Mitsubishi Electric’s Jeff Whiting points out that controlling the power consumption of motors is just the tip of the iceberg when it comes to the energy saving benefits of variable speed drives (VSDs).
We’ve all heard the facts and figures many times. Motors account for 65% of all industrial power consumption, only 25% of motors are fitted with variable speed drives, and yet it’s well known that a VSD can slash that motor’s energy usage. The UK government has recently woken up to the fact that VSDs, along with a wholesale switch to LED lighting and thermal insulation in commercial buildings, represents one of the best ways to reduce the UK’s carbon footprint.
However, as the world economy recovers from its battering of the last couple of years, a more sophisticated definition of green manufacturing is emerging where its recognised that best practice environmental measures can also boost productivity.
For instance, the stopping of large machines would traditionally be achieved with some sort of mechanical brake which uses friction to bring it to a stop. An inherent by-product of which is of course 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.
This of course saves energy but the regeneration function also makes it possible to achieve smaller, less expensive drive systems and simpler, more compact switchgear layouts.
Increased control of the whole production line leads to 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 a company more able to meet customer requirements. However, it also means less energy is used per finished product.
We can apply the same thinking to the wider production cycle, which is characterised more and more these days 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. However, today’s competitive global markets demand flexibility if a company is to thrive, or even to survive.
Inflexibility in machinery and processes can lead to significant costs in product changeovers, in terms of manpower and lost production. However, tighter control means that 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.
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. When we’re 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.
The bigger picture
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. However, when you consider the likely number of motors around a typical industrial site, then you can see that these motors will be contributing to a measurable temperature rise.
This can be critical in temperature sensitive environments such as cosmetics production, where 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.
A much more efficient solution would be to reduce the heat signature of the motors themselves, or even capture that energy – and here again VSDs 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.
It’s no secret that VSDs have a significant role to play in making plants and processes more efficient. And, although it may be the energy saving impact of not running a motor at fixed speed that grabs most of the headlines, 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.