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What is a VFD? Variable Frequency Drive (VFD,VSD)

What is a VFD

Table of Contents

This article will discuss what a VFD is, how it operates and what its benefits are.

What Is A VFD?

A Variable Frequency Drive (VFD) is an energy conservation system that controls the speed of an electric motor by varying the frequency and voltage of the motor.

Newer, more efficient types of motors draw significant amounts of power when they run at full capacity. A variable frequency drive regulates the flow of electricity instead of just delivering it at one set voltage. Since electricity follows the path of least resistance, the benefits are felt immediately in reduced operating costs and energy consumption.

Why to use VFD?

Commonly, induction motors are supplied from a utility distribution network with a fixed voltage level and constant frequency of 50/60 Hz.

This fact significantly affects the possibility of AC motors speed control, i.e., the fixed voltage and frequency values result in fixed AC motor speed independently on the load connected to the motor’s shaft.

From the efficiency point of view, this could be extremely inconvenient as the motor’s energy (power) consumption is fixed at the rated value even for relatively small loads.

Before VFDs appeared, the only way to control the induction motor’s speed was by changing the rotor’s current (flux).

This method requires additional equipment, i.e., brushes and reels, which require maintenance (extra costs) and significantly decrease the robustness.

Furthermore, this approach is not feasible with the most robust induction motors – squirrel cage induction motors where the rotor’s windings can not be accessed.

For those reasons, some alternative ways of induction motor speed control have been researched for many decades.

Development of the VFD

The development of power electronics and solid-state technology introduces new possibilities related to induction machines’ speed control.

The basic idea was to vary the values of voltage and frequency, which feed the machine, to regulate the shaft’s speed and adjust it to the load requirements.

Following this idea appeared VFDs – Variable Frequency Drives. These devices are also called variable speed drives, adjustable speed drives, adjustable frequency drives, AC drives, micro-drives, and inverters.

The operating principle of VFDs is shortly described in the following paragraphs.

How VFD work?

As stated above, the induction motor speed control could be achieved by varying the voltage and frequency of its power supply.

Considering the fixed voltage and frequency values at the utility connection points, obviously, some additional conversion that will ensure flexibility of voltage and frequency is obligatory.

VFD Charts

Conventional VFDs, as their first component, contain an AC/DC converter that converts AC utility voltage to some defined DC value. AC/DC conversion could be implemented utilizing a variety of typologies.

Primarily, the six-pulse converter converts AC voltage to the appropriate DC value. The six-pulse converter is implemented as a bridge consisting of six diodes (one pair of diodes per phase of a three-phase AC power supply) which conducts the current in only one direction.

At the connection point of the bridge, capacitors usually act as an energy buffer utilized to decrease voltage ripples that occur as a consequence of switching.

The previously obtained DC voltage is converted again back to the desired AC waveform at the second step.

You will probably wonder why it is converting back or why we have performed AC/DC conversion if we still need AC output to feed the motor.

The answer lies in the starting point – we want to vary the frequency and voltage level of the AC voltage used to feed the motor.

Therefore, after we have ensured filtered DC voltage (with reduced voltage peaks) at the outputs of the AC/DC converter, we have to perform another conversion and utilize another converter in the next step.

This, the “second” converter, is in charge of DC/AC conversion and is called an inverter.

Unlike the first AC/DC converter, implemented using diodes (uncontrollable switching components), the implementation of the inverter (DC/AC) has to be based on controllable switching components such as IGBTs (Insulated Gate Bipolar Transistors) or MOSFETs (Metal-oxide–Semiconductor Field-effect Transistors).

Similar to the AC/DC converter, the inverter is also implemented in bridge topology with two transistors per phase.

Closing an adequate group of transistors ensures inverting the sign of the output voltage (+/-), and varying the transistors switching times leads to the variable frequency of the output.

By closing one group of transistors (commonly the top group in the bridge), the output voltage equals the DC input value and is positive.

