1. Overview of Carrier Frequency of Low-Voltage Frequency Inverters
For frequency converters with voltage ≤500V, almost all of them nowadays use AC-DC-AC main circuit, and their control method also uses sinusoidal pulse width modulation, i.e., SPWM. Its carrier frequency is adjustable, generally from 1-15kHz, which can be easily selected manually.
However, in actual use, many users simply follow the original settings provided by the inverter manufacturer without adjusting them according to the actual site conditions. This results in improper selection of the carrier frequency value, affecting the correct and effective working state. Therefore, correctly selecting the carrier frequency value of the inverter is crucial during its use. This article provides a basis for correctly selecting the carrier frequency value by considering the following aspects.
2. Carrier frequency and inverter power consumption
The power loss of the IGBT power module is related to the carrier frequency, and the power loss increases with the increase of the carrier frequency. This leads to decreased efficiency and increased heat generation in the power module, which is detrimental to operation. Of course, the higher the operating voltage of the inverter, the greater the impact on power loss. The higher the carrier frequency, the greater the inverter 's losses and the lower the output power. If the ambient temperature is high, the dead zone of the two inverter transistors during the alternating conduction process of the inverter bridge will decrease, which can lead to a short circuit in the bridge arm and damage the inverter in severe cases.
3. Carrier frequency and ambient temperature
When the inverter is in use, the carrier frequency requirement is high and the ambient temperature is also high, which is very unfavorable to the power module. In this case, the allowable constant output current of the inverter should be appropriately reduced according to the carrier frequency and ambient temperature of the inverter with different power, so as to ensure the safe, reliable and long-term operation of the power module IGBT.
4. Carrier frequency and motor power
For motors with higher power, a relatively lower carrier frequency should be selected to reduce interference (the impact on other equipment). This principle is generally followed, but specific values may vary between manufacturers. For example, the following relationship is provided for reference in Japan: carrier frequency 15kHz 10kHz 5kHz
Motor frequency ≤30kW 37-100kW 185-300kW (Example), Finland VACON carrier frequency 1-16kHz 1-6kHz; Motor power ≤90kW 110-1500kW (Example), Shenzhen Ansheng (formerly Huawei) carrier frequency 6kHz 3kHz 1kHz
Electric motor power: 5.5-22kW, 30-55kW, 75-200kW
For example, Chengdu Jialing Company's JP6C-T9 series
Carrier frequency 2-6kHz 2-4kHz
Electric motor power: 0.75-55kW, 75-630kW
5. Carrier frequency and length of secondary output lines (U, V, W) of the frequency converter
Carrier frequency 15kHz 10kHz 5kHz 1kHz
Line length: <50M> 50-100M> 100-150M> 150-200M
6. The effect of carrier frequency on inverter output current
As is well known, the inverter (DC/AC conversion) section of a frequency converter uses IGBTs to generate a sinusoidal current waveform through the motor windings via sinusoidal pulse width modulation (SPWM). The carrier frequency directly affects the quality of the current waveform and the magnitude of interference. The carrier frequency is a highly sensitive and direct factor; therefore, during operation, it is crucial to first correctly select the carrier frequency value before considering the addition of various harmonic suppression devices, such as AC reactors, DC reactors, filters, and sequence reactors, as well as installation, wiring, and grounding measures. This approach is more reasonable and effective; it is essential to avoid reversing the order of importance in problem-solving. A higher carrier frequency results in a smoother, more sinusoidal current waveform, leading to fewer harmonics and less interference. Conversely, a lower carrier frequency results in a lower waveform. When the carrier frequency is too low, the effective torque of the motor decreases, losses increase, and the temperature rises. Conversely, when the carrier frequency is too high, the inverter's own losses increase, the IGBT temperature rises, and the rate of change of the output voltage (dv/dt) increases, significantly impacting the motor insulation.
(1) The higher the operating frequency, the larger the duty cycle of the voltage wave and the smaller the higher harmonic components of the current. That is, the higher the carrier frequency, the better the smoothness of the current waveform.
(2) The higher the carrier frequency, the smaller the current that the inverter can output;
(3) The higher the carrier frequency, the smaller the capacitive reactance of the wiring capacitor (because Xc=1/2πfC), and the greater the leakage current caused by the high-frequency pulse.
7. The effect of carrier frequency on motors
The higher the carrier frequency, the less vibration, the lower the operating noise, and the less heat the motor generates. However, the higher the carrier frequency, the higher the frequency of harmonic currents, the more severe the skin effect of the motor stator, the greater the motor losses, and the lower the output power.
7.1 The impact of carrier frequency on motor noise
The noise of an electric motor comes from three sources: ventilation noise, electromagnetic noise, and mechanical noise. We will not discuss ventilation and mechanical noise here, but will only analyze the electromagnetic noise problem after using a frequency converter.
