What is a variable speed drive?
25 November 2015

While a variable speed drive is a necessity for many aspects of global manufacturing, its use is ubiquitous across a wide variety of industry applications. So let’s look at why a variable speed drive might be required to perform motor control and how its use impacts our world.

Wherever movement is required in industry, from lifting a heavy load onto a cargo ship to turning a ventilation fan, an electric motor (Link to blog "What is an electric motor?") is generally required. Rotational movement is created at the motor’s shaft, which is commonly converted into the linear movement required for most industry machinery by coupling together with a mechanism such as a ball screw.

This is essentially the same principle as how a rack and pinion system operates. An electric motor can be turned on and off but without the use of peripheral equipment it will always operate at maximum capacity, therefore providing only a single speed or torque (the force required to rotate an object). This isn’t a problem for some purposes, but for a conveyor belt to repeatedly stop and start or run at a desired speed, or for an elevator to gradually decrease speed and stop dead at an exact point, control of the motor is required. This is where a variable speed drive comes in.

A variable speed drive (also known as a VSD, variable frequency drive, VFD, adjustable speed drive, inverter, converter or simply, ‘a drive’) is a device which is used to control the speed or torque generated by an electric motor. Its construction consists of a series of PCBs (printed circuit boards) to control the drive’s ‘power stage’, which receives and then feeds electrical current to the motor to control its speed and/or torque. These components are protected in a casing which can vary in its construction dependent on the environment in which the drive will be installed, and the drive can vary in its size dependent on the power it will receive required for the size of motor (or motors) it will control. Generally speaking, the larger the power required, the larger the motor and the larger the drive. The user can interact and program the variable frequency drive by using a keypad and screen attached on or near to the drive (also known as a Human Machine Interface (HMI), or by a PC or laptop.

So let’s look at why a variable speed drive might be required to perform motor control and how its use impacts our world.

Modern manufacturing demands high productivity – to manufacture as much as possible as fast as possible – with a high degree of accuracy and reliability, and a ‘Pick and Place’ machine is an excellent example. Typically used for manufacturing PCBs, which are made in their millions to control industrial applications or the gadgets of our daily lives, a pick and place machine is responsible for selecting specific components and moving them at high speed into specific, highly exacting locations. A variable speed drive will typically control an electric motor, which moves the vacuum nozzle or other form of collection tool used to ‘pick’ the component, and then ‘tell’ the motor and coupled mechanism where to ‘place’ the component – all achievable in fractions of a second.

Manufactured items usually need to be packaged for transit and a packaging machine can construct, fill and fasten packages by ‘bending’ pre-formed cardboard blanks. Bending and forming the packaging material into a box, as well as filling and then fastening it, is the result of moving the packaging machine’s ‘arms’, propelled by the motor’s rotational movement and ultimately controlled by the variable speed drive.

Packaged manufactured goods then need to be exported. The lifting power of a port crane, carrying containers from a truck to a container ship, relies on a high-powered electric motor to move cargo. The port crane’s motor is able to raise the load under the control of a variable speed drive, and as a result of the drive’s ability to control torque – which the electric motor by itself cannot achieve – the port crane has the crucial capacity to ‘hold’ its load in place and lower it in a measured and secure way.

While a variable speed drive is a necessity for many aspects of global manufacturing, its use is ubiquitous across a wide variety of applications. Stage lighting in a theatre, for example, is one of the more unusual and interesting cases. Lighting and other apparatus – even to the hoists which propel performers acrobatically through the air – are powered by electric motors which must be commanded when to stop and start with the speed of their movement and their position controlled: this is done by variable speed drives.

Outside of controlling mechanical motion, another of the primary uses of a variable speed drive is to save energy. An electric motor-powered heating, ventilation or refrigeration system may not be required to operate at 100% capacity for 100% of the time. When a refrigeration system reaches a pre-set specific ambient temperature, it may not be necessary or desirable for system to continue to operate at its maximum capacity. An electric motor without the control of a variable speed drive has no way of regulating its performance and cannot deviate from maximum capacity operation. Alternatively, by introducing a variable speed drive into the system, when a set condition such as temperature is achieved, the drive can cut or reduce the motor’s output and therefore save energy. According to The Carbon Trust, independent experts on carbon reduction, by reducing the speed of a motor controlling a fan or pump from 100% to 80%, it’s possible to save up to 50% of energy. Electric motors are responsible for up to 40% of the world’s electrical power consumption but the majority of the world’s motors are not controlled by variable speed drive technology. Therefore, drives have further potential in reducing the world’s energy consumption and carbon footprint.

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