What is a VFD?

A VFD is a type of motor controller that drives an electric motor by varying the frequency and voltage of its electrical power supply. The VFD also has the capacity to control ramp-up and ramp-down of the motor during start or stop, respectively.

Even though the VFD controls the frequency and voltage of power supplied to the motor, we often refer to this as speed control, since the result is an adjustment of motor speed.

There are many reasons why we may want to adjust electric motor speed.
For example, to

• Save energy and improve system efficiency
• Convert power in hybridization or electrification applications
• Match the speed, torque or power of the drive to the process requirements
• Improve the working environment
• Lower noise levels, for example from fans and pumps
• Reduce mechanical stress on machines and motor windings to extend their lifetime
• Shave peak consumption to avoid peak-demand prices and reduce the motor size required

Today’s variable speed drives integrate networking and diagnostic capabilities to boost control performance and increase productivity. So: reduced energy consumption, intelligent motor control, and reduction of peak-current drawn are three great reasons to choose a VFD as the controller in every motor-driven system.

• VFDs are most commonly used to control fans, pumps and compressors. These applications account for 75% of all drives operating globally
• Conveyors, centrifuges, cranes, lifts and industrial processing equipment which need precise motion control are also common applications for VFDs
• Electric propulsion systems for electric and hybrid ships are another common use where VFDs contribute with essential power conversion in electrification of our future maritime transport system
• Soft starters and across-the-line contactors are other, less sophisticated types of motor controllers. They are used to protect motors on stopping and starting, in simple applications

  -            A soft starter is a solid-state device and provides a gentle ramp-up to full speed during startup of an electric motor

  -            An across-the-line contactor is a type of motor controller that applies the full line voltage to an electric motor

Explore Danfoss Drives products for more information. We offer both single-phase VFDs and three-phase VFDs

Find out more about soft starters from Danfoss Drives

VFDs trim your power bill.

A variable frequency drive can use VFD control to vary the power supplied to match the energy requirement of the driven equipment, and this is how it saves energy or optimizes energy consumption.

The drive can dramatically reduce energy consumption when compared to direct-on-line (DOL) operation, where the motor runs at full speed regardless of the demand. Using a VFD, speed control does not waste power (unlike regulation with a throttle valve or damper, for example), but instead adjusts the motor power to match exactly the actual demand. Using a variable speed drive, power or fuel savings of 40% are common. The roll-on effect means that the use of drives also reduces NOx emissions and CO2 footprint of the systems in which it is installed.

Case story: Read how VFDs dramatically reduce energy consumption at this Roca Group factory

A widespread use for VFDs is in power conversion for hybrid or electric systems using energy storage. Growth in electrification means power converters are in high demand at sea and on land, for fast charging, DC grids, electrolysis in Power-to-X, smart grids, energy storage and more. 

Explore the wide range of power conversion applications:

• Electrification through power conversion
• Power conversion for shore supply and vessel charging
• Industrial DC backup as an alternative to UPS – ensuring no more power dips
• Case story: How a RoPax ferry in Finland uses VFDs to run on battery power
• Case story: Danfoss iC7-Hybrid powers Everfuel HySynergy Power-to-X project
• Case story: Mobile energy storage units power events in the Netherlands

There are many diverse types of drive available, all widely used in industry. Both single-phase VFDs and three-phase VFDs have evolved to suit a wide variety of applications. The differences in drive types include the methods used for frequency and voltage control; and the ways in which drives mitigate harmonic distortion.

Learn more about harmonic mitigation solutions from Danfoss Drives

The three major variable frequency techniques used in drives are called pulse width modulation (PWM), current source inverter, and voltage source inverter. The PWM technique is the most common. It requires switching the VFD’s inverter power devices – transistors or insulated-gate bipolar transistors (IGBTs) – on and off many times, to generate the proper root-mean-square (RMS) voltage levels. Controlling and varying the width of the pulses is how PWM varies the output frequency and output voltage to achieve VFD control.

