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Post at 17 May 2024

What is VFD?

VFD stands for Variable Frequency Drive. It’s basically an electronic device that controls the speed of AC electric motors. They work by changing the frequency of the electricity that the motor receives. This allows the motor to run at different speeds, which can be useful for a variety of applications.

Here are some of the common names for VFDs:

  • Adjustable-speed drive
  • Adjustable frequency drive
  • AC drive
  • Micro drive
  • Inverter drive
  • Drive

VFDs are used in a wide range of applications, including:

  • Industrial fans and pumps
  • Conveyor belts
  • Machine tools
  • Textile machinery
  • HVAC systems (heating, ventilation, and air conditioning)

How VFD works

A VFD controls an AC motor’s speed by manipulating the incoming AC power through a series of stages:

  1. Rectification: The VFD first converts the incoming fixed-frequency AC voltage to DC voltage using a rectifier circuit. This rectifier acts like a one-way valve, allowing current to flow only in one direction.
  2. DC Bus: The raw DC voltage from the rectifier isn’t perfectly smooth. The DC bus stage, which includes capacitors and inductors, filters out the fluctuations in the DC voltage, creating a smoother and more stable DC supply.
  3. Inversion: Here’s where the magic happens! The inverter section takes the smooth DC voltage and converts it back into AC voltage. But unlike the fixed-frequency AC you started with, the inverter can control both the frequency and voltage of the output AC.
  4. Output to Motor: The inverter’s output, a variable-frequency AC voltage, is what ultimately powers the motor. By adjusting the frequency of this AC output, the VFD controls the speed of the motor. There’s a relationship between the frequency and the speed: a higher frequency translates to a faster motor speed.

Here’s an analogy: Imagine a cyclist pedaling a bike. The faster the cyclist pedals (higher frequency), the faster the wheels turn (motor speed). The VFD acts as the control system, adjusting the pedaling rate (frequency) to achieve the desired bike speed (motor speed).

VFD categorize

Mechanical VFDs:

  • These adjust speed through mechanical means, like changing the diameter of pulleys or using hydraulics. They are less common than electrical VFDs. Here are some sub-types of mechanical VFDs:
    • Variable pitch drive: Uses adjustable pulleys to change speed.
    • Traction drive: Adjusts the diameter of contacting rollers for variable speed.
    • Hydraulic hydrostatic drive: Uses a variable displacement pump to control motor speed.

Electrical VFDs

  • These are the most common type and use electronic components to control the AC power going to the motor. They achieve this by converting AC to DC and then back to AC with variable frequency. There are three main types of electrical VFDs:
    • Voltage Source Inverter (VSI): The most common electrical VFD, it uses a DC voltage source and inverter to control the output AC frequency.
    • Current Source Inverter (CSI): Maintains a constant current output to the motor, offering simplicity and effectiveness for specific applications.
    • Pulse Width Modulation (PWM) Drives: Manage both frequency and voltage for AC motors, creating a more precise wave-like current output for superior performance.

Benefits of VFD

Energy Savings: VFDs are champions of energy efficiency. By precisely controlling the motor speed to match the application’s needs, they significantly reduce energy consumption compared to running motors at full speed all the time. This translates to lower electricity bills and a positive environmental impact.

Soft Starts and Stops: Unlike fixed-speed operation where motors experience a sudden jolt when starting and stopping, VFDs offer a smoother experience. They can gradually ramp up the motor speed during startup and slow it down during stopping, minimizing wear and tear on the motor and mechanical components connected to it. This extends the lifespan of your equipment and reduces maintenance costs.

Precise Speed Control: VFDs excel at providing very precise control over motor speed. This is crucial for many industrial applications where consistent and accurate speed is essential for product quality and process efficiency. Imagine a conveyor belt in a factory. A VFD allows for fine-tuned control of the belt speed, ensuring optimal product movement without damage.

Improved Process Control: The ability to precisely control motor speed using VFDs leads to better overall process control in various industries. For instance, in a chemical processing plant, a VFD can regulate the speed of pumps to ensure precise flow rates of liquids, leading to consistent product quality.

