VFD Drives

By Paul Wright, P.Eng.

VFD drives or variable speed drives, or AC drives, or adjustable frequency drives, have become ubiquitous in applications that use AC motors. Originally designed in the 1980s to reduce energy consumption on centrifugal fans and pumps, VFDs have since found their way into a wide range of production equipment. With improved reliability, VFDs now provide better motor speed control, reducing system downtime and extending the life between maintenance shutdowns.


Figure 1. VFDs replace Inlet & Outlet Dampers and Throttling Valves


Variable frequency drives, also known as VFD drives or AC drives, have become ubiquitous in applications that use AC motors. Originally designed in the 1980s to reduce energy consumption on centrifugal fans and pumps, VFDs have since found their way into a wide range of production equipment. With improved reliability, VFDs now provide better motor speed control, reducing system downtime and extending the life between maintenance shutdowns.

VFDs have several benefits, including unlimited starting without overheating the motor, electrical and mechanical soft starting, unlimited full speed drive  direction changes, longer equipment life, less wear and tear, and the ability to operate at speeds faster than 60 Hz, increasing production rates. Additionally, VFDs eliminate water hammer in pumping systems, increase the life of pump impellers, and convert AC to DC link.

The benefits of VFD drives are not just limited to energy savings. They also offer a wide range of features and benefits that can enhance the performance and efficiency of industrial applications. Some of the other advantages of VFD drives include:

  • Unlimited starting without overheating the motor
  • Electrical and mechanical soft starting, with no significant inrush current or line voltage drop during starts
  • Mechanical soft starting that increases life for gearboxes and belt drives
  • Unlimited speed direction changes
  • Motor operation above 60 Hz, increasing production rates
  • Longer equipment life with less wear and tear
  • Elimination of water hammer in pumping systems
  • Increased life of pump impeller, with less wear at reduced speed
  • Conversion of AC to DC link
  • Phase converter control
  • Output frequency control
  • Improved sine wave
  • Better pulse width modulation



VFDs offer several ways to control motor speed, including single phase input converter control, output frequency control, improved sine wave, better pulse width modulation, and variable torque control. They can also vary the frequency and voltage of an AC motor, providing more precise control.

Despite being the most expensive method for starting a motor, there is no application of an AC motor that is not suitable for VFD operation. The benefits that VFDs provide for a given application justify the upfront cost.

For example, a user had a gearbox on a fixed speed application that was wearing out over several years of operation. The teeth on the gears were becoming brittle and started to chip during each start. The gear box required an overhaul, but the new set of gears had a six-month delivery time frame. The user purchased a VFD to provide for a mechanical soft start to reduce the across the line starting stress of the gears and reduce further damage until the replacement gear components arrived. After the gears were replaced, the user decided to leave the VFD in place to potentially delay or prevent a future gear box failure.

In another example, a fixed speed centrifugal centrifuge had been operating successfully for several years until the induction motor failed. The user replaced the motor with a standard motor from a local distributor, but the motor overheated upon the first start. The new motor did not produce enough constant torque to accelerate the high inertia load before it overheated and tripped out. The user had a VFD for a different application and temporarily installed it until the proper replacement motor arrived. Upon the first start, the user found that there was no significant heating in the motor, and the motor full load current draw was 15 amperes lower than the original motor. The user decided to keep the VFD in place because of the soft starting of the VFD and the energy saving of not using a motor 250% oversized for the application. The VFD also provided faster stopping, reducing the time of the high unbalance that caused the centrifuge to violently shake, reducing equipment damage and personnel safety risk.

Some customers have inherited VFDs on what they perceive to be fixed speed applications only and question why the consultant specified the VFD for this application. The reasons may include the VFD's ability to prevent motor overheating on starting, increase motor insulation life to 20 years instead of 10-12 years, start the motor any time without causing under frequency and voltage power dips, prevent belt slippage on starting, offer programmable ramp-up time for a very slow increase of air pressure, eliminate the cost of a mechanical damper and the reduction in air flow efficiency, and slow down the motor when full air flow is not required rather than just stopping the motor.

In summary, VFDs offer several benefits, including precise control over motor speed, improved energy efficiency, and extended equipment life. They are suitable for any application of an AC motor and can justify their upfront cost through the benefits they provide.

What are VFD drives, and how do they work?

VFD drives, also known as variable frequency drives or AC drives, are electronic devices used to control the speed and torque of an AC motor by varying the frequency and voltage supplied to the motor. They work by converting the AC voltage supplied by the power source into a DC voltage and then converting it back into an AC voltage with variable frequency and voltage levels. This allows for precise control of the motor's speed and torque.

What are the benefits of using VFD drives in industrial applications?

There are many benefits of using VFD drives in industrial applications, including:

  • Energy savings
  • Increased motor life and reduced maintenance costs
  • Better process control and product quality
  • Increased productivity and throughput
  • Reduced wear and tear on mechanical components
  • Improved safety and reduced downtime
  • How do you select the right VFD drive for your application?
  • When selecting the right VFD drive for your application, there are several factors to consider, including:
  • Motor type and power conversion rating
  • Environmental conditions and installation location
  • Required control functions
  • System voltage and frequency
  • Required overload capacity and protective features
  • Budget and cost-effectiveness
  • What are some common problems with VFD drives, and how can you troubleshoot them?
  • Some common problems with VFD drives include:
  • Overheating and thermal shutdown
  • Motor starting issues
  • Electrical interference and noise
  • Fault codes and alarms
  • Voltage and frequency fluctuations

To troubleshoot these issues, it is important to consult the manufacturer's manual and follow their troubleshooting steps. In addition, it is important to regularly maintain the VFD drive and its components to prevent issues from arising.

How can VFD drives improve energy efficiency in industrial applications?

VFD drives can improve energy efficiency in industrial applications by reducing the amount of energy consumed by the motor. By varying the frequency and voltage supplied to the motor, VFD drives can match the motor's speed and torque to the specific needs of the application, resulting in less wasted energy and improved overall efficiency.

What safety precautions should you take when working with VFD drives?

When working with VFD drives, it is important to follow these safety precautions:

  • Wear appropriate personal protective equipment (PPE)
  • Turn off power to the VFD drive and all associated equipment before working on it
  • Follow proper lockout/tagout procedures
  • Be aware of electrical hazards and follow safe electrical work practices
  • Follow proper installation and grounding procedures
  • Do not exceed the VFD drive's rated capacity or specifications
  • What are some best practices for maintaining VFD drives?
  • Some best practices for maintaining VFD drives include:
  • Regularly inspecting the VFD drive and its components for signs of wear or damage
  • Keeping the VFD drive clean and free of debris
  • Following proper grounding and installation procedures
  • Regularly testing the VFD drive's protective features, such as overcurrent and overvoltage protection
  • Following the manufacturer's recommended maintenance schedule and procedures
  • Training personnel on safe and proper operation and maintenance of VFD drives.