On the other hand, by switching the transistors’ second (bottom group), the output voltage value is negative with a value equal to the DC input.

Using the previously described switching procedure, the alternating output voltage could easily be achieved by controlling the switching time of the transistors. We can achieve any defined frequency level.

For example, if we want to achieve a lower frequency, we will slowly switch the transistors. In opposite, to achieve a higher frequency of output AC voltage, the switching components (transistors) have to be switched more quickly.

Together with the desired frequency level, we must adjust the voltage level at the desired value to keep the voltage/frequency ratio constant.

This is performed using a PWM technique (Pulse Width Modulation).

Shortly, this technique is used to achieve a pulsing waveform of the output voltage, which in summary, leads to the desired effective value of AC output voltage retrieved as an average value of pulsing outputs.

The benefits of a VFD?

Although the VFD seems quite complicated, based on many different components, the benefits of its utilization in AC motor drives exceed its costs.

First, the efficiency aspect plays a crucial role in VFD’s cost-benefit evaluation. As we described above, VFDs are introduced as a very reliable and flexible solution in cases where the motor’s full speed is not necessary.

If we consider that induction motors make almost 65-70% of the load in modern industries, the savings achieved by using VFDs are significant. Moreover, they could easily be quantified in a short period.

Besides obvious energy savings, there are additional efficiency improvements manifested through smoother control of motor drives.

Using VFDs enables smooth control of AC motors and eliminates jerks on startups, leading to decreased number of faults and mistakes.

Additionally, smoother control of induction motors eliminates the necessity for complicated mechanical gearboxes, which are often a source of faults and production outages.

Finally, the development of microcontrollers also improved VFDs. As a result, modern VFDs are intelligent devices compatible with different communication protocols such as EthernetIP, Modbus TCP, Modbus RTU, Profibus, Profinet, etc.

In practice, VFDs act as local controllers in charge of controlling AC motors with flexible control logic.

Besides main control functions, modern VFDs provide self-diagnostic and monitoring functions, direct connection of different sensors, and predefined maintenance schedules.

From an automation point of view, VFDs are easily integrated into any modern SCADA system, which supports standard communication protocols.

Furthermore, communication interfaces integrated into VFDs enable easy connection to some superior controller (PLC) and implementation of complex control schemes.

As VFDs communicate with superior SCADA using standardized communication protocols, modern SCADAs treat the VFDs as all other connected, intelligent devices – i.e., nodes characterized by specific attributes, measurements, commands, and hierarchy status.

How to select a VFD?

As the VFDs are still quite expensive, specific attention has to be paid to selecting the most suitable VFD for some particular purpose and operating environment.

However, no specific procedure will ensure optimal VFD selection, but some guidelines can significantly simplify it.

The first step in choosing the most suitable VFD is to get more details about the motor you want to control. Some basic parameters of the motor, such as rated power (horsepower), rated voltage, full load current (ampacity), and rated speed (in RPM), play a crucial role in the selection of appropriate VFD.

Besides the controlled motor, getting as many details as possible related to the load driven by the motor is essential.

The following parameters could be classified into this group of factors: load factor, speed ranges, and overall control scheme utilized to control and monitor the considered process.

These parameters are also crucial for auxiliary equipment necessary to ensure a whole motor drive’s reliable and efficient operation.

Commonly, accessories that are used together with VFDs include circuit breakers (or disconnectors), auto switches, bypass equipment, reactors, different filters utilized to suppress harmonics, and other devices for speed reference setting (speed switches, analog speed followers, display units, touchscreen HMIs, etc.).

All of the equipment mentioned above affects the final price, so the selection procedure is a task of compromise between available budget and technical and commodity requirements.

As there is no unified algorithm to select appropriate VFD and auxiliary equipment, each project should be analyzed as an independent case study with all of its attributes, constraints, and requirements.