The output voltage and current of the frequency converter contain a certain component of high-order harmonics, which increases the high-order harmonic flux in the air gap of the motor, thus increasing noise. Its characteristics are:
(1) Due to the resonance between the lower high-order harmonic components output by the inverter and the rotor's natural frequency, the noise near the rotor's natural frequency increases.
(2) Due to the high-order harmonics output by the frequency converter, the core, housing, bearing housing, etc. resonate, and the noise near the natural frequency increases.
(3) Noise is directly related to the carrier frequency. When the carrier frequency is high, the noise is relatively low.
(4) Tests showed that when the motor was running at a variable frequency, the noise was only 2dB greater than when it was running at a power frequency of 50Hz, so the impact was not significant and the absolute value was around 70dB.
(5) Using a variable frequency motor can reduce noise by 6-10 dB under the same operating parameters.
7.2 Carrier Frequency and Motor Vibration
The causes of motor vibration can be divided into two categories: electromagnetic and mechanical. Here, we will not discuss mechanical causes, but only analyze electromagnetic causes:
(1) Due to the resonance between the lower-order higher harmonic components and the rotor, the vibration components near its natural frequency increase.
(2) Vibration occurs due to the influence of pulsating torque generated by higher harmonics.
(3) When using a frequency converter, the vibration is slightly larger when working at the same 50Hz frequency. In particular, when the working frequency is 20Hz, the vibration will increase to a full amplitude of 7um. When the working frequency is 80Hz-120Hz, the full amplitude will increase to 6um. The vibration is slightly more severe for motors with smaller pole numbers than for motors with larger pole numbers.
(4) Vibration reduction can be achieved by using an output AC reactor.
(5) Give a smaller value for v/f.
(6) Using a variable frequency motor can reduce vibration.
(7) For high-speed grinding machines, a special motor with low noise and low vibration can be used.
7.3 Carrier frequency and motor heating
Because the inverter uses sinusoidal pulse width modulation, its current output waveform is approximately sinusoidal. The harmonic components are shown in Figure 3. A certain amount of higher-order harmonics will inevitably be generated, and the waveform will not be smooth, with glitches appearing. This will inevitably cause an increase in output current of up to 10%. Since heat generation is proportional to current I², under the same operating frequency and load, the motor temperature rise will be slightly higher after using a frequency converter. To minimize this loss, the carrier frequency should be as high as possible, which is beneficial for operation, or a variable frequency motor should be selected. Specific solutions include:
(1) Use a higher carrier frequency as much as possible to improve the output current waveform.
(2) Add input and output AC reactors or active filters, etc.
(3) Select a variable frequency motor.
(4) The operating frequency of the inverter should be lower than 20Hz, while the production equipment should be low-speed and have a large load. A speed reducer should be added after the output shaft of the motor to increase the operating frequency (inverter) and increase the output torque. This will help to solve the load requirements, inverter permissions, and the vibration, noise, heat generation, operating frequency and carrier frequency of the motor in a unified and reasonable manner.
8. Imbalance between carrier frequency and inverter input three-phase current
The input section of the frequency converter is a 6-pulse three-phase bridge diode rectifier circuit, i.e., AC/DC conversion. Since diodes are non-linear components, the internal impedance of each component will not be consistent during actual assembly, causing three-phase mismatch. Furthermore, because the input current is non-sinusoidal, this leads to an imbalance in the three-phase current input to the frequency converter. This is especially true when there is a significant imbalance in the input voltage, for example, a difference of 3-5%. In this case, the maximum difference in the three-phase input current can reach 10-20%, which is quite common. To improve and minimize the imbalance in the three-phase input current, the following methods are typically used:
(1) Improve the quality of the power grid to make its imbalance as small as possible.
(2) Select high-end, high-quality frequency converters.
(3) Increase the carrier frequency value as much as possible.
(4) Change the phase sequence of the three phases R, S, and T (the phase of the inverter input voltage does not need to be adjusted).
(5) Select a variable frequency motor
The principle is to minimize the three-phase imbalance using the methods described above; achieving absolute balance is difficult. However, the three-phase output current of the frequency converter is basically balanced. It is important to note that when measuring the input or output voltage and current of the frequency converter, it is best to use a voltage or current meter with filtering that only reflects the fundamental frequency (50Hz), such as a clamp meter or universal meter. Otherwise, the measured value will be larger than the actual value, which should also be noted.
9. Carrier frequency and electromagnetic interference
The higher the carrier frequency, the more severe the interference of high-frequency voltage on electronic equipment through electrostatic induction, electromagnetic induction, and electromagnetic radiation.