Reduced Torque Fluctuations: VFDs can help minimize torque fluctuations experienced by motors during operation. This smoother torque delivery translates to improved machine performance and reduced strain on mechanical components.

Additional Advantages:

  • Extended motor life: Reduced wear and tear due to soft starts and stops contribute to a longer lifespan for your motors.
  • Lower noise levels: Smoother operation often leads to quieter operation of motors equipped with VFDs.
  • Versatility: VFDs are adaptable to a wide range of applications across various industries.

Overall, VFDs offer a compelling value proposition by delivering energy savings, improved process control, extended equipment life, and a range of other benefits. They are a worthwhile investment for many applications that require precise control of AC motor speed.

Disadvantages of VFD

While VFDs (Variable Frequency Drives) offer significant advantages, there are also some potential drawbacks to consider before implementation:

  • Higher Initial Cost: VFDs typically have a higher upfront cost compared to simple on/off controls for AC motors. However, the energy savings and extended equipment life they provide can often offset this initial investment over time. 
  • Complexity: VFDs can be more complex to install and configure compared to basic motor controls. They may require specialized knowledge or assistance from a qualified electrician or VFD expert for proper setup and programming. 
  • Harmonics: The way VFDs convert AC power to variable frequency can introduce harmonic currents into the electrical system. These harmonics can potentially disrupt the operation of other sensitive electronic equipment. Mitigation strategies, such as harmonic filters, may be necessary depending on the specific application and electrical setup. 
  • Heat Generation: VFDs generate heat during operation due to the power conversion process. Proper ventilation and heat dissipation considerations are crucial to ensure optimal performance and lifespan of the VFD. 
  • Potential Motor Derating: In some cases, using a VFD with a specific motor may require derating the motor’s horsepower rating. This means the motor may not be able to deliver its full rated power when controlled by the VFD. Consulting the VFD and motor manufacturer’s specifications is important to ensure compatibility and avoid exceeding the motor’s capabilities. 
  • Electromagnetic Interference (EMI): VFDs can generate electromagnetic interference (EMI) that may interfere with the operation of nearby electronic devices. Proper grounding and shielding techniques may be required to minimize EMI issues. 
  • Maintenance Considerations: While VFDs generally require less maintenance than traditional motor controls due to reduced wear and tear on motors, they still have electronic components that may require periodic inspection and maintenance. 

By carefully weighing the advantages and disadvantages of VFDs in the context of your specific application, you can make an informed decision about whether they represent a worthwhile investment. If the benefits of precise speed control, energy savings, and extended equipment life outweigh the potential drawbacks, then VFDs can be a valuable addition to your system.

Discover: What is the Difference Between VFD and Servo Drives?

Applications of VFD

VFDs (Variable Frequency Drives) have a wide range of applications across various industries due to their ability to precisely control the speed and torque of AC motors. Here are some of the most common applications:


  • Material Handling: VFDs are extensively used in conveyor belt systems to control the speed of material movement, optimizing production flow and reducing energy consumption.
  • Manufacturing: In machine tools, VFDs ensure precise speed control for cutting, drilling, and other processes, leading to improved product quality and efficiency.
  • HVAC Systems: VFDs regulate the speed of fans and pumps in heating, ventilation, and air conditioning systems, allowing for efficient temperature control and reduced energy use.
  • Chemical and Oil & Gas: VFDs precisely control pumps for precise flow rates of liquids in various processes within these industries.

Other Applications:

  • Water and Wastewater Treatment: VFDs manage the speed of pumps for water treatment and wastewater processing, optimizing energy use based on demand.
  • Building Automation: VFDs control ventilation fans and pumps in buildings, leading to improved comfort and energy savings.
  • Agriculture: VFDs regulate irrigation pumps for efficient water usage and precise control over crop watering.
  • Renewable Energy: VFDs are crucial components in wind turbine systems, controlling the speed of the generator based on wind conditions to optimize power generation.

