22 June 2016 Energy savings, that is the reduction of electricity consumption, is something we’ve heard a lot about in recent times. The interest in this topic appears to be growing and while one aspect has been the greenness of our planet, the other has been the rising cost of energy. These costs have been climbing steadily, and this seems set to continue for the foreseeable future. However, up until the 1990s, energy saving scarcely rated a mention in the press, and many people were indifferent to the concept. This was partially due to the environmental impacts not being well understood, but also because the cost of energy was relatively low. Often, the cost of energy saving devices, and the effort associated with installing and running them, was greater than the cost of the energy that was saved. But since the last decade, there has been a remarkable turnaround, with global warming, dwindling fossil fuel supplies and as governments look to force the issue by placing a price on carbon. Energy saving is now at the forefront of both domestic users and industry in general. And as a significant portion of energy is consumed by motors, the spotlight has turned to them, and the drives that control them. The development of motors over the last few decades has been somewhat gradual, with the ubiquitous threephase, squirrelcage synchronous induction motor still the main stay of industry. It’s design has remained largely unchanged. On the other hand, permanent magnet (PM) motors, where the rotor is made of rare earth material such as neodymium-iron, is gaining traction in the market. PM motors are more expensive than HARRY MULDER (B Comp. Sc) – Engineering manager for Omron Electronics Harry Mulder has been involved in the industrial control industry for nearly 30 years, the last 25 of which have been with Omron Electronics. With a degree in computer science, his experience includes sales, engineering and product management of industrial programmable controllers, HMIs, networking and software. He currently manages an engineering team across four states but still likes to get involved with day-to-day problem solving. ENERGY SAVINGS THROUGH THE EFFICIENT USE OF DRIVES an induction motor of equivalent power rating. But this is more than offset by their greater efficiency, which lowers running costs. They are also usually more compact than induction motors, do not require a fan and have been used extensively in servo systems, where they can perform very precise control associated to builtin encoder feedback. They still require a drive to supply them, but many variable speed drives (VSDs) can be set to handle induction and PM motors. However, it’s VSDs that have gained most attention recently. While the installation of a drive is optional (as many induction motors can run direct on line (DOL) or connect straight to mains), drives are seen now as mandatory throughout industry. Their controlled up and down ramping capabilities, ability to easily vary motor speed, be networkable to controllers, as well as a myriad of other features make them indispensable. DOL is hardly used any more. Considerable product development has gone into VSDs over the last few years, with many features now aimed solely as reducing the energy footprint of the drive. One small advantage of this is that internal heat dissipation is reduced, simplifying product design and presumably reducing the manufacturing overheads. Here is a summary of some of the other enhancements of drives, designed specifically for energy management: • Many VSDs now have a dual rating: constant torque and variable torque. When a drive is running in its variable torque mode (also called ‘energysaving’ mode), it can temporarily greater output torque than when running in constant torque mode. This is because of the nature of certain loads, such as fans and pumps, allow the drive to reduce its output torque once the motor reaches its target speed. This means a smaller drive can be installed. Dual rating works well for certain applications, but it relies on the load inertia being smaller, and also stable and once at speed. Stalling of the motor can occur if the load changes appreciably during operation. Some applications such as lift control or transfer conveyor need a constant torque and cannot use variable torque mode. • Tuning of motor parameters in the drive ensures it closely matches the actual motor, and thereby both maximising performance and reducing energy. These parameters can be difficult to understand and hard to set up, so the drive should assist the user by providing an auto-tune function. To maximise its effectiveness, autotune should have both static (when the motor is still) and dynamic (where the motor is moving) tuning modes. • Sleep mode is another relatively new feature available in modern drives. It works in conjunction with the inbuilt PID loop, and can make the drive go into a ‘hibernate’ mode after a specified period BY HARRY MULDER MOTORS, DRIVES & TRANSMISSIONS
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