TechArchive

Original article date: June 2000

You can do your bit towards cutting greenhouse gases by employing energy saving variable speed drives and high efficiency AC motors, says ABB’s JAMES HAIGH

You’ve probably noticed that energy saving is back on the agenda. Big time. Government initiatives at the Rio Summit and the Kyoto Protocol are aimed at cutting energy consumption and reducing emissions of greenhouse gases.

Widespread use of variable speed drives and high efficiency AC motors across industry would be one of the most effective ways to achieve great energy savings. In fact, so much energy is wasted by inefficient constant speed and mechanical control mechanisms that every industrialised nation around the world could make several power stations redundant, simply by using variable speed drives instead.

In pump and fan applications, this can cut the energy bill by as much as 60%, Yet 97% of all motors in applications under 2.2kW have no form of speed control at all, equating to some 37 million industrial motors sold annually worldwide.

With drives in this power band now available from as little as US$150, an argument for investment in variable speed drives can viably be made on energy grounds alone. Drives are becoming smaller, so it’s easy to integrate them into even the most space constrained of applications. And as the size and cost come down, so new applications are opening up in areas where no one would have thought to employ a variable speed drive in the past.

In an ideal world, we would be approachingthe point where energy was applied with pinpoint accuracy when and where needed, and never wasted. But all too often, control is still performed with throttling valves in pump systems or vanes in fan applications, while the demands for rotating machinery are solved by gears or belt drives. An estimated 65% of industrial energy is used by electric motors, and some 20% of this is lost by wasteful throttling mechanisms.

So, in the right applications, variable speed drives can make a huge difference. Just a small reduction in speed can make a big difference on the energy consumption, and as many fan and pump systems, for example, run at less than full capacity a lot of the time, a variable speed drive can make huge savings. A variable speed drive can also make a motor stop when it is not required. And regulating motor speed has the added benefit of easily accommodating production rises without extra investment, as a speed increase of say 5-20% is no problem with a variable speed drive.

As an example of what can be achieved, Heathrow airport has introduced a HVAC system using variable speed drives. When the room is at the required temperature, the two air handling fans run at their minimum setting. As the room temperature rises, the fans speed up, maintaining the room temperature at its set point under varying conditions. An occupancy controller has also been installed, using infrared sensors – one t the ticket desk area and another at the far end of the departure lounge. when the room is empty, the fans revert to low running speed. At the same time, the occupancy controller sends a signal to the temperature controller, which establishes the setpoint at a lower standby level, further increasing energy savings. when the sensors detect the room is used, the setpoint temperature is raised and the room temperature is quickly returned to comfort conditions. The energy savings achieved varied between 70% and 80% in three tests caried out.

Recently introduced drive technologies such as ABB’s DTC (Direct Torque Control) can provide significant energy savings, as well as giving more accurate speed and torque control across a wider speed range. Further energy savings can be achieved by using regenerative drives. In many applications, a variable speed drive can be shared between several motors, or the motors can be run in tandem.

Efficient use of energy can also mean less maintenance, as runing the motors on less than full speed gives a lower temperature rise in the motor, so increasing the life of the windings, the bearings and the bearing grease. But above all, lower energy consumption means that less energy needs to be generated to run the motors, so reducing emissions of CO 2 .

• ABB

June 2000