The Science and Technology Facilities Council (STFC) is saving a significant amount of its energy costs after fitting three ABB-Baldor direct drive cooling tower motor and drive systems to the cooling towers of the ISIS neutron source’s proton accelerator.
The ISIS neutron source is used by thousands of scientists every year to carry out cutting edge research in Harwell near Oxford. Three towers form part of the accelerator’s primary water-based cooling system.
Previously, the fans in the cooling towers were run at full speed using a conventional system consisting of a fixed speed AC induction motor connected by a drive shaft and disc coupling into a right angle gearbox. This reduced the motor’s speed to that required by the fan, typically around 6:1. With this arrangement, the gearbox requires regular inspection and maintenance of the lubrication and seals. The possibilities of gearbox oil leaks are also potentially harmful to the environment. Misalignments in the power transmission system can cause vibration, wear and noise, while the mechanically complex power transmission system introduces significant energy losses.
Installed some 15 years ago, the motors were approaching the end of their lives and needed replacing. The system provided no opportunity to vary the speed of the fans as the load on the cooling system changed.
The ABB-Baldor system incorporates a permanent magnet (PM) motor. These are a viable alternative to conventional AC induction motors and deliver significant energy efficiency advantages, even compared with the latest premium-efficiency types of motors. This higher efficiency is even more pronounced when run at low frequencies which is the case for these direct drive cooling tower motors.
Using a variable-speed drive with a special control algorithm, the PM motor can achieve the same low output speed as that previously attained by using the gearbox. As such, the gearbox is eliminated and the PM motor takes its place on the base. This allows the motor to directly drive the fan, thereby removing the need for the drive shaft and coupling used in the previous arrangement. This further increases efficiency and reduces noise while substantially reducing drive system maintenance.
Variable-speed control of fans can save an enormous amount of energy compared with older fixed speed solutions. ABB’s algorithm provides sensorless control of the PM motor. This is optimised to manage the large inertia of the fan and has a low starting current.
The three drives were installed in about a week.
Two duty fan units are run 24/7, with one in standby mode. Compared to the previous situation, in which the motors were run at 45 Hz, giving a fan speed of 220 rpm, the ABB-Baldor drives now run the motors at 9 Hz, giving a fan speed of 140 rpm. This gives a normal current load of only 10 A, while even the highest ambient temperatures and relative humidity need around 17 A.
Direct drive fans bring several benefits to the end user. As well as the lower running costs, it also offers higher reliability, greatly reduced maintenance and less noise.
The ability to vary the speed of the fans means that the water in the cooling towers can be evaporated at the rate that matches demand. This means that lower amounts of chemicals can be added to the water to combat the threat of Legionella and other pathogens, saving more expense.
ABB (www.abb.com) is a leader in power and automation technologies that enable utility and industry customers to improve performance while lowering environmental impact. The ABB Group of companies operates in around 100 countries and employs about 150,000 people.
