The Engineering Behind VSDs (Variable Speed Drives)

After our previous post looking at what a variable speed drive (VSD) is, Geraint Thomas, a technical writer for Control Techniques, looks in more detail at the engineering behind it.

With increasing pressure on industry to lower its energy usage, the variable speed drive (VSD, sometimes known as variable frequency drive, VFD) has become a valuable means of making this happen in everyday industrial applications. Let’s take a look at where motor control technology was and where it is now.

From the first AC versions, to today’s modern plant, motors are an established part of the operation and production of items used by us every day. Before we had VSDs, AC motors were controlled simply by wiring the motor directly to an electrical AC supply. This method was known as direct online (DOL), which provided the motor with the full frequency of the supply (50 Hz in Europe and 60 Hz in America), nothing more, nothing less. Then, to control the speed of the motor, a mechanical system was employed to take the rotational motion and alter the output depending on the production machine’s requirements. This system is commonly known as a gearbox.

As the knowledge and understanding of power electronics grew, and the technology associated with it became increasingly more advanced, the first VSD was born. It was admittedly quite different to the VSD’s seen today in modern factories, but did share some key similarities. Fundamentally, the way they operate is the same but with the use of analogue control instead of the digital operation we see today.

Rapid growth in the world of micro-processors fuelled incredible technological advancement in the VSD, pushing it beyond the age of analogue and into the new digital world, creating a product that can not only control a motor but have its own form of intelligence. VSDs subsequently became better, faster and smaller, enabling them to be used in a whole host of industries and applications.

How do VSDs work?

There are three key parts within the power electronics of the VSD which we will look at separately.

1. Input bridge
2. DC Link
3. Output bridge

The AC supply comes into the input bridge, where there sits a diode bridge rectifier which converts AC current into DC current. The current is then typically smoothed by a DC capacitor bank, although if the drive is rated over 2.2 kW then the supply current is smoothed by an inductor. The DC voltage is then stored within the DC Link, before entering the output stage where there are 3 pairs of insulated gate bi-junction transistors (IGBTs). Older versions would have used MOSFETs or thyristors to achieve the same effect. The output stage then uses the IGBTs to switch on and off, which ‘chops’ the current to the correct frequency. This is done by a process called Pulse Width Modulation (PWM).

As the PWM is producing a triangular waveform in a sinusoidal character, the motor will run as normal as it will only see the wave in the form of sine wave.

How do VSDs (variable speed drives) save money?

Imagine a motor being run DOL, with the speed control managed by a gearbox.  The motor will be running at full speed, constantly, using the maximum amount of electricity all the time.

Now imagine replacing the gearbox with a VSD to control the speed of the motor. The motor will not have to run at full speed and use the gear box to slow the system down. Instead, the variable speed drive can alter the frequency or speed of the motor by adjusting the PWM within the output stage of the VSD. This means less power will be pulled from the electrical supply, in turn meaning greater efficiency and lower costs.

Typically, up to 5,700 kWh of savings can be achieved from a 15 kW motor compared to using the motor without a VSD. When you consider motors account for 65% to 70% of the total electricity used in industry, you can see how incorporating VSDs to these motors could deliver huge savings.