DRIVES & CONTROLSEnergy EfficiencySnake oil and disbeliefNo matter how you focus on energy issues, the use of inverter<strong>drives</strong> makes real sense. They offer significant operational costand carbon savings and make control systems more responsiveto changing needs, says Jeff Whiting of Mitsubishi ElectricIn this current market, where energy costsare so volatile, it is vital to take whateveropportunities we can to reduce powerconsumption and therefore costs. Even inthe unlikely event that costs are not an issue,taking the situation from a green viewpoint, it isclear that we need to take carbon reductionseriously. This is high on the government agendaand it is clear that the carbon element of anyoperation will certainly begin to bite over the nextfew years.Manufacturers of variable speed <strong>drives</strong> orinverters claim that significant energy savingscan be realised in most fan and pumpapplications, simply by fitting their equipment tothe motors. In fact the high value of savings oftenpresented by manufacturers seems almost tocreate a credibility gap. As a ‘dyed in the wool’<strong>drives</strong> person, I have seen legitimate energysavings claims treated with suspicion rather likethe ‘snake oil’ solutions of the travelling WildWest shows.Dramatic energy reductionThe reality is that where inverters are usedappropriately, they deliver the projected savingsand very often exceed them dramatically.Manufacturers continue to stand by theirprojections and are often willing to put theirmoney where their mouth is by loaning <strong>drives</strong> toprove the projected claims. And it should be noted thatthe government has already recognised the importance ofinverter <strong>drives</strong> for energy saving and carbon reduction.Drives are included in the Carbon Trust Energy <strong>Technology</strong>List, and attract tax benefits where they are implemented.Let’s consider something as simple as an air handlingunit to understand the cost savings. Based on a typical110kW motor, the inclusion of an inverter drive and<strong>controls</strong> is likely to add about £9000 to the cost of theproject and would typically have a life expectancy inexcess of 10 years. A 110kW motor operating 24 hoursa day will have an annual energy cost of £70,000 attoday’s prices.Clearly, with this level of annual costs, saving energyshould be part of the long term strategy as even a smallsaving will make considerable impact on the lifetimecosts of the system. Given that a pump, fan or othermotor installation is likely to have a working life of at least10 years, the financial savings on the initial out lay canbe truly massive. And even in short-lived installations aninverter can be very cost effective, with the paybackperiod for installing an inverter often measured in months;indeed it rarely goes beyond a year. If energy prices rise,the payback period becomes shorter and shorter. Inreality, there are generally large savings to be achieveddue to the nature of the most installations and thecharacteristics of most fan and pump systems. Motors arefixed speed devices which run at a speed determined bythe supply frequency (50Hz in the UK), but fitting aninverter allows the speed to be adjusted, and the slowerthe motor goes the less energy it consumes.Quantifying the savingsThe amount of energy saved by slowing down a motor canbe quite considerable. For most fan and pumpapplications the consumed power falls in proportion tothe cube of the speed reduction which in real worldlanguage means that if you can run the fan or pump at90% of its rated speed then theoretically the powerconsumed would fall by 27%. The inverter itself wouldadd a few losses into the system, so a realistic savingwould be around 24% – a great saving, which based onour 110kW motor would equate to £16,800 per year incost reduction at today’s prices. Payback for fitting theinverter of about 7–8 months with an ongoing reductionin operating costs of at least £16800 for the life of theequipment. The question becomes, will the fan or pumpsystem operate effectively at the reduced speed to meetthe needs of the process? A review of actual installationsshows that most motors do run too fast, so there wouldappear to be room for energy savings. When systems aredesigned, they are generally sized for peak operationalloading, with an extra margin “to be on the safe side”.This is partly because motors only come in standard sizesand installing one that is too small results in poorperformance and high maintenance. Government figureshave identified that today most installed motor drivenfans and pumps are 25% oversized even for peakoperation.The oversized fan or pump does have ‘spare capacity’and traditionally operates with the excess air flow (orother output) being restricted or vented off throughmechanical restrictors such as baffles or valves. A verysmall degree of power reduction is experienced in thistype of system, but it pales into insignificance whenconsidering the savings which can be achieved usinginverters to adjust the motor speed and therefore therequired airflow.The inverter can be integrated into monitoring andcontrol systems to run at the optimum workingconditions, meeting operational requirements whilstmaking significant energy savings.Automatic motor optimisationModern inverters such as Mitsubishi’s FR Series invertershave further tricks up their sleeves as they are able totune themselves completely to the motor and dynamicallyreduce the power fed to the motor for the current loadconditions. This automatic motor optimisation cantypically save a further 3–10% of the power without anyloss of motor performance. It gives exceptional resultswhere systems have been oversized at it reduces heat andmagnetic losses in the motor itself.There are many motors installed throughout industrythat are contenders for being fitted with an inverter. Theexample quoted was based on one motor; but typically aninstallation – be it for a manufacturing plant, a processingfacility or a building’s environmental control – will includeseveral motors, each of which can be fitted with aninverter and gain significant energy savings.Indeed the situation is such that if you are starting anenergy reduction programme or just want to reduceoperational costs, a good first step would be to identify allmotors over 1.1kW and assume that they should all befitted with an inverter. You probably would not do theseall at once, but would do them in sequence, starting withthose that are either easiest or those that will provide thebest payback. With payback within a year, it would bepossible for the energy programme to become self-fundingafter the initial installations.Some people may always be concerned about ‘snakeoil’ where such large savings are claimed. As for me, I justhate paying energy companies and the taxman money Idon’t need to. Best of all, I know I am helping createa sustainable environment for us all.MORE INFORMATION:Enter J285 on the enquiry card, or visit ‘latest issue stories’www.industrialtechnology.co.uk for further details from Mitsubishiand more stories on <strong>drives</strong> and control systems28INDUSTRIAL TECHNOLOGY • October 2008
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