Influence of inverter carrier frequency on motor operation - News - Global IC Trade Starts Here Free Join

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Most of the inverters use PWM modulation for the inverter. That is to say, the voltage output by the inverter is actually a series of pulses, and the width and interval of the pulses are not equal. Its size depends on the intersection of the modulated wave and the carrier, which is the switching frequency. The higher the switching frequency, the greater the number of pulses in a cycle, the better the smoothness of the current waveform, but the greater the interference to other devices. The lower the carrier frequency or the poor setting, the motor will make an unpleasant noise.
By adjusting the switching frequency, the noise of the system can be minimized, the smoothing of the waveform is best, and the interference is also minimal.
1 low-voltage inverter overview For inverters with voltage ≤500V, almost all of the main circuits of AC-DC-AC are used today. The control mode also uses sinusoidal pulse width modulation, ie SPWM, whose carrier frequency is adjustable, generally from 1 -15kHz, easy to make artificial selection. However, in actual use, many users only follow the original set value of the inverter manufacturing unit, and do not adjust according to the actual situation on site. As a result, the carrier frequency value is improperly selected, and the effect is correct and the effective working state is felt. Therefore, it is also important to correctly select the carrier frequency value of the inverter during the use of the inverter. This article provides a basis for considering the following aspects and selecting the carrier frequency value correctly.
2 Carrier frequency and power loss The power loss of the IGBT of the power module is related to the carrier frequency, and the carrier frequency increases and the power loss increases. This reduces the efficiency. The second is that the power module increases heat, which is unfavorable for operation. Of course, the higher the operating voltage of the inverter, the greater the power loss. Correct
3 Carrier frequency and ambient temperature When the inverter requires high carrier frequency and the ambient temperature is high, it is very unfavorable for the power module. At this time, the carrier frequency of the inverter with different powers is used. The height and the ambient temperature are appropriately reduced for the allowable constant output current of the inverter to ensure safe, reliable and long-term operation of the power module IGBT.
4 Carrier frequency and motor power motor power is relatively high, the relative selection of carrier frequency is lower, the purpose is to reduce interference (the impact on the use of other equipment), generally comply with this principle, but the specific values ​​of different manufacturers are also different.
For example, Japan has the following relationship for reference carrier frequency 15 kHz 10 kHz 5 kHz
Motor frequency ≤ 30kW 37-100kW 185-300kW
Example, Finland VACON
Carrier frequency 1-16kHz 1-6kHz
Motor power ≤90kW 110-1500kW
For example, Shenzhen Ansheng (formerly Huawei)
Carrier frequency 6kHz 3kHz 1kHz
Motor power 5.5-22kW 30-55kW 75-200kW
For example, Chengdu Jialing Company JP6C-T9 series carrier frequency 2-6kHz 2-4kHz
Motor power 0.75-55kW 75-630kW
5 carrier frequency and secondary output of the inverter (U, V, W) length carrier frequency 15kHz 10kHz 5kHz 1kHz
Line length <50M >50-100M >100-150M >150-200M
6 Carrier frequency The waveform of the secondary current output from the inverter is well known. The inverter (DC/AC conversion) part of the inverter is a sinusoidal current waveform generated by the IGBT through a sinusoidal pulse width modulation SPWM, then the carrier frequency is Directly affect the quality of the current waveform, as well as the size of the interference, and the size of the carrier frequency is more sensitive and direct, so in the operation process, we must first correctly select the size of the carrier frequency value, and then consider adding various suppression harmonics. Wave devices, such as AC reactors, DC reactors, filters, alternative reactors, and installation wiring, grounding, etc., so that the treatment is more reasonable and more effective, and must not be inverted to deal with the problem, this is Very important principle. When the carrier frequency is high, the current waveform is sinusoidal and smooth. Such harmonics are small, the interference is small, and vice versa. When the carrier frequency is too low, the effective torque of the motor is reduced, the loss is increased, and the temperature is increased. On the contrary, when the carrier frequency is too high, the loss of the inverter itself is increased. The IGBT temperature rises, and the rate of change dv/dt of the output voltage increases, which has a great influence on the motor insulation.
7 Carrier frequency noise of the motor The noise of the motor comes from three aspects: ventilation, electromagnetic noise and mechanical noise. The ventilation and mechanical noise are estimated here. Only the inverter is used to analyze the electromagnetic noise problem.