Tap into Drives knowledge articles, to learn more about optimizing system efficiency with VFDs

Energy bills are the biggest cost after wages in many industrial companies​. You can reduce your energy bill by 20-60% by installing VFDs in the following applications:​

• HVAC systems or air transfer in data centers, other buildings, car parks, or tunnels​
• Pumps for liquids​ and water management
• Air compressors
• Other compressors on site, such as cold storage​

“If all fans and pumps (globally) were equipped with VFDs, the savings would equal the entire power generation capacity of the EU. By employing drives, GHG emissions can reduce by 40% by 2040.”
Source: OMDIA COP26 global report

Poor or outdated control of driven loads such as fans, pumps, conveyors and compressors is a major source of wasted energy​. Many fan, pump and compressor applications still run direct-on-line (DOL) and operate at full speed. Even new equipment is built to run at fixed, full speed. In 2023, over 74% of new equipment was installed DOL, so there is still plenty of scope for improving energy efficiency. ​

You can typically expect to save 20-60% on the energy bill, by specifying that all fans, pumps, and chillers be fitted with VFDs. For each individual electric motor load, electricity savings due to VFD control can be as high as 80%.

By applying variable speed control to fans and pumps, you can typically achieve energy savings of 40%. This is defined by the variable torque curve. For fans and pumps, a 20% reduction in motor speed results in 50% energy savings. 

Discover the energy savings achieved by fan control in air handling units (AHU), at this factory in Illinois, USA

VFDs also play a crucial role in industrial applications such as cranes and conveyors. In these applications, process optimization and precise speed and position control are the primary reasons for using VFDs. For example, the VFD offers integrated condition monitoring and anti-sway functionality. For conveyors, the VFD is equipped with integrated high-precision positioning control as an alternative to servo solutions. VFD-integrated condition monitoring (CBM) with a remote monitoring service work hand-in-hand not only to improve reliability but also create new benefits in terms of visualization and logging. While VFDs can additionally help to save energy, the savings for conveyors and cranes are unlikely to reach the 40% achieved with fans and pumps.

Learn more about the value of VFDs in conveyor and industrial crane applications

In marine propulsion, electric motors and VFDs are rapidly replacing diesel engines and mechanical systems. Without VFDs, controlling a ship's speed and direction would be impossible.

As well as optimizing energy efficiency, VFDs offer a range of other benefits in marine applications. The advantages include integrated functional safety, easy integration into ship systems, compact control in limited spaces below deck, and improvement of power quality on board.

Many ships still use diesel generators to make electricity. However, fully electric ferries are quickly becoming more common.

Explore our marine case stories to learn how variable frequency control and power conversion optimize vessel performance and reduce emissions, on board and on shore.

Did you know that 26% of existing installed AC motors are fitted with VFDs? And 75% of these motors drive variable torque loads (fans, pumps and compressors). Control of variable torque loads has the potential to save the most energy, in an industrial plant. Investing in variable frequency drive (VFD) technology to control these applications, along with potential utility rebates, is one of the fastest ways to reduce your energy bills and decarbonize your plant.​

Source: OMDIA COP26 global report

Discover our series: Decarbonization Explained

Europe has raised the bar on motor efficiency, with minimum IE4 motor rating mandatory for motors rated 0.75 to 375 kW, from July 2023.
However, upgrading a DOL motor from IE1, IE2, or IE3 to IE4 can actually increase the energy used on a fan or a pump, due to higher slip. Therefore, we recommend installing a VFD on these motor upgrades, to ensure success in reducing system energy consumption.

Source: EU MEPS, EcoDesign regulations Europe

Did you know that 95% of electric motors are oversized or sized for the peak-load scenario? Many existing systems still operate with mechanical constriction (valves or dampers) to adapt the oversizing for partial loading. VFDs can replace the valve control to save a surprising amount on running costs for motor or driven load. And this allows you to reduce the dimensioning of your electric motor at the same time.

When investing in VFDs, the typical return on investment is as short as 6-12 months.
Example: A 7.5 kW pump running 8,000 hrs annually consumes electricity costing $6,000 per year, based on electricity charge of 10 cents per kWh.

• Installing a VFD to reduce motor speed by just 5Hz (10%) delivers 27% savings in electricity consumption: $1,600 savings per annum
• Total savings during lifespan: $40,500
• Payback time: 6-12 months