Drives & Controls - Delivering energy understanding to the top table


Speak of deploying widespread AC inverter technology, or calculating cube law considerations for a plant's fans and pumps and the average finance director would keel over. However, as GAMBICA's variable speed drives group argues, until such topics are understood at the top table, industry will fail to maximise potential energy efficiencies

In most developed countries, industry accounts for about half the electricity consumed. Of this industrial consumption, two thirds is typically used in powering electric motors. For this reason, the control of motors must be placed high on any energy efficiency agenda. Yet, because motor control involves highly technical issues, the overwhelming majority of boards of directors and senior business managers continue to focus energy efficiency attention on readily understood areas such as insulating building fabric and reducing lighting consumption.

For many, energy measures revolve around the consideration of thermal issues in the building fabric with remedies such as insulation, glazing, and heat loss countermeasures. These are really only passive countermeasures that compensate for energy loss rather than the active control of the energy deployed.

Under the Kyoto Protocol industrialised countries have agreed to reduce their collective emissions of greenhouse gases by 5.2% by 2008-2012 compared to the year 1990 (however, compared to the emissions levels expected by 2012 prior to the Protocol, this limitation represents a 29% cut). The target in Europe is an 8% reduction overall with a target for CO2 emissions to fall by 20% by 2020.

The cost of an electric motor can be very deceptive; in particular, the annual energy cost to run the motor can be up to ten times its purchase price. Indeed, the running costs of a fully loaded motor operating at 50Hz can range from over £1,000 a year for a 2.2 kW motor to over £18,000 a year for a 37 kW one. Consider that, for example, a typical 11kW AC induction motor, that can be purchased these days for as little as £300, could cost as much as £30,000 in electricity consumed over a 10 year lifetime.

Fit a variable speed drive and simply by slowing the motor by 20% the energy bill is halved. About 105TW of electricity is consumed by British industry each year. It is estimated that two thirds of that is consumed powering electric motors; and of those motors about three quarters of them power fans, pumps or compressors in continuous duty variable torque applications. It is these variable torque applications where the greatest savings can be made from VSDs.

In other words, if every fan, pump and compressor motor was equipped with a VSD and had a 20% reduction in speed, the total consumption would fall by about 26TW per annum. Put another way, the savings would enable the closure of the UK's largest coal fired power station, saving 20 million tonnes of CO2 per annum. Even more dramatic to consider is that this figure effectively means that the CO2 emission attributable to  3.1 million homes would be negated. In other words, the whole of Greater Manchester and Birmingham would be rendered carbon neutral!

If the targets of those nations compliant with the Kyoto Protocol are to be met, greater attention must be placed on broader energy efficiency regimes. These have to include motor speed control.

Most of the bodies charged by government with communicating and effecting change in the approach to energy consumption are naturally focussed on conservation in the broadest public arena. For this reason, the public relations and other promotion has been almost exclusively targeted at the general public and on measures such as insulation, heating and lighting reduction. This is true even of the respected Carbon Trust and Energy Saving Trust in the UK. However, with ambitious national carbon reduction targets and current consumption trends failing to meet the rate of decrease needed indicates that the UK will fall drastically short within the 2020 deadline. Bodies such as Gambica, which represents most of the significant motor speed control manufacturers, know that more must be done.

That government may impose further levies, legislation or penalties relating to energy use is speculation, but as the 2020 deadline looms, it remains a possibility. Against this backdrop, is the almost exponential rise in energy prices that are directly impacting on industry, commerce and consumers by its significant and proportionate affect on costs.
In the simplest terms there are a number of things business can do: ignore energy issues completely and either pass on costs as increased prices or accept continuously reducing margins (rather than invest in energy efficiencies); take passive measures such as installing energy saving luminaires, improving insulation; take active energy initiatives and address all aspects of consumption.

Hence, we return to the topic of motor control. This for industry, commerce and large commercial buildings represents low hanging fruit in terms of making substantial energy savings. In many respects fitting motor speed controls is like changing to energy saving light bulbs in the home - but with on-going automatic savings.

First, a quick physics lesson
Consider that billions of electric motors are in daily (often continuous use) every day. In most countries, fewer than 10% of the motors have any form of control. In continuous duty applications such as powering fans, pumps and compressors, it is possible to effect incredible savings by reducing the motor speed by a very small amount. This is because in such applications, the fundamental physical laws governing centrifugal fans and pumps also preside over the potential savings that can be achieved. In laws of physics, power is consumed as a cube of output. Indeed, affinity laws dictate that while motor torque varies with the speed squared, power varies with the speed cubed. Hence, the cube law impacts greatly the energy efficiency in such equipment.

On loads of this type any speed reduction will save large amounts of energy (that is, a 20% speed reduction will result in a 50% power saving). Remember, torque varies with the speed squared, power with the speed cubed. This means that variable torque loads, such as fans and pumps offer the greatest potential for energy saving.
Culture gaps inhibit industrial energy conservation
Why there has not yet been a huge uptake of motor speed controls lies largely with ignorance of the technology and ambiguity within industry over who owns the task of implementing energy efficiency. To explain this statement more clearly, in a manufacturing plant for example, the plant engineers are often acutely aware of the energy efficiency benefits of installing variable speed drives (VSDs) to AC motors. In the boardroom, managers may be alert to the need to economise on energy for any number of reasons: escalating fuel bills; corporate social conscience; or even just because it's good marketing sense to be seen to be green. The problem is plant engineers are never measured on their ability to save energy; while directors and managers are unaware their plant engineers could do so. It is a simplistic view, but one borne out by evidence throughout manufacturing industries.
Energy intensive industries such as metals manufacturing, glass and plastics processing and food and beverage production understand the need for energy management because their processes involve great amounts of heat. These businesses have traditionally sought ways to maximise their return on investment from the energy used in their primary processes. However, even these energy aware businesses often fail to realise how much more can be saved through building controls and a company wide energy policy.

Of far greater significance overall is the use of variable speed drives in a broad range of processes.

While in many countries industrial energy use has now been slightly outweighed by that consumed by commercial and residential buildings, it is a fact that industry consumes huge amounts of electrical power. About two thirds of that is typically consumed powering electric motors. Of these, an overwhelming majority can be made significantly more energy efficient by controlling their switching on and off or by controlling their speed.

This is a relatively simple task of equipment retrofitting, yet it is clear that most manufacturing and process plants fail to take the step. The reason is often because those that control the costs of an industrial operation are not communicating with those charged with the management of the production processes.

For example, if a painting plant uses hundreds of AC motors on fans, pumps and compressors (continuous duty applications) it could readily benefit from the use of variable speed drives. However, while the plant manager, as an engineer, understands this, he or she is invariable responsible only for improving productivity or output and not for the overhead costs. Higher management is concerned with paying the overheads but remains unaware that such a saving could be made because it is never on the agenda in engineering meetings.

In industry, senior management and plant engineers must learn to talk if an holistic and comprehensive energy efficiency policy is to be achieved. In no other sector is the communication gap wider, than between those charged with making energy decisions and engineers in industry who know how energy can be saved.