Towards a new age of electric motor efficiencies

Electric motor minimum energy performance standards (MEPS) are now well established across Australia. Alongside moves to harmonise efficiency standards on a global scale over the coming years, innovative research and new directions are already emerging, writes SEW-Eurodrive's engineering manager, Frank Cerra.

The Australian introduction of MEPS for electric squirrel-cage induction motors is, in some respects, old news. This Australian Greenhouse Office (AGO) initiative, which was first legislated in October 2001, represented an important first step for Australia. Since this inauguration, both the Australian and international MEPS scene have been anything but stagnant.

In April 2006, the two Australian efficiency levels were ratcheted up a notch. A revised edition of the standard (AS/NZS1359.5:2004) saw the previous 'high-efficiency' (HE) levels specified in AS/NZS 1359.5:2000 effectively become the new MEPS levels (often referred to as 'MEPS2'), coupled with an entirely new, and more stringent HE level. This new HE level targeted a further 15% reduction in losses on the 2001 MEPS 'high-efficiency' levels. In the long term, these new standards should reduce total Australian energy consumption by 8900 GWH, and cut the country's greenhouse emissions by 7.7 MT [1].

Internationally, there are moves to harmonise the disparate electric motor efficiency standards, such as those of the US and Europe, into common International Electrotechnical Commission (IEC) standards over the coming years. Yet research and development work is already being undertaken by global industry bodies that goes beyond the bounds of the electric motor itself. This work is exploring the domain of what is often described as 'total drive efficiency'.

Beyond MEPS

While current MEPS-compliant electric motors realise a 2 to 3% efficiency improvement over old 'standard' efficiency motors, it is valuable to see how such figures sit within the context of a total drive train efficiency upgrade. While MEPS compliance is vital, it is equally vital to look beyond the motor itself and consider the drive train as a whole.

Figure 1 depicts a real-world example of such a total drive train efficiency upgrade, drawn from the Australian mining industry. The top-most illustration in Figure 1 shows the pre-upgrade conveyor head-drive installation of a very old system. Originally, the drive train comprised a MEPS-compliant motor, coupled to the head-drive via a vee-belt, worm-gear unit and chain/sprocket drive. The inherent efficiencies across the three-stage drive train resulted in an overall drive efficiency of 60.8%.

A high-efficiency helical-bevel gear unit close coupled to the MEPS-compliant motor replaced this inefficient drive train. The resultant efficiency of this replacement assembly was 88%. Most important are the energy savings achieved as a result of this almost 30% efficiency improvement. Based on an energy cost of $0.10/kWh, the new installation saves almost $2500 per annum in reduced energy costs. The estimated return on investment in this particular scenario is around two to three years. There would also be a significant reduction in greenhouse gas emissions.

It should be noted that the average life expectancy of a typical induction motor can be around 20 years [2]. As a result, the energy savings determined here offer commercial benefits well beyond the ROI period.

More telling is just how this total drive train efficiency might vary if MEPS-compliant motors were not used. Using a pre-2006 MEPS-compliant motor (non-compliant by today's rulings), the total drive train efficiency with the close-coupled helical-bevel gear unit would only drop to 86%. This figure is still significantly higher than the vee-belt/worm-gear/chain/sprocket combination with MEPS-compliant motor. This underscores the importance of looking beyond the motor itself, considering the entire drive train and using modern techniques in drive applications.

A further consideration is the application of frequency inverter technology, used in conjunction with high-efficiency electro-mechanical systems. Studies have shown that pumped flow regulation systems can achieve significant efficiency improvements by replacing traditional 'single-speed pump plus throttle valve' technology with a pump driven by a variable speed inverter [3]. This efficiency improvement can reduce energy consumption to a fraction of what would be required if the pump speed regulation is coupled with high-efficiency motors, pumps, couplings and piping systems.

Towards the future

These 'total drive efficiency' perspectives are central to studies currently being undertaken by a wide range of influential industry groups and conferences. These include the European Commission's Energy Usage Product Directive, the International Copper Association's Energy Efficiency in Motor Drives Systems (EEMODS), and the Asia-Pacific Partnership on Clean Development and Climate (AP6) initiative. The latter is most important from an Australian environment perspective, as it represents the major industrial groups in our region: Australia, China, India, Japan, Korea and the US.

Such industry groups are seeking out energy-saving opportunities within specific electric motor application areas such as fans, pumps, HVAC, compressed air systems, refrigeration and so on. The results of these studies will pave the way for the future, and could represent the foundation for a new wave of global 'total system MEPS' standards.

A 2007 study from the European Commission's Energy Usage Product Directive notes that energy costs for an electric motor in continuous operation represent 96% of its total life-cycle costs [2]. Capital cost of the motor itself represents just 2.5%. This is largely because of the inherent robustness and longevity of the squirrel-cage induction motor — the same study pegs the motor's average life at somewhere between 12 and 20 years.

This suggests that the performance class of the electric motor — and the design of drive-train to which it is coupled — need to be carefully selected and implemented. Initial capital expenditure will typically be a negligible factor in terms of total life-cycle expenditure. By contrast, the very nature of the technology applied, both at the motor level and across the total drive train, is crucial to operational component of total life-cycle cost.

The MEPS world is fast moving towards a uniform standard. The next major challenge will be to address total life-cycle/total drive train efficiency issues. The potential benefits and ramifications, both environmentally and economically, are substantially greater than those capable from a focus on the electric motor alone.

References:

[1] Regulatory impact Statement for Minimum Energy Performance Standards for Electric Motors, Syneca Consulting, December 2003.

[2] EUP Lot 11 Motors, Report No. 3, Analysis of existing technical and market information, ISR- University of Coimbra, Aníbal T. de Almeida et al, April 2007.

[3] Motor Challenge - Energy Efficient Motor Driven Systems, European Copper Institute, April 2004.

Figure 1: Conveyor head-drive upgrade — legacy 'vee-belt/worm-gear/chain +sprocket' installation (top-most); helical-bevel gear unit upgrade (below).