Variable frequency drive

Variable Frequency Drive

Also called variable speed drives, frequency converters, or inverters AC, a Variable Frequency Drive (VFD) is responsible for controlling the speed of electric motors supplying the energy needed for each process in question.

What is a Variable Frequency Drive?

A frequency converter is an electronic device that can be used to regulate the speed of electrical machines such as a three-phase motor. It generates an alternating voltage from the alternating voltage at a fixed speed, the amplitude and frequency of which can be changed. A VFD is connected upstream of motors to regulate the output frequency and voltage. In this process, the motor’s torque does not change, but it has a much larger speed range.In addition, the direction of rotation can be changed directly with a frequency converter. There are current-controlled frequency converters suitable for applications in the upper megawatt range and voltage-controlled frequency converters used in the lower megawatt or kilowatt range.

VFD Applications

Frequency converters are used in various (industrial) sectors. Important: If electrical machines (or three-phase motors) are connected directly to the AC voltage network, only a fixed speed is available. This depends on the number of poles and the fed-in frequency of the local power grid. However, frequency converters are usually used if a variable alternating voltage is required for the corresponding application and thus a controllable speed.

The areas of application are:

  • Category: Pumps
  • Ventilation systems
  • Processing machines
  • Transport systems and conveyor belts
  • Cranes
  • Handling systems such as lifting devices

Uses include:

  • for speed control of motors, for building management (pump & fan applications), or for water & wastewater technology
  • can be used for the low voltage, medium voltage, and DC voltage range
  • for control cabinet installation as well as motor assembly and decentralized installation
  • Wall mounting as well as decentralized installation (as a motor starter or standard converter)

How to Select VFD Specifications

Installing a VFD solves the main problem with synchronous or asynchronous AC motors – speed control without compromising mechanical performance. VFDs are also successfully used in electric drives based on electric machines with permanent magnets.

There are several things to consider when choosing a VFD. They include:

Driver Type

The first thing that is taken into account when choosing a drive is the type of drive motor. They produce single and three-phase converters, devices for permanent magnet synchronous motors, asynchronous motors with squirrel-cage and phase rotor, and electric machines of other types. The device manual indicates the types of motors with which the inverter can work.


There are general industrial and specialized VFDs. The first are universal devices. Such inverters are widely used in the drive of machine tools, other equipment without special requirements for starting, braking, and other characteristics.

They also produce converters for a decentralized electric drive of autonomously operating equipment, frequency converters for centralized, automated control systems, and control of technological parameters. When choosing devices for the process control system, it is necessary to consider the supported communication interface with sensors and primary converters, upper-level equipment (control panels, PC). The communication protocols of the automation system and the inverter must match.

Specialized converters have a built-in set of functions designed for the specific drive. They include:

  • For lifting equipment with high overload capacity, regenerative braking capability, high starting torque, and other special options.
  • For ventilation systems and pumping stations with group control functions, skipping resonance frequencies, etc.

There are also special devices for elevators, draft systems of boiler houses and heat and power plants, fire-fighting equipment. The use of such frequency converters greatly simplifies setup and programming and reduces the cost of a variable frequency drive.


The service life and correct operation of the electric drive depend on the right choice of the power of the frequency converter. The inverter with overpowered power cannot provide an emergency stop of the motor in case of overload. A device with underpowered power will constantly shut down when operating in high dynamic mode.

For motors with normal starting conditions and work on a load with a constant torque, select a converter with a power equal to the electrical power of the power unit. It is important not to confuse electrical characteristics with mechanical shaft power.

Operating Conditions

The design of the VFD must correspond to the environmental conditions. Manufacturers produce inverters for all conditions with a degree of protection from IP00 for installation in a cabinet up to IP66 for open areas. They also consider the ambient temperature, the degree of heating of the inverter in all operating modes. In some cases, additional fans may be required to remove heat. Frequency converters with liquid cooling are produced for metallurgical workshops and other premises with high temperatures.

VFD Panel

Manufacturing plants are known for their rough work environment, usually characterized by dirt, moisture, and corrosion. VFDs usually have protective shells but will still need protection from such elements. That is why a VFD panel is essential.

A VFD panel is made up of an enclosure and other equipment like:

  • Motor starters
  • Harmonic filters
  • PLC for advanced control
  • Temperature control equipment
  • Bypass control that will ensure VFD is running in all conditions
  • Protection equipment like stabilizers, circuit breakers, and relays
  • Communication equipment such as modems and switches

The uses of a VFD panel include:

  • Adjustable speed
  • Visual control
  • Power switching and protection
  • Protection from environment

Advantage of VFD

  • High control accuracy
  • Energy savings in the event of variable loads (i.e., running the electric motor at partial load).
  • Equal to the maximum starting torque.
  • Possibility of remote diagnostics of the drive via an industrial network
  • phase failure detection for input and output circuits
  • accounting of engine hours
  • aging of main circuit capacitors
  • fan failure detection
  • Increased equipment resource
  • Reducing the hydraulic resistance of the pipeline due to the lack of a control valve
  • Smooth engine start, which significantly reduces engine wear
  • A VFD usually contains a PID controller and can be connected directly to a controlled variable (e.g., pressure) transmitter.
  • Controlled braking and automatic restart on power failure
  • Catching a rotating electric motor
  • Stabilization of rotation speed when load changes
  • Significant reduction of the acoustic noise of the electric motor (when using the “Soft PWM” function)
  • Additional energy savings from optimizing the excitation of el. engine
  • Allows you to replace a circuit breaker

Disadvantage of VFD

  • Most VFD models are a source of interference (RFI Filters required)
  • Relatively high cost for a high-power VFD (payback period is at least 1-2 years)
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