These are just a few examples, and VFDs can be found in many other applications where precise control of motor speed and torque is necessary. Their ability to save energy, improve process efficiency, and extend motor life makes them a valuable tool across various industries.

Common VFD Errors and Troubleshooting Tips

Variable Frequency Drives (VFDs) offer significant benefits, but like any equipment, they can encounter errors. Here’s a breakdown of some common VFD errors and troubleshooting approaches:

  1. Ground Fault:
  • Error: This error indicates a potential issue with the grounding of the VFD or motor.
  • Possible Causes: Loose grounding connections, damaged grounding wire, internal motor grounding fault.
  • Troubleshooting: Tighten all grounding connections, inspect the grounding wire for damage, and ensure proper grounding of the motor using a qualified electrician. If the issue persists, consult the VFD or motor manufacturer’s service manual for further troubleshooting steps.
  1. Overcurrent Fault:
  • Error: This error signifies that the motor is drawing more current than the VFD is rated for.
  • Possible Causes: Mechanical overload on the motor (e.g., seized bearing, jammed conveyor), incorrect VFD settings for motor parameters, faulty motor windings.
  • Troubleshooting: Check for any mechanical issues causing excessive load on the motor. Verify the VFD settings match the motor’s nameplate data (voltage, current, horsepower). If the problem persists, consider reducing the motor load or consult a qualified electrician to diagnose potential motor issues.
  1. Undervoltage Fault:
  • Error: This error indicates that the incoming AC voltage to the VFD is lower than the minimum required for operation.
  • Possible Causes: Power supply voltage fluctuations, loose connections in the power supply wiring.
  • Troubleshooting: Check the power supply voltage and ensure it meets the VFD’s specifications. Tighten all connections in the power supply wiring. If the issue is due to external power fluctuations, consider voltage regulation equipment to stabilize the power supply.
  1. Overvoltage Fault:
  • Error: This error signifies that the incoming AC voltage to the VFD is higher than the maximum allowable voltage.
  • Possible Causes: Power supply voltage spikes, faulty voltage regulator in the VFD.
  • Troubleshooting: Similar to undervoltage fault, check the power supply voltage. If experiencing voltage spikes, consult an electrician to diagnose the source of the problem. In rare cases, a faulty voltage regulator within the VFD may be the culprit, requiring service by a qualified technician.
  1. Communication Error:
  • Error: This error indicates an issue with communication between the VFD and other control systems (e.g., PLC).
  • Possible Causes: Incorrect communication settings (baud rate, protocol), loose or damaged communication cables.
  • Troubleshooting: Double-check the communication settings in both the VFD and the control system to ensure they match. Inspect the communication cables for any damage or loose connections. Refer to the VFD and control system manuals for specific communication setup instructions.

General Tips:

  • Consult the VFD Manual: The VFD manufacturer’s manual provides specific error codes and troubleshooting steps for that particular model.
  • Check Control Panel Displays: VFDs often display error codes or messages on their control panels, providing valuable clues about the issue.
  • Reset the VFD: Sometimes, a simple reset of the VFD can resolve temporary glitches. Refer to the manual for the proper reset procedure.
  • Seek Professional Help: For complex errors or if troubleshooting steps are unsuccessful, consider contacting a qualified electrician or VFD service technician for further assistance.

Remember, these are just some common VFD errors. The specific troubleshooting approach may vary depending on the error code displayed and the VFD model you’re using. By following these tips and consulting the manufacturer’s resources, you can effectively address many VFD errors and keep your system running smoothly.

How to choose the right VFD for your application?

Choosing the right VFD (Variable Frequency Drive) requires careful evaluation to ensure optimal performance, compatibility, and value for your specific application. Here’s a breakdown of key factors to consider when selecting a good VFD:

Power Supply Compatibility:

  • Match Voltage and Phase: The VFD’s input voltage and phase configuration must be compatible with your available power supply. Common options include 200V/3-phase/60Hz or 400V/3-phase/50Hz.

