  |
|
|
<< back to Energy Home << back to Articles |
|
|
Author: James Haigh, General Manager - Drives, ABB Automation 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 world-wide. In the past, this might have been understandable, as a small drive cost in the region of US$500 per kW. But over the past few years, drives across the range have become smaller and cheaper, and now start at around US$150 per kW. This can make investment in a variable speed drive a viable proposition on energy grounds alone. As the new generation of drives is smaller, installation might also be possible in places where a space constraint was an issue in the past. They are also more energy efficient than their predecessors, and there is a growing retrofit market where old drives are replaced with new units, freeing up valuable floor space and saving energy at the same time. As the size and cost of drives are coming down, they are used in new, small scale operations where no one would have thought to apply a variable speed drive in the past, such as potters' wheels, spa baths, oven hobs, honey centrifuges and chicken slicers. The drives themselves are becoming leaner too, not only smaller in size but more energy efficient to manufacture with smaller circuit boards and enclosures made of recyclable plastic. In an ideal world, we would be approaching the point where energy was applied with pinpoint accuracy when and where needed, and never wasted. Simple systems sap energy 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. Though these arrangements may seem cost effective at first, they are energy wasting as well as noisy. Running a motor at full speed while throttling the output has the same effect as driving a car with one foot on the accelerator and the other on the brake; a part of the produced output immediately goes to waste. An estimated 65% of industrial energy is used by electric motors, and some 20% of this is lost by wasteful throttling mechanisms. This indicates that in the right applications, variable speed drives can make a huge difference. In particular, variable speed drives can dramatically reduce energy consumption in fan and pump systems. A pump or fan, running at half speed, consumes only one-eighth of the energy compared to one running at full speed. As a small reduction of the speed can make a big difference on the energy consumption, and as many fan and pump systems run at less than full capacity a lot of the time, a variable speed drive can make huge savings. This is particularly so when compared to a motor that is continuously running at full speed. Improved process control A variable speed drive can also make it possible to stop a motor completely, when it is not required, as soft start is one of the built-in features included in the drive. By matching the performance of the motor to the needs of the process, variable speed drives can give major savings, compared to the wasteful practice of running the motor at full speed against a restriction to modulate output. Speed control with belt drives, gearboxes and hydraulic couplings all add to the inefficiency of the system to varying degrees, and require the motor to run at full speed all of the time. In addition, mechanical drives can be noisy as well as difficult to service, situated as they are between the motor and the driven machinery. Further efficiency benefits Efficient use of energy also means less maintenance, as running the motors on less than full speed gives a lower temperature rise in the motor, increasing the life of the windings, the bearings and the bearing grease. Variable speed drives also eliminate the mechanical and electrical stresses brought on by direct on-line starting, reducing leaks and other symptoms of wear. It can also give higher productivity, as the accurate control of speed and torque means the process can be easily optimised for the best quality output. Higher productivity means less energy is used producing each unit, further compounding energy savings. But above all, lower energy consumption means that less energy needs to be generated to run the motors, thereby reducing emissions of CO2 and other pollutants to the atmosphere. Top of the page |
|