Most frequency converters use PWM modulation. This means the voltage output by the frequency converter is actually a series of pulses, with varying pulse widths and intervals. The magnitude of these pulses depends on the intersection of the modulating wave and the carrier wave, which is the switching frequency. A higher switching frequency results in more pulses per cycle, leading to a smoother current waveform, but also greater interference to other devices. A lower carrier frequency, or a poorly set one, will cause unpleasant noise from the motor. Adjusting the switching frequency can minimize system noise, achieve optimal waveform smoothness, and minimize interference.

1. Overview of Carrier Frequency in Low-Voltage Frequency Converters

For frequency converters with voltages ≤500V, almost all modern converters use an AC-DC-AC main circuit, with Sinusoidal Pulse Width Modulation (SPWM) as the control method. Its carrier frequency is adjustable, typically from 1 to 15kHz, and 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 leads to improper carrier frequency selection, affecting the correct and effective operating state. Therefore, correctly selecting the carrier frequency value of the inverter is crucial during its use. This article provides the following aspects to consider as a basis for correctly selecting the carrier frequency value.

2. Carrier Frequency and Inverter Power Consumption

The power loss of the IGBT power module is related to the carrier frequency. As the carrier frequency increases, the power loss increases, leading to decreased efficiency and increased heat generation in the power module, which is detrimental to long-term operation. Of course, the higher the inverter's operating voltage, 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 requires a high carrier frequency and the ambient temperature is also high, it is very detrimental to the power module. Therefore, for inverters of different power ratings, the allowable constant output current of the inverter should be appropriately reduced depending on the carrier frequency used and the ambient temperature to ensure the safe, reliable, and long-term operation of the IGBT power module.

4. Carrier Frequency and Motor Power

For motors with higher power, a relatively lower carrier frequency should be selected to reduce interference (impact on other equipment) and lower power consumption and heat generation. This principle is generally followed, but specific values may vary between manufacturers.

5. Relationship between Carrier Frequency and the Length of the Secondary Output Lines (U, V, W) of the Inverter 

Generally, as the output line length increases, the leakage current of the inverter will increase. Therefore, if the output line is relatively long, the carrier frequency should be appropriately reduced.

Carrier Frequency: 15kHz 10kHz 5kHz 1kHz

Line Length: <50m >50~100m >100~150m >150~200m

6. The Influence of Carrier Frequency on Inverter Output Current

As is well known, the inverter (DC/AC conversion) section of an inverter uses IGBTs to generate a sinusoidal current waveform through the motor windings via SPWM (Sinusoidal Pulse Width Modulation). Therefore, the carrier frequency directly affects the quality of the current waveform and the magnitude of interference. The carrier frequency is quite sensitive and direct; therefore, during operation, the correct carrier frequency value must be selected first, followed by consideration of adding various harmonic suppression devices, such as AC reactors, DC reactors, filters, zero-sequence reactors, and installation wiring and grounding measures. This approach is more reasonable and effective; it is crucial to avoid reversing the order of importance in problem-solving.

When the carrier frequency is high, the current waveform exhibits good sinusoidal characteristics and is smooth. This results in lower harmonics and less current interference, and vice versa. 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-order harmonic components of the current; that is, the higher the carrier frequency, the smoother the current waveform.

(2) The higher the carrier frequency, the smaller the allowable output current of the inverter.

(3) The higher the carrier frequency, the smaller the capacitive reactance of the wiring capacitor (because Xc=1/2πfC), and the larger the leakage current caused by high-frequency pulses.

7. The Influence of Carrier Frequency on the Motor

The higher the carrier frequency, the smaller the motor vibration, the lower the operating noise, and the less heat generated by the motor. However, the higher the carrier frequency, the higher the frequency of the harmonic current, 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

Motor noise originates from three sources: ventilation noise, electromagnetic noise, and mechanical noise. Ventilation and mechanical noise will not be discussed here; only the impact of frequency converters on electromagnetic noise will be analyzed:

The output voltage and current of the frequency converter contain a certain amount of high-order harmonics, which increases the high-order harmonic flux in the motor's air gap, thus increasing noise. Its characteristics are:

(1) The lower high-order harmonic components output by the frequency converter resonate with the rotor's natural frequency, increasing noise near the rotor's natural frequency.

