Variable interest

With an eye on both energy efficiency and cost savings, all the signs point to a major shift towards the use of variable speed drives in a range of applications.


With energy saving top of the environmental agenda, the public is fast becoming aware that just by switching off the TV standby setting when not in use means a cut in electricity bills.

This same need to save cash has been the imperative behind the increased use of variable speed AC drives in many applications. Running units at full power is very wasteful and variable speed drives can save a lot of money.

The EU has recognised this and could soon be adopting legislation requiring users to install the most energy-efficient motor configurations possible, as the US Environmental Protection Agency has done.

A recent study on high-power AC drives by US-based Automation Research Council (ARC) confirms the growing popularity of variable speed drives, saying, ‘The worldwide market for high-power AC drives is expected to grow at a compound annual growth rate of 9.1 per cent over the next five years and is forecast to be nearly $7.4bn (around £4bn) in 2011.’ A report covering low-power drives forecast similar figures for that sector.

The savings are made possible by controlling the output of, for example, a pump or fan with the drive so that only the required power output is used with no wasted energy. Mike Bannister, product manager for drives and automation products at Lenze UK explained: ‘The old-style fixed units were run at maximum load with some kind of baffling to reduce the flow to the required level. The laws governing the loads needed to move fluids state that the load is related to the power by a cubic law. This means that, for example, if you can lessen the load by 10 per cent you can reduce the power consumed by some 30 per cent.’

Another consideration is the over-dimensioning of motors to ensure they are well within spec. Steve Ruddell, general manager of ABB UK’s drives and motors business said: ‘If a motor is over-dimensioned we can sometimes see energy savings of 50-60 per cent when a properly matched variable speed drive is employed.’

Drives are being used to save cash in many places. For example Invertek drives has installed its Optidrive VTC inverters at the Rural Water Scheme in the south-west of Ireland with a range of energy-saving features.

Meanwhile Honeywell’s literature, like many others’, emphasises the cost-saving potential of the use of variable speed drives when used to control pumps, fans and compressors.

Users of the Valhalla dark water ride on Blackpool’s Pleasure Beach — who usually end up rather damp — are also benefiting from the use of variable speed drives.

The ride features an open-framed air conditioning system, which uses four high-volume cross-flow fans to create a vortex of warm air, drying riders after their Viking voyage.

An ABB variable speed drive is used for the dryer to avoid riders being blasted by a high-speed jet of hot air as soon as they enter the drying cabinet at the end of the ride. The drives are used in two modes, idle and operational. Idle is used when the dryer is unoccupied. The drive operates the fans at a low speed to continually recycle the air.

In operational mode, the fans are driven to full speed, recycling the air at an increased rate. Together with infrared lamps, this produces conditions similar to that of a warm, windy day. After drying is complete, the infrared lighting is reduced and the fans are returned to idle.

Variable speed drives also allow the use of frequencies above 50Hz. This gives higher fan speeds and provides up to 20 per cent more drying capacity compared to running at 50Hz, due to the improved air flow. The fans achieve an air velocity of nine m/s, changing the air in the dryer four times a second.

Drives are following the general automation trends of everything becoming smaller, faster, cheaper and easier to integrate. A recent example is the launch of a drive from Parker SSD Drives which emphasises the ease of set-up and configuration and reduced costs, as well as interoperability options for all the major communications protocols.

This, along with many similar claims for new products, ensures payback times will be even lower in future as drives find more applications.

Increased intelligence — which seems to be the way forward — is another feature of recent new products. As Lenze’s Bannister said: ‘As drives get smarter and cheaper we can expect to see them used more and more on a small scale. They are already being used in roller shutter doors and running machines, for example, and this use will expand in the coming years.’

ABB’s Ruddell said: ‘Drives will soon become self-commissioning and will be able to call on other instrumentation for diagnostic information, with self-checking and healing processes becoming standard features.’