const control mode can basically meet the requirements of this type of load. The following is an introduction to the selection of VFD (Variable-frequency Drive) according to the main characteristics of this type. The problem.

3.1.1 Avoid overloading

Fans and water pumps are generally not easy to be overloaded. When selecting the capacity of the VFD (Variable-frequency Drive), ensure that it is slightly greater than or equal to the capacity of the motor; at the same time, the overload capacity requirements of the selected VFD (Variable-frequency Drive) are also low. It takes 1 minute to reach 120%. However, when selecting and presetting VFD (Variable-frequency Drive) function parameters, attention should be paid to the fact that since the resistance torque of the load is proportional to the square of the rotational speed, when the operating frequency is higher than the rated frequency of the motor, the resistance torque of the load will exceed rated torque to overload the motor. Therefore, it is necessary to strictly control the maximum operating frequency not to exceed the rated frequency of the motor.

3.1.2 Matching of VFD (Variable-frequency Drive) acceleration time and deceleration time when starting/stopping

Due to the relatively large moment of inertia of the load of fans and pumps, it is a very important issue to match the acceleration time and deceleration time of VFD (Variable-frequency Drive) when starting and stopping. When VFD (Variable-frequency Drive) is selected and applied, the acceleration time and deceleration time of VFD (Variable-frequency Drive) should be calculated according to the load parameters to select the shortest time, so as not to occur when VFD (Variable-frequency Drive) starts Overcurrent tripping and VFD (Variable-frequency Drive) deceleration does not cause overvoltage tripping. But sometimes in the production process, the start-up time of fans and pumps is very strict. If the time calculated above cannot meet the demand, the VFD (Variable-frequency Drive) should be redesigned and selected.

3.1.3 Avoid resonance<