What is a variable speed drive?

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Variable speed drives or also know as frequency converters convert alternating currents (AC) of one frequency to another frequency. Originally, these devices were electro-mechanical machines and they are sometimes referred to as “dynamic” VSD’s. Due to the invention of solid state electronics it is now possible to build completely electronic VSD’s - these are referred to as ‘static’ VSD’s.

Over the years, the principle of converting fixed mains voltage and frequency into variable quantities has near enough stayed the same however, some improvements have been made since the first VSD’s. The first ever VSD’s featured thyristors and analogue technology whereas, modernised VSDs are microprocessor-controlled digital units.

The degree of automation in industry is always increasing meaning there is a constant need for more automated control and a steady increase in production speeds therefore, engineers are always trying to improve the efficiency of production plants.

The VSD-controlled, three-phase motor is an element in all automated process plants, commercial and public buildings. Motor designs such as, Permanent Magnet Motors, EC motors and Synchronous Reluctance Motors require regulation with VSD’s, a lot of motors cannot even be operated directly from the 3 phase standard power supply.

Nowadays, frequency converters can be applied to adjust and maintain the speed or torque of a driven machine with an accuracy within ±0.5%. Manufacturers of motors use a variety of concepts in order to achieve high efficiency in electric motors. It can sometimes be hard for the user to see the main benefit from one technology to another however, it is often observed by the user that energy efficient motors need high technology controls.

In theory, all motors can be employed with control algorithms specially adapted to each motor type. When manufacturing FCs some companies design them to be compatible with certain motor technologies, but a lot of manufacturers have different algorithms built in and are selectable during installation. For the person installing the VSD it is important that it is easy to commission based on the date, normally available for the motor type which is used. After installation, it is vital that the user is confident that the system is really as easy as expected, therefore it is essential that there is online measurements of actual energy consumption and easy access to important data about the operation.

In order to ensure that all Government aims of reducing energy consumptions are met, there is a big motivation for a complete set of regulations.

You must bear in mind that all system components are important for potential energy savings. According to the German Association of Electrical and Electronics Manufacturers (ZVEI), approximately 10% of the savings can be achieved by using high-efficiency motors, 30% of savings are achieved by variable speed, but as much as 60% of the potentials savings are achieved by looking at the overall system and optimising accordingly.

Speed Control of Electric Motors

There are many different terminologies used for systems that can control or alter the speed of electrical motors, these include:

  • Variable Speed Drive (VSD)
  • Adjustable Speed Drive (ASD)
  • Frequency Converter (FC)
  • Variable Frequency Drive (VFD)
  • Adjustable Frequency Drive (AFD)

VSD and ASD refer to speed control in general where as AFD and VFD are directly connected to adjusting the feeding frequency of a motor, but also the abbreviation “drive” is used as well. The terminology Frequency Converter is widely used, this covers the power electronic part of the devices and the supporting components like current sensors, I/Os and Human Machine Interface (HMI).

Why use speed control?

There are many different reasons as to why you may need to adjust the speed of an application:

  • Lower noise levels, for example on fans and pumps
  • Save energy and improve efficiency of systems
  • Match the torque or power of a drive to the process requirements
  • Improve the working environment
  • Reduce mechanical stress on machines
  • Match the speed of the drive to the process requirements

Depending on the application one or the other benefit is predominant. The speed control is proven to bring significant advantages in many different applications.

How to adjust the Motor Speed

In order to realise the speed controlled in industry there are three main technologies. All of them have their own unique features:


  • Friction drives (metallic)
  • Belt and chain drives (with adjustable diameters)
  • Variable speed gear

Engineers still favour mechanical solutions, this is mainly because of their simplicity and low cost.


  • Servo systems (for example servo amplifier and servo PM motor)
  • VSD with electrical motor
  • DC motor with control electronics
  • Slip-ring motor (slip control with wound-rotor induction motor)

Originally, electrical devices were complicated to handle and expensive, and they were usually used for the most challenging jobs where there was no more alternatives.


  • Hydro-dynamic type
  • Static types

They are often recommended in conveyor applications, mainly for earth-moving and mining equipment. This is due to the “soft start” capability of the hydraulic unit.

How do I chose a VSD?

Check out our other article ‘How to choose the right AC inverter drive’ for all the things you should consider when choosing a variable speed drive.

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