The output voltage and current of the inverter contain a certain amount of higher harmonics, which increases the higher harmonic flux of the motor air gap, so the noise becomes larger. Its characteristics are:
(1) The noise near the natural frequency of the rotor increases due to the resonance of the lower harmonic component of the inverter output and the natural frequency of the rotor.
(2) Since the harmonics output from the inverter cause resonance of the core, the casing, the bearing housing, etc., the noise near the natural frequency increases.
(3) The noise is directly related to the carrier frequency. When the carrier frequency is high, the relative noise is small.
(4) It is tested that when the motor is running at variable frequency, the noise is only 2dB larger than the operating frequency of 50Hz. The absolute value is about 70dB.
(5) The frequency conversion motor can reduce the noise of the same operating parameters by 6-10dB.
8 Carrier frequency and vibration of the motor The cause of the vibration of the motor can be divided into electromagnetic and mechanical. It is estimated here and not for mechanical reasons, only for electromagnetic reasons:
(1) The vibration component near the natural frequency increases due to the lower harmonic component and the resonance of the rotor.
(2) Vibration occurs due to the influence of the pulsating torque generated by the higher harmonics.
(3) When the inverter is operated at the same 50Hz frequency, the vibration will be slightly larger, especially when the operating frequency is 20Hz, the vibration will increase to the full amplitude of 7um, the operating frequency of 80Hz-120Hz will increase to 6um, and the number of motor poles Smaller than the largest number is slightly more serious.
(4) The output AC reactor can be used to reduce vibration.
(5) Give v/f a smaller value.
(6) The inverter motor can reduce vibration.
(7) For high-speed grinding machines, special motors with low noise and low vibration can be used.
9 Carrier frequency and motor heating Because the inverter adopts sinusoidal pulse width modulation, its current output waveform is approximate sine wave, the harmonic component is shown in Figure 3. There must be a certain component of each higher harmonic generation, and the waveform is not enough. Smooth burrs appear, which will cause the output current to increase by up to 10%, and the heating is proportional to the current I2. Therefore, under the same load at the same working frequency, the temperature rise of the motor is slightly higher after using the inverter, so as to be as small as possible. For this part of the loss, it is necessary to make the carrier frequency value as large as possible, which is beneficial to the operation, or use a variable frequency motor. The specific solution is:
(1) Use a higher carrier frequency whenever possible to improve the output current waveform.
(2) Add input and output AC reactors or active filters.
(3) Use variable frequency motor.
(4) The operating frequency of the inverter should be lower than 20Hz, and the production equipment should be low speed. When there is a large load operation, the first output reducer is added after the output shaft of the motor to improve the working frequency (inverter). And increase the output torque, in order to solve the load requirements, the permission of the inverter, and the reasonable solution of the motor vibration, noise, heat, operating frequency, carrier frequency.
10 Carrier frequency and the unbalance of the input three-phase current of the inverter The input part of the inverter is a 6-pulse three-phase bridge diode rectifier circuit that is AC/DC conversion. Since the diode is a nonlinear component, in actual assembly, each component The internal impedance will not be consistent, causing three-phase mismatch, and the input current is non-sinusoidal, which causes the imbalance of the three-phase current input to the inverter, especially when there is a large imbalance in the input voltage, for example: There is a difference of 3-5%, so that the three-phase input current may have a maximum difference of 10-20%, which is often possible. To improve the imbalance of the input current three-phase, as much as possible, usually Use the following method:
(1) Improve the quality of the grid so that its imbalance is as small as possible.
(2) Use high-end quality brand inverters.
(3) Increase the carrier frequency value as much as possible.
(4) Change the phase sequence of three phases of R, S and T (inverter input voltage phase does not need to be phased)
(5) The principle of using the variable frequency motor to reduce the three-phase unbalance as much as possible by the above method is difficult to achieve with absolute balance. However, the inverter output three-phase current is basically balanced. It is also necessary to measure the input or output voltage and current of the inverter. It is best to use one, which only reflects the fundamental (50Hz) filtered voltage and current meter. The clamp-type ammeter is universal or the table is suitable, otherwise the measured value is larger than the actual value, which should also be noted.