Motor Characteristics:

  • Match Motor Nameplate Data: Refer to the information plate on your motor for crucial details like voltage, current, horsepower (or kW), and frequency. The VFD’s output voltage and current rating must be equal to or slightly exceed the motor’s requirements for continuous operation. A mismatch can lead to overloading and potential damage.

Application Requirements:

  • Torque Considerations: Understand the torque profile your application demands. Constant torque applications like conveyors might function with a standard VFD. For variable torque loads like fans and pumps, a VFD sized for the peak torque is recommended to handle fluctuations.

Motor Type Compatibility:

  • AC Induction Motors: Most VFDs are compatible with standard AC induction motors. However, verify compatibility if you have a permanent magnet synchronous motor (PMSM) or brushed DC motor, as they may require specific VFD types.

Control Features:

  • Basic vs. Advanced: Consider the level of control needed. Basic VFDs offer simple speed control, while advanced models provide features like programmable acceleration/deceleration ramps, communication protocols for integration with other control systems, and motor protection functionalities.

Environmental Conditions:

  • Choose Appropriate Enclosure: Select a VFD with an enclosure rating suitable for the environment it will be installed in. Consider factors like dust, moisture, or extreme temperatures that could affect the VFD’s performance and lifespan.

Additional Considerations:

  • Manufacturer Reputation: Choose a VFD from a reputable manufacturer known for quality, reliability, and technical support.
  • Future Expansion: If you anticipate future motor upgrades, consider a VFD with some headroom in terms of power capacity to accommodate potential growth.
  • Cost vs. Value: While initial cost is a factor, consider the long-term benefits of a VFD. Energy savings, extended motor life, and improved process control can offer significant cost savings over time.

Here are some additional tips for evaluating a good VFD:

  • Consult a VFD Expert: For complex applications, seeking guidance from a VFD manufacturer representative or a qualified distributor can ensure you select the most suitable model for your specific needs. They can provide expert advice on factors like compatibility, sizing, and programming.
  • Research and Compare VFD Options: Explore different VFD models from various manufacturers, comparing their features, specifications, and pricing. Online resources, manufacturer websites, and industry publications can be valuable sources of information.
  • Read User Reviews: Checking online reviews from other users can provide insights into the real-world performance, ease of use, and reliability of different VFD models.

By carefully evaluating these factors and conducting thorough research, you can make an informed decision and select a good VFD that delivers optimal performance, efficiency, and value for your application.

Rockwell Automation Brand: Allen-Bradley VFD

Rockwell Automation is a leading global provider of industrial automation and digitalization solutions. Within their portfolio, the Allen-Bradley brand is a powerhouse for industrial automation products, including Variable Frequency Drives (VFDs).

Here’s a breakdown of Rockwell Automation’s Allen-Bradley VFDs:

  • Brand Recognition: Allen-Bradley is a well-established and respected brand in the industrial automation sector. Choosing an Allen-Bradley VFD provides confidence in quality, reliability, and support.
  • Product Range: Rockwell Automation offers a comprehensive range of Allen-Bradley VFDs to cater to various application needs. These VFDs range from basic and user-friendly models for simple applications to feature-rich options for complex industrial processes.
  • PowerFlex Family: The Allen-Bradley brand uses the PowerFlex name for their VFD product line. Popular PowerFlex VFD series include:
    • Maximize Performance with Allen-Bradley® PowerFlex® Drives

      The PowerFlex® family of AC and DC drives delivers solutions that directly impact your bottom line. We offer a versatile range of control options, from basic speed regulation to advanced torque and speed control. This flexibility, combined with industry-standard motor control, empowers you to optimize any application and achieve peak productivity.

      Here’s what sets PowerFlex® apart:

      • Wide Range of Control Technologies: We offer a variety of control options to perfectly match your application’s needs.
      • Enhanced Application Flexibility: Unique control features within the PowerFlex® family allow you to tailor performance for your specific needs.

      By choosing PowerFlex® drives, you gain the tools to optimize your operation and achieve lasting success.

