Plant & Works Engineering
Why it’s time to believe in VSDs
Published:  05 August, 2010

As a plant or maintenance engineer one has to deal with countless technologies, from boilers to robotics and bearings to valves. As a result, it isn't always easy to stay on top of the current best practice with regard to each one. The variable speed drive (VSD) is one such technology that, once understood and applied, provides remarkable energy savings and impressive financial ROI (Return On Investment). But how can you predict the level of savings that will be delivered in advance? What"s the best argument to present to the board to obtain funding? And how can you ensure that those energy savings are sustained over the lifetime of the system?

I’m often told that the variable speed drive is a misunderstood technology. However, it’s my feeling that, certainly within engineering, the fundamentals are actually very well understood. The problem is that on some level people don’t believe that energy savings of up to 70% can be delivered by such a relatively low cost device. It just seems like too good a deal.

Nevertheless even that barrier is breaking down. Up until the beginning of the recession, VSD use was increasing steadily year on year and seemed to be reaching a tipping point. Like all capital expenditure in engineering this dipped once the recession took hold. But there is real evidence that the financial benefits of applying variable speed control are so significant that doing so could help manufacturers come out of this financially turbulent time in the best shape possible. In fact, one could argue that installing VSDs wherever you can is an insurance policy against rising energy prices and increasing costs.

Despite this, only about ten per cent of motors in use in British industry already have variable speed drives fitted. Of those that remain about half would benefit from VSD control. It seems clear that a progressive company can gain advantage over their competitors by ensuring that their own equipment and anything they manufacture swells that 10% figure and helps improve their own bottom line by reducing spending on energy.  

The levels of saving a VSD can deliver are, of course, dependent on the current usage of the application on which they are going to be fitted. Assessing this is the first step towards estimating the savings you might be able to make. You should establish the size of the motor the VSD is going to control, how many hours it is running for and whether it is running at full speed or not at present.

One also needs to assess the type of load; variable torque applications, such as pumps and fans, will deliver a return on investment that is a multiple of the speed reduction. For instance, reducing speed by 20% will produce a 50% saving. In contrast, constant torque applications, such as conveyors or air compressors, will deliver a saving that directly matches the speed reduction; a 20% reduction will result in a 20% saving.

When you understand the load itself, you can then assess by how much you can decrease it. If the VSD is replacing a mechanical throttle this reduction will be a set rate, but if the reduction is based on other factors, such as required chilling on an air conditioning system for instance, then it will be variable according to the environment. In the latter case, there may well be times when the motor has to run at full capacity but also occasions when it can be either off or running at a minimal level.

When you have established the level of saving, the next step is to obtain the funding to do the work. Normally this involves making your financial team or board aware of the savings that are possible and how those savings can be funded.

The key message to present to a financial team or board meeting is that ROI is normally delivered in less than twelve months. Another good argument to present is the encouragement offered by the Carbon Trust. The first level is the Enhanced Capital Allowance scheme, which means a business can claim 100% first-year capital allowances on their spending on VSDs. Then, more importantly, there is the loan scheme, which allows small companies and SMEs to use an interest free loan to pay for the VSD and its installation. In effect, you borrow the money to buy the equipment and the VSD itself then covers the repayments, by reducing expenditure on energy.

In the face of such a win-win argument can your board really refuse to provide the budget? If someone presented you with a 70%, or even 40%, saving funded by a 'free’ loan, could you really refuse the request for the initial capital expenditure? The facts are all in place, there is no reason not to ‘believe’ in these potential savings any longer.

If there were anything more required to convince your board of the benefits of VSD control, McKinsey and Co have developed a carbon abatement curve to help prioritise energy saving technologies. On the X axis it shows the technology and on the Y axis the cost of implementation. On the right of the curve are the expensive ways of saving energy and on the left the most cost effective. Motor system efficiency is on the very far left of the graph. This demonstrates that even the very highest level of management consultancy now believes in the power of the simple VSD to provide significant and demonstrable energy savings.

The key to ensuring that these demonstrable energy savings are sustainable over time is good monitoring. If you understand that whenever the process being controlled is altered the energy usage needs to be managed, you are half way to ensuring that the lifetime energy savings can be optimised. You should begin to consider energy usage as a quality throughput in the same way as the percentage of right first time product for instance.

There may well be instances where a motor has been specified to cope with a maximum level of output that is very rarely, or never, reached in reality. For example a cooling application might have a maximum capacity that is never used. This isn’t over specification in the motor per se, because the capacity might be there to cope with a conceivable extreme situation, but it is an opportunity to save energy at other times.

In terms of highly localised monitoring, the complexities of motor management need to be managed on a case by case basis. This article isn’t the place to analyse all the things that need to be done to continually keep a motor running at its optimum capacity. The list of tasks, from bearing maintenance to regularly checking the settings, is just too long.

However, while the minutiae of motor maintenance might be a complex issue, the fundamentals of VSD control are not. The energy saving potential is very easily understood and explained to a finance team. The financial argument is straightforward, thanks to the Carbon Trust. Now it is merely a case of believing and translating that belief into action that could save your business significant sums and substantially decrease its carbon emissions.

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