The higher the switching frequency, the smaller the harmonic of the output waveform after passing through the motor winding. The reason is that the higher the switching frequency, the higher the harmonic frequency, the greater the harmonic impedance and the smaller the harmonic current. The smaller the harmonic current is, the smaller the harmonic loss on the motor is, the smaller the torque ripple is, and the better the motor performance is. However, the higher the switching frequency, the greater the loss of the inverter, the lower the efficiency of the inverter, more importantly, the greater the heating, the inverter may not be able to bear. In addition, according to the fundamental frequency of the operation, the switching frequency should be an integral multiple of the fundamental frequency, and this multiple is still an integral multiple of 3, which can effectively reduce the low-order harmonics that have great impact on the motor!
By adjusting the switching frequency, the noise of the system can be minimized, the smoothness of the waveform is the best, and the interference is the least.
For the voltage & le; 500V frequency converter, almost all adopt the AC-DC-AC main circuit, and its control mode also uses sine pulse width modulation (SPWM). Its carrier frequency is adjustable, generally from 1-15khz, which can be easily selected by human. But in actual use, many users just according to the original set value of the inverter manufacturer, and did not adjust it according to the actual situation of the site. As a result, due to the improper selection of carrier frequency value, the correct and felt effective working state is affected. Therefore, how to correctly select the carrier frequency value of the frequency converter in the process of using the frequency converter is also an important matter. In this paper, we should consider the following aspects and choose the carrier frequency correctly.
1. Carrier frequency and power loss
The power loss of power module IGBT is related to carrier frequency, and with the increase of carrier frequency and power loss, the efficiency will be reduced; on the other hand, the heating of power module will increase, which is unfavorable to the operation. Of course, the higher the working voltage of frequency converter, the greater the power loss. For different voltage, power inverter with the increase of carrier frequency
2. Carrier frequency and ambient temperature
When the frequency converter is in use, the carrier frequency requirement is high, and the ambient temperature is also high, which is very unfavorable to the power module. At this time, with the carrier frequency and ambient temperature of the frequency converter with different powers, the allowable constant output current of the inverter should be appropriately reduced to ensure the safe, reliable and long-term operation of the power module IGBT.
3. Carrier frequency and motor power
For the motor with high power, the carrier frequency should be lower than the carrier frequency to reduce the interference (impact on the use of other equipment). This principle is generally followed, but the specific value varies from manufacturer to manufacturer.
4. Carrier frequency and the length of secondary outlet (U, V, w) of frequency converter
Carrier frequency 15KHz 10kHz 5KHz 1kHz
Line length < 50m > 50-100m > 100-150m > 150-200m
5. The waveform of carrier frequency to the output secondary current of frequency converter
As is known to all, the inverter (DC / AC conversion) of the inverter is generated by IGBT through SPWM. Then the size of carrier frequency directly affects the quality of current waveform and the size of interference. Moreover, the size of carrier frequency is more sensitive and direct. Therefore, it is necessary to correctly select the value of carrier frequency during operation, Then consider adding various harmonic suppression devices, such as AC reactor, DC reactor, filter, other sequence reactor, installation and wiring, grounding and other measures. This treatment is more reasonable and more effective, and it is not allowed to deal with the problem without inversion, which is a very important principle. When the carrier frequency is high, the current waveform is sinusoidal and smooth. When the carrier frequency is too low, the motor's effective torque will decrease, the loss will increase and the temperature will increase. On the contrary, when the carrier frequency is too high, the inverter's own loss will increase, the IGBT temperature will rise, and the output voltage change rate DV / dt will increase, which will have a great impact on the motor insulation.
6. Noise of carrier frequency to motor
The noise of motor comes from ventilation noise, electromagnetic noise and mechanical noise. The ventilation and mechanical noise are not discussed in this paper, but the electromagnetic noise after using frequency converter is analyzed.
The output voltage and current of the inverter contain a certain component of high-order harmonic, which increases the high-order harmonic flux in the air gap of the motor, so the noise becomes larger. Its characteristics are as follows: (1) due to the resonance between the lower high-order harmonic component output by the frequency converter and the natural frequency of the rotor, the noise near the natural frequency of the rotor increases. (2) Due to the high-order harmonic output of the frequency converter, the resonance of the core, housing, bearing seat, etc., increases the noise near the natural frequency. (3) The noise is directly related to the carrier frequency. When the carrier frequency is high, the relative noise is small. (4) The test results show that when the motor is running at frequency conversion, the noise is only 2 dB higher than that at 50 Hz power frequency. The visible effect is not great, and the absolute value is about 70 dB. (5) Using frequency conversion motor can reduce the noise of 6-10db under the same operating parameters.
7. Carrier frequency and motor vibration
The vibration causes of motor can be divided into electromagnetic and mechanical reasons. Here, the mechanical reasons are estimated and not discussed. Only the electromagnetic causes are analyzed as follows: (1) due to the resonance between the lower higher harmonic component and the rotor, the vibration component near the natural frequency increases; (2) the vibration occurs due to the influence of pulsating torque produced by higher harmonic. (3) When the frequency converter is used, the vibration is slightly larger when working at the same frequency of 50 Hz, especially when the working frequency is 20 Hz