VFD Carrier Frequency Explained – Noise, Efficiency & Motor Heating
Carrier frequency is one of the most important yet often misunderstood settings inside a variable frequency drive (VFD). It plays a major role in how quietly the motor runs, how efficiently the drive operates, and how much heat is generated in both the motor and inverter.
If you are commissioning a new drive, troubleshooting motor noise, or trying to optimise performance, understanding carrier frequency can help you get better results from your motor control system.
In this guide, we explain what VFD carrier frequency is, how it affects noise, efficiency, and motor heating, and how to choose the right setting for your application.
If you are sourcing new drives, you can browse our full range of inverter drives, including reliable Allen Bradley inverter drives such as the PowerFlex 40 series and the modern PowerFlex 523 drives.
What Is VFD Carrier Frequency?
Carrier frequency refers to the switching frequency used by a VFD when converting DC voltage into a pulse width modulated (PWM) waveform to drive an AC motor. In simple terms, it determines how quickly the inverter’s power transistors switch on and off to create the output voltage supplied to the motor.
This switching action happens thousands of times per second. The carrier frequency is usually measured in kilohertz (kHz) and typically ranges between about 2 kHz and 16 kHz depending on the drive model.
Although the motor ultimately sees a smooth average voltage, the switching behaviour has a significant impact on motor noise, electrical losses, and heat generation inside the drive and motor.
Why Carrier Frequency Matters
The carrier frequency setting inside a VFD influences three main things:
- Motor noise levels
- Drive efficiency and heat generation
- Motor heating and insulation stress
Because of this, choosing the correct carrier frequency is often a balancing act between quieter operation and maintaining good thermal performance.
How Carrier Frequency Affects Motor Noise
One of the most noticeable effects of carrier frequency is motor noise. When a motor is driven by a VFD, the switching waveform can create audible electrical noise commonly described as a high-pitched whine.
This happens because the PWM switching produces small torque ripple and magnetic vibration in the motor.
Low Carrier Frequency
At lower carrier frequencies (for example 2–4 kHz), switching noise tends to fall within the audible range of human hearing. This often produces a noticeable whining sound from the motor.
While this is electrically efficient, it may not be desirable in environments where noise reduction is important.
High Carrier Frequency
Increasing the carrier frequency shifts the switching noise above the audible range, which makes the motor sound significantly quieter.
This is why many installations increase the carrier frequency in offices, laboratories, food processing facilities, and other noise-sensitive environments.
How Carrier Frequency Affects Drive Efficiency
Higher carrier frequency improves acoustic performance but increases switching losses inside the drive.
Every time the inverter’s IGBTs switch on and off, a small amount of energy is lost as heat. When switching happens more frequently, those losses increase.
This means that:
- Lower carrier frequency generally improves drive efficiency and reduces inverter heating.
- Higher carrier frequency increases switching losses and raises internal drive temperature.
For this reason, some drives automatically reduce carrier frequency under heavy load to protect internal components.
How Carrier Frequency Affects Motor Heating
Carrier frequency can also influence motor temperature, particularly with older motors that were not originally designed for inverter operation.
Higher switching frequencies can increase electrical losses within the motor due to additional harmonic currents and insulation stress.
While modern inverter-rated motors are designed to handle this better, it is still important to monitor motor temperature if higher carrier frequencies are used.
Excessive motor heating may occur if:
- The carrier frequency is set too high
- The motor cable length is very long
- The motor insulation is not inverter rated
- The motor is already operating near its thermal limit
Typical Carrier Frequency Ranges
Although the exact values vary by manufacturer and drive model, the following ranges are typical in many industrial VFDs:
- 2–4 kHz – Lower noise control, highest efficiency
- 4–8 kHz – Balanced setting for many applications
- 8–16 kHz – Quieter motor operation but higher drive heating
Most installations work well with the factory default setting unless noise reduction is required.
When to Increase Carrier Frequency
Increasing carrier frequency can be beneficial in situations where motor noise needs to be reduced.
This is commonly done in applications such as:
- Building ventilation systems
- Laboratory equipment
- Food processing plants
- Packaging machinery near operators
- Office or commercial HVAC systems
In these environments, quieter motor operation often outweighs the slight increase in drive heating.
When to Reduce Carrier Frequency
Lower carrier frequencies are often preferred when thermal performance and drive efficiency are the priority.
This may be the case in:
- High-power industrial installations
- Applications with heavy loads
- Hot ambient environments
- Long motor cable installations
- Situations where inverter cooling is limited
Reducing carrier frequency can help prevent overheating and improve overall reliability.
Carrier Frequency and Cable Length
Long motor cables can increase voltage stress on the motor due to reflected wave phenomena caused by fast switching edges from the VFD.
Higher carrier frequencies may increase this effect, particularly on older motors or installations with long cable runs.
In these cases, engineers may reduce the carrier frequency or install output filters to protect the motor insulation system.
Carrier Frequency on Allen Bradley PowerFlex Drives
Many industrial systems use Allen Bradley drives because they provide flexible parameter control and strong reliability.
Popular ranges such as the PowerFlex 40 and PowerFlex 523 allow users to adjust carrier frequency settings through simple drive parameters.
This makes it easy to balance motor noise and drive efficiency depending on the application requirements.
If you are selecting new equipment, you can explore our full range of inverter drives including industrial-grade Allen Bradley inverter drives suitable for a wide range of motor control applications.
Tips for Choosing the Right Carrier Frequency
Choosing the correct carrier frequency does not need to be complicated. In most cases, the best approach is to start with the manufacturer’s default value and adjust only if needed.
Some useful guidelines include:
- Start with the default carrier frequency recommended by the drive manufacturer.
- Increase frequency if motor noise is a problem.
- Reduce frequency if the drive or motor is running too hot.
- Monitor motor current and temperature when changing settings.
- Avoid unnecessarily high switching frequencies in heavy-duty industrial systems.
Common Carrier Frequency Mistakes
Although the parameter is simple to change, incorrect settings can sometimes cause unexpected issues.
Common mistakes include:
- Setting carrier frequency too high without considering drive cooling
- Ignoring motor temperature after changing settings
- Increasing switching frequency on long cable installations
- Assuming higher frequency always improves performance
Remember that the quietest motor is not always the most efficient or reliable configuration.
Final Thoughts
Carrier frequency is a small parameter that can have a big impact on how a VFD system behaves. By understanding how it influences motor noise, inverter efficiency, and thermal performance, you can fine-tune your drive settings for better overall results.
For many applications, the default setting works perfectly well. However, when noise reduction or thermal optimisation is required, adjusting carrier frequency can help achieve the right balance between performance and reliability.
If you are selecting new equipment or upgrading an existing system, explore our full range of inverter drives, including reliable Allen Bradley inverter drives such as the PowerFlex 40 and PowerFlex 523 ranges.
