Carrier frequency is the process of signal transmission. Instead of directly transmitting the signal, it loads the signal onto a fixed frequency wave. This process is called loadingFixed frequency.Strictly speaking, a lower signal frequency is modulated to a relatively higher frequency. The higher frequency modulated by a lower frequency is called carrier frequency, also called fundamental frequency。
The carrier frequency of frequency converter (switching frequency) is mostly converted by PWM modulation.That is to say, the voltage output by the inverter is actually a series of pulses, and the pulse width and interval are not equal.Its size depends on the intersection of modulation wave and carrier wave, that is, switching frequency.The higher the switching frequency, the more pulses in a cycle, the better the smoothness of current waveform, but the greater the interference to other equipment.The lower the carrier frequency or the bad setting, the motor will make unpleasant noise.By adjusting the switching frequency, the noise of the system can be minimized, the smooth waveform is the best, and the interference is also the smallest.[1]
Design parameters
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Voltage and frequency
For frequency converters with voltage ≤ 500V, nowadays almost all of them use AC-DC-AC main circuit, and their control mode is also selectedSinusoidal pulse width modulationThat is, SPWM. Its carrier frequency is adjustable, generally from 1-15kHz, which can be easily selected manually.However, in actual use, many users only follow the original set value of the frequency converter manufacturer and do not adjust it according to the actual situation on site, which results in improper selection of carrier frequency value and affects normal use. Therefore, it is also important to correctly select the carrier frequency value of the frequency converter during the use of the frequency converter.
Power and frequency
The power loss of the power module IGBT is related to the carrier frequency, and with the increase of the carrier frequency, the power loss increases, which reduces the efficiency and increases the heating of the power module, which is adverse to the operation. Of course, the higher the working voltage of the inverter, the greater the power loss.The frequency converter with different voltage and power will change with the increase of carrier frequency and power loss.
Temperature and frequency
When the frequency converter is used with high carrier frequency requirements and high ambient temperature, it is very unfavorable to the power module. At this time, the allowable constant output current of the frequency converter with different power should be properly reduced with the use of carrier frequency and ambient temperature, so as to ensure the safe, reliable and long-term operation of the power module IGBT.If the motor power is high, the carrier frequency is relatively low, so as to reduce interference (impact on the use of other equipment). This principle is generally followed, but the specific value varies from manufacturer to manufacturer.[2]
Carrier frequency application
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Frequency converter
The carrier frequency of the inverter is the number of times that the inverter's power switching devices (such as IGBT) are turned on and off. Therefore, it is also calledswitching frequency 。It mainly affects the following aspects: the power loss of the power module IGBT is related to the carrier frequency, the carrier frequency increases, the power loss increases, and the power module heats up, which is bad for the inverter;Influence of carrier frequency on the waveform of inverter output secondary current: when the carrier frequency is high, the current waveform is sinusoidal and smooth.In this way, the harmonics are small, but the interference is relatively large, otherwise, it is poor. When the carrier frequency is too low, the effective torque of the motor decreases, the loss increases, and the temperature increases. On the contrary, when the carrier frequency is too high, the converter's own loss increases, the IGBT temperature rises, and the change rate of the output voltage dv/dt increases, which has a great impact on the motor insulation.Assuming that the carrier frequency of SPWM wave is fc and the fundamental frequency is fs, fc/fs is calledCarrier wave ratioN，For three-phase inverter, when N is an integer multiple of 3, the output does not contain the third harmonic and the integer multiple of 3.In addition, the harmonic frequency is near the integer multiple of the carrier frequency in the harmonic concentration, that is, the harmonic frequency is: kfc ± mfs, and k and m are integers.The carrier wave ratio of the actual SPWM wave is not necessarily an integer. At this time, in order to reduce theSpectrum leakage, the length of Fourier window can be appropriately increased, and the PWM of multiple fundamental cycles can beFourier transform(FFT or DFT).
alternator
The electric drive control system is a motor system that takes electric energy as its energy source and carries out energy transformation through the motor body, driver, controller, sensor and other links.along withMicroelectronicsThe rapid development of power electronics technology, motor manufacturing technology and new material technology, especially the continuous progress and deepening of modern control theory and technology research, electric drive control system has been widely used in industry, agriculture, aerospace and other fields, becoming the mainstream of transmission systemPermanent magnet synchronous motorHigh performance control of the system, knowingMotor rotorThe position signal of is essential.In traditional drive systems, these signals are usuallyPhotoelectric encoderorResolverThis kind of mechanical sensor has problems such as mechanical installation, cable connection and fault, which reduces the reliability of the system and increases the volume and cost of the system, which limits the application range of the permanent magnet synchronous motor system.
No position sensor
In order to solve various defects caused by mechanical sensors, noneposition sensorControl technology has been widely studied and applied.The research on sensorless control technology can be divided into two categories: fundamental excitation estimation method and high-frequency signal component method.The fundamental excitation estimation method is mainly based on the fundamental dynamic model of the motor.This kind of method has good dynamic performance, but it is sensitive to the change of motor parameters and has poor robustness.In this kind of method, the fundamental wave parameters required for rotor position estimation are proportional to the motor speed, which limits its application in the zero speed and low speed ranges. It is only applicable to sensorless operation of motors in the middle and high speed ranges. In order to achieve accurate estimation of rotor position signals in the full speed range, some literatures have proposed the high-frequency signal component method.This kind of method requires that the motor has a certain degree of spatial saliency. The position signal of the rotor is obtained by tracking the spatial saliency effect of the motor rotor, so it is insensitive to the changes of motor parameters and has good robustness.In this kind of method, the carrier frequency component method uses the carrier frequency component signal of the inverter itself to realize the position sensorless operation of the system without additional high-frequency excitation, which has become position sensorless.[3]