(2) The high-order harmonics output by the frequency converter cause resonance in the core, housing, bearing housing, etc., increasing noise near the natural frequency.

(3) Noise is directly related to the carrier frequency; higher carrier frequencies result in lower relative noise.

(4) Tests show that when the motor operates at a frequency converter, the noise is only 2dB higher than when operating at the mains frequency of 50Hz, indicating a relatively small impact; the absolute value is approximately 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

Motor vibration can be divided into electromagnetic and mechanical causes. Here, we will only analyze the electromagnetic causes, excluding mechanical causes:

(1) Due to the resonance between lower-order harmonic components and the rotor, the vibration components near its natural frequency increase.

(2) Vibration occurs due to the pulsating torque generated by higher-order harmonics.

(3) When using a frequency converter, the vibration is slightly greater at the same 50Hz frequency, especially at 20Hz where the full amplitude increases to 7µm, and at 80Hz~120Hz where the full amplitude increases to 6µm. The vibration is slightly more severe for motors with fewer poles than those with more poles.

(4) An output AC reactor can be used to reduce vibration.

(5) Lower the V/F setting.

(6) Using a variable frequency motor can reduce vibration. (7) For high-speed grinding machines, low-noise, low-vibration dedicated motors can be used.

7.3 Carrier Frequency and Motor Heating

Since the inverter uses sinusoidal pulse width modulation, its current output waveform is approximately sinusoidal, inevitably generating a certain amount of higher harmonics and resulting in an uneven waveform with glitches. This will cause an increase in output current of up to 10%. Since heat generation is proportional to the square of current I, under the same operating frequency and load, the motor temperature rise is slightly higher after using a frequency converter. To minimize this loss, the carrier frequency should be as high as possible for better operation, or a variable frequency motor should be selected. Specific solutions include:

(1) Using a higher carrier frequency to improve the output current waveform.

(2) Installing input and output AC reactors or active filters, etc.

(3) Selecting a variable frequency motor. (4) The inverter's operating frequency should be below 20Hz, while the production equipment requires low speed and operates under heavy load. Therefore, an additional speed reducer should be installed after the motor output shaft to increase the operating frequency (inverter) and output torque. This allows for a unified and reasonable solution considering load requirements, inverter specifications, and factors such as motor vibration, noise, heat generation, operating frequency, and carrier frequency. 

8. Carrier Frequency and Inverter Input Three-Phase Current Imbalance

The inverter's input section 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 inverter. Especially when there is a significant imbalance in the input voltage, for example, a difference of 3-5%, the maximum difference in the three-phase input current can reach 10-20%, which is frequently observed. To improve and minimize the three-phase input current imbalance, the following methods are typically used:

(1) Improve the power grid quality to minimize its imbalance.

(2) Select a high-end, high-quality inverter.

(3) Increase the carrier frequency value as much as possible.

(4) Reverse the phase sequence of the R, S, and T phases (the phase of the inverter input voltage does not need to be considered).

(5) Select a variable frequency motor.

The principle is to minimize the three-phase imbalance using the above methods. Absolute balance is difficult to achieve. However, the three-phase output current of the inverter is basically balanced. It is important to note that when measuring the input or output voltage and current of the inverter, it is best to use a voltage and current meter, clamp meter, or universal meter that only reflects the fundamental frequency (50Hz) with filtering. Otherwise, the measured value will be larger than the actual value. This point should also be noted.

9. Carrier Frequency and Electromagnetic Interference

The higher the carrier frequency, the more severe the interference to electronic equipment from high-frequency circuits through electrostatic induction, electromagnetic induction, and electromagnetic radiation.