      PowerFlex 4M is the most cost-effective of the PowerFlex family of drives and features feed-through wiring and Zero-Stacking (TM) for ambient temperatures

      PowerFlex 400 is ideal for pump and fan applications. Built-in features such as purge and damper input provide a cost-effective solution for speed control in a broad range of variable torque fan and pump applications.

      PowerFlex 520-series combines powerful performance with flexible control and focuses on ease of use. Each of the three drives in this family offers a unique set of features to distinctively match the needs of your application.

      PowerFlex 755 AC drives include multiple control, hardware, and safety options. Built-in EtherNet/IP delivers real time operating data and easily integrates into the Logix control system. Ideal for applications requiring position, speed, or torque control up to 1500 kW/2000 Hp. DC input options available.

      PowerFlex 753 AC drives feature standard built-in I/O plus three options slots for communications, safety, and additional I/O.

      PowerFlex 755TS Driveincorporates TotalFORCE technology into a traditional six-pulse drive. TotalFORCE can now be used in a wider range of applications including fan, pump, and conveyor applications to more advanced motor control processes that require high performance features typically found in specialized drive solutions. PowerFlex 755TS offers real-time operational intelligence, automated commissioning, and optimization.

      PowerFlex 755TL Drive Provides harmonic mitigation and power factor correction by using active front end technology. By reducing the adverse effects of harmonic distortion, the drive helps to improve energy efficiency, reduce energy costs and minimize power distribution issues on the factory floor.

      PowerFlex 755TR Drive- Features built-in regeneration capability that helps decrease energy consumption by delivering regenerative energy from motors back to the incoming supply. Line regeneration reduces the need for braking resistors and associated cooling equipment and helps avoid wasteful dissipation of energy. This drive also offers harmonic mitigation.

      PowerFlex 755TM Drive System -Select from a series of predesigned configurations for regenerative, and non-regenerative common bus supplies and common bus inverters to optimize your system design and power consumption. A common bus drive system offers advantages such as design flexibility, energy optimization and reduced installation costs. PowerFlex 755TM systems provide harmonic mitigation and built-in regeneration capability.

      PowerFlex 755T Drives Configured to Order – Our configured to order drives are factory delivered for PowerFlex 755TL, TR, and TM drives in frames 7…10. Packaging options include NEMA Type 1 and 12 enclosures. Options include: input circuit breaker and/or fuses, door-mounted push buttons and selector switches, optional doormounted HIM, and output contactor and/or sine wave (DV/DT) filter.

      Armor PowerFlexVFD – you need for On-Machine in One Drive

      • Integral Ethernet switch, safety, & security
      • Easy configuration using Studio 5000 AOP
      • Lower your design and installation costs
      • VFD control is based on the PF525
      • Improve OEE with ADC & predictive functions

      PowerFlex® 6000TCommon control architecture across low voltage and medium voltage drives

      Common control hardware architecture, firmware, and network interface software with PowerFlex TotalFORCE® drives like PowerFlex 755T

      • Reduce integration, operation, and support costs
      • Optimized performance with flexible motor control modes for Induction Motors• Streamlined design and commissioning experience
      • Device health optimization with predictive and built-in maintenance alerts, maximizing availability
  • Integration and Support: Rockwell Automation offers a strong ecosystem of complementary products from Allen-Bradley, including motors, human-machine interfaces (HMIs), and programmable logic controllers (PLCs). This allows for seamless integration and simplifies system design. Additionally, Rockwell Automation provides comprehensive technical support and resources for their Allen-Bradley VFDs.

Servo Dynamics: Your Trusted Partner for Allen-Bradley in Vietnam

Established in 2006, Servo Dynamics has become a leading provider of industrial automation and control solutions in Vietnam. As an authorized distributor for Rockwell Automation, a global giant in industrial automation, Servo Dynamics offers the complete range of Allen-Bradley products, including their industry-renowned Variable Frequency Drives (VFDs).

With over 18 years of experience serving the Vietnamese market, Servo Dynamics possesses unmatched expertise in Allen-Bradley technologies. Their team of qualified engineers provides comprehensive support, from product selection and application design to installation, commissioning, and after-sales service.

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