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Energy saving: time to return to the basics?

03 January, 2020

ABB’s Steve Hughes discovers that the energy-saving benefits of variable-speed drives are still not widely understood.

I recently made a presentation to an audience with a vested interest in energy efficiency at the Emex show, where I extolled the virtues of digital energy. I was struck by questions from the audience that were truly “back to basics” in terms of how VSDs actually save energy, where they can and can’t be used, and so on. Here are three examples:

Are there any applications where you wouldn’t use a VSD? There are more than 750 million motors in use around the world with about 12% operating with VSDs. Of course, applications that run at a fixed speed won’t always benefit from a VSD. Such constant-torque loads include compressors, conveyors, mixers, agitators, crushers and surface winders. With these applications, the torque requirement is the same throughout the speed range. The power change is proportional to the speed of change. 

While applying VSDs can still deliver savings, they are not as dramatic as in variable-torque applications. However, some applications, such as compressors and hydraulic power packs, can achieve significant savings, depending on the on- and off-load cycle times. 

The main benefits of VSDs in constant-torque applications are precise speed control, and starting/stopping with controlled acceleration/deceleration. As well as reducing the effects of over-sizing, the improved control reduces mechanical stresses and, thus, maintenance requirements, giving higher production availability and lower lifetime costs. 

Can VSDs be fitted with a bypass?  Yes, but the counter-question is: why do that? Apparently there are some engineers who refuse to install VSDs unless they are fitted with a bypass for fear that if the drive fails, the process stops. This, it turns out, isn’t an unusual perception. ABB carried out an energy assessment at a large chemicals site, guaranteeing a specific level of energy saving. Drives were installed but are now being run at 50Hz because of fears that they may jeopardise the process. In another case, a famous London landmark is stacked with VSDs, yet the motors they are meant to be controlling all run at 50Hz. While energy managers may want to install VSDs, they need to work closely with other stakeholders to solve any challenges at the outset.  

How reliable and resilient are VSDs?  VSDs have a mean-time-between-failure (MTBF) of more than 500,000 hours, or nearly 62 years. Failure rates depend largely on the environment in which the VSDs operate. There are many factors to consider, with most VSD damage being heat-related. The second-highest cause of failure is neglect caused by poor or no maintenance. Simple measures include keeping the drives dust-free to allow air to flow freely to cool them properly. Keep them dry to avoid PCB corrosion and keep connections tight to prevent arcing damage. The final factor affecting reliability is the application. A 250kW VSD driving a water pump or fan will have an easier life than one driving a demanding application such as a centrifuge. This leads to the subject of digital energy where we can measure environmental impact, availability, stress levels and how much time a VSD is in a fault condition. 

Today’s engineers are confronted with an avalanche of digital technologies – IIoT, machine learning, augmented reality, big data, edge computing, and so on – all vying for attention, and leaving little time for energy-saving basics. The irony is that a focus on minimising energy wastage can result in some truly amazing savings, releasing the capital needed to fund future digital projects.

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Birmingham 2020The next Drives & Controls Exhibition and Conference will take place in Birmingham, UK, from 25-27 January, 2021. For more information on the event, visit the Show Web site


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