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Drives may be greener than you think

01 February 2011

Controlling the power consumption of motors is only the first of many environmental credentials of the inverter, says Mitsubishi Electric’s Jeff Whiting.

Motors account for 65% of all industrial power consumption, and yet only 25% of motors are fitted with variable speed drives.

Using a variable speed drive to control a motor, with an appropriate speed profile for the task, will allow users to cut that motors energy usage. However, a more sophisticated definition of green manufacturing is now emerging. And this time it makes even better business sense, because, while measures such as the Climate Change Levy and the CRC Energy Efficiency Scheme effectively penalise companies financially for not reducing their energy consumption, in our more sophisticated picture of green manufacturing, best practice environmental measures can actually boost productivity. Variable speed drives can really make the difference.

Consider, for example, the stopping of large machines, or indeed any shaft driving a load that needs to be brought to a controlled stop. Traditionally, this would be achieved with a mechanical brake. But these work by clamping the shaft and using friction to bring it to a stop – an inherent by-product of which is heat, or wasted energy. However, a key feature of many modern variable speed drives is regenerative braking which converts braking energy back into electrical energy. This energy can then be fed back into the main supply or shared with other drives by connecting their power reserves together.

Not only does this save energy in its own right, but the regeneration function also makes it possible to achieve smaller, less expensive drive systems and simpler, more compact switchgear layouts.

It seems obvious, but better control of a motor on any machine or process, optimising speed and torque, means better controllability. When you apply that tighter control to the whole production line, what you immediately see is significantly increased useful output, with far fewer reject products, and a dramatically reduced need for any product rework. How many products, for example, are thrown away at the start of the production cycle as the machinery is tuned and optimised? How many more are rejected as processes drift out of tolerance? Variable speed drives can help in optimising machinery and processes from the minute they are turned on, and in keeping them at optimum efficiency throughout the production cycle.

A reduction in reject parts and in the need for rework can significantly impact on a company’s bottom line. If a process is making greater numbers of useful products for a higher proportion of time, that makes you more competitive and better able to meet customer requirements. But it also means that you’re using less energy per finished product.
The same thinking can be applied to the wider production cycle, which today is characterised by frequent line changeovers that cater for short runs of many different products. The requirements of the customer and the need to optimise production efficiency can appear to be in conflict, since maximum efficiency is gained on the longest possible production run of a single product. But today’s competitive global markets demand flexibility if a company is to thrive, or even to survive.

In machinery and processes without inherent flexibility, there are significant costs in product changeovers, in terms of manpower and lost production. But once you have tighter control of those processes, changeovers from one product run to another become recipe based, with complete lines reset at the touch of a button. What would have required time-consuming retuning of motor speeds and profiles can now benefit from automatic adjustment. The recipes for each product to be made on the line will store all the relevant parameters and settings, and these can automatically reset the likes of variable speed drives as required.

This optimisation of the production cycle can mean the difference between having to manufacture for stock and being able to manufacture to order – or at the very least to a more optimised inventory schedule. Because when we are simply manufacturing for stock, inevitably there will be over-production of some items which will then just sit on shelves losing value. Each of those products in the warehouse represents some degree of wasted energy in manufacturing.

We can look at the wider plant environment, too, because every motor – regardless of its efficiency rating – generates heat. Outside of specific hazardous areas, it is unlikely that the heat produced represents much of a problem to the machine itself or to personnel. But when you consider the number of motors there are likely to be around a typical industrial site, then you can see that these motors will be contributing to a measurable temperature rise.

In some controlled environments, that can be critical. In temperature sensitive environments such as cosmetics production, overall temperature has to be closely controlled within specific tolerances. If one process is generating excess heat, then another process has to be introduced to bring the temperature down – most likely some form of force air recirculation or air conditioning. And this, of course, is using energy.

Much more efficient would be to reduce the heat signature of the motors themselves – or even capturing that energy – and here again variable speed drives come into their own. The variable speed drive more closely matches the motor to the load, and so the motor generates less heat. Not only is the motor being run more efficiently, less work has to be done to compensate for the heat generated.

We can see then that variable speed drives have a hugely significant role to play in making industrial plants and processes more efficient. It may be the energy saving impact of not running a motor at fixed speed that grabs most of the headlines, but when we consider a more sophisticated picture of green manufacturing, it becomes clear that variable speed drives are making an even greater contribution to energy efficiency than might first be considered.


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