To summarize, VFDs are a very powerful and versatile tool that can be used to automate AC motors. In addition, they eliminate the need for complicated mechanical gearboxes, which are often a source of faults and production outages.

With modern VFDs, you can control your motor with some flexible logic and provide self-diagnostic and monitoring functions, direct connection of different sensors, and predefined maintenance schedules.

From an automation point of view, they’re easily integrated into any SCADA system that supports standard communication protocols such as EthernetIP or ModbusTCP; furthermore, most modern SCADAs also treat them like all other connected, intelligent devices.

In this article, we’ve covered what a VFD is and how a VFD is used to control AC motors. We hope that the information provided has been helpful and insightful for you.


The most common application for a variable frequency drive is to control speed on AC motors used on pumps, fans, conveyors, and other systems that require precise speed control in their operation. A VFD can also be used as an adjustable load for power generation, such as wind turbines.

A VFD is a device that changes frequency, while an Inverter changes the voltage. A VFD will change frequencies from 0 to 60Hz or from 50Hz to 60Hz, but it will not be able to output a different voltage than what was inputted. An inverter will allow you to change the voltage.

The controller in a variable frequency drive is used to set parameters such as maximum and minimum speeds and acceleration/deceleration ramp times. The controller uses the information the motor’s encoder provides to adjust these parameters for optimal performance.

The variable frequency drive enables operation on AC motors.

VFDs are usually sold with different options depending on the size of the motor. A typical VFD for a 3-phase AC motor is around $4000-$7000, but this can vary depending on your specific motor parameters.

Harmonic distortion is the addition of multiple frequencies to the main voltage waveform. This can cause instability and overheating in your equipment resulting in damage and poor performance. The best way to eliminate this possibility is to use a VFD with harmonic suppression built into its design.

Resonance occurs when there is synchronization between the frequency of inputs to outputs. It causes the motor to vibrate at its natural frequency, which can cause damage if not addressed. This is why it is essential to use a VFD with anti-resonance features.

Your VFD should include input and output terminals for lightning protection, current overloads, over-voltage, under-voltage, and over-speed. You should also ensure that your VFD has an alarm that sounds in the incorrect state.

A servo drive produces less horsepower than a VFD. The motor will not stop until out of range, even if the command stops. The motor feedback responds to sudden changes quickly because it uses current control, whereas a VFD uses voltage control. A servo drive is typically more expensive than a VFD.

Variable-frequency drives control frequency, whereas variable-speed drives control both voltage and frequency. Variable-frequency drives are typically used with rotating equipment such as AC motors and generators. Variable-speed drives can be used with either AC or DC motors and can be fixed or variable frequency.

A variable-frequency drive has a wide range of applications, including fans, pumps, and machine tools. However, the most common use of a VFD is with an AC motor to allow the motor to run at the proper speed while using less energy. This saves money on utility bills and can also lead to more consistent products.

Single-phase or three-phase, high horsepower or low horsepower, variable frequency drives come in all shapes and sizes. Choosing the proper VFD for your application is crucial to its success.

Variable-frequency drives are typically more efficient than standard AC motors, which leads to lower energy costs. Variable-frequency drives also ensure machine availability, allowing consistent speeds up to rated load.

These drives can help you maintain constant speeds under varying load conditions, producing more consistent product quality.

A single-phase variable frequency drive requires only one line voltage connection and typically comes with an input line voltage isolation transformer. However, Single-phase units cannot supply the power for multiple motors on a single unit, so they are usually used in small production facilities or single-motor systems.

Three-phase variable frequency drives require three separate line voltage connections, typically with one to three-phase transformers attached. Single-phase drives only operate on a single line voltage connection and require an input line voltage isolation transformer. Single-phase VFDs are typically used in smaller production facilities or for smaller motors.

There are two common modes for VFDs: torque mode and speed mode. In torque mode, the motor will run at its synchronous speed – no faster or slower. In speed mode, the motor will run slower than its synchronous speed when commanded to reduce energy costs.

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