There is a straightforward way for businesses to cut their energy bills and reduce their carbon footprint. AC variable speed drives are the key, as Stephen Takhar explains.
There are many ways for organisations to cut energy usage, but one option is really too good to miss, and that's the installation of AC variable speed drives.

Particularly attractive savings can be made in hvac plant, which invariably incorporates motors driving pumps and fans. In the past, these motors were equipped with simple fixed speed starters and diverters or throttling were used to regulate flow.

Running the motors at full speed all the time is, however, bad news, as plant is designed on a worst-case basis, and the motors are sized for the coldest and hottest days of the year. At all other times, they're doing far more work than necessary.

The logical solution is to rip out all the single speed starters, fit modern AC variable speed drives and regulate the speed of the motors so that it matches the actual demand. It's easy to see that this will save energy, but just how much energy might be a surprise.

The energy used by a fan or a centrifugal pump varies as the cube of its speed. When it's running at half speed, therefore, it uses 0.5 x 0.5 x 0.5 = 0.125 times as much energy as it does at full speed. In other words, cutting the speed by 50 per cent reduces energy usage by 87.5 per cent.

So why aren't AC drives being fitted everywhere? Probably the biggest issue is that organisations are concerned about the cost and disruption associated with installing new drives. In fact, variable speed drives are not expensive and installation is almost always straightforward. The best drives also qualify for the Enhanced Capital Allowances (ECA) scheme, which means that 100 per cent of their cost can be offset against tax in the year in which they are installed.

But the real clincher must surely be the ongoing savings on energy costs - in most cases, the cost of fitting AC variable speed drives is recovered in two years or less.

Modern drives are small and easy to accommodate, and they are available in enclosed versions that can, for example, be mounted directly on a wall without the need for further protection.

There may be a need to fit other devices such as flow sensors, but this is no problem for an experienced drive installer.

The arguments in favour of replacing fixed speed starters with AC variable speed drives are convincing, but that not all drives are created equal and it's important to choose the right drives for the job. Invariably, these will be types specifically designed for hvac applications.

The best hvac drives include functions like real-time clocks to shut the system down automatically outside normal occupancy hours, PID controllers that allow the motor speed to be controlled directly from flow or pressure sensors, and a sleep option that completely stops the motor when demand falls to zero.

The best hvac drives also have good EMC performance so that even if they are used in large numbers on a site they do not adversely affect the supply quality. Finally, they use components chosen to provide long reliable working lives. In particular, the latest drives use film capacitors rather than electrolytic types.

Film capacitors not only have longer lives, they are also more environmentally friendly as they have no electrolyte to create a pollution hazard when they are ultimately discarded. Fitting AC variable speed drives to hvac plants really is an energy- and costsaving opportunity too good to miss. The drives are inexpensive, reliable and easy to install with a minimum of disruption.

But the best results will be obtained by choosing dedicated drives from a supplier with a proven track record.

• Stephen Takhar is managing director of Vacon UK