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Don’t Jump to Conclusions over Drive Efficiency, say Eaton.

Friday, October 12th, 2012

Thanks to Eaton for the content of this blog post.

Fitting variable speed drives to motors is often put forward as a way of saving energy, and this idea appears to have influenced the wording of part of the European Ecodesign Directive. However, as Phil George of Eaton’s Electrical Sector explains, variable speed drives are not always the most energy efficient choice.

Electric motors are everywhere – they literally drive the wheels of industry, they power the HVAC plant on which modern commercial operations rely and they have a myriad other applications. Since motors are so widely used, achieving the highest possible energy efficiency from them is a key concern, both from the point of view of cutting energy bills for the users and of minimising environmental impact.

In response to this need for efficiency, the International Electrotechnical Commission (IEC) has produced a new international standard that identifies three energy efficiency classes for three-phase motors. This has been adopted in the UK as BS EN 60034-30:2009. The classes defined are IE1, standard efficiency; IE2, high efficiency; and IE3, premium efficiency.

A fourth class, IE4, super premium efficiency, will be added at some time in the future. These classes supersede the older CEMEP EFF1 and EFF2 efficiency classes for motors.
Using higher efficiency motors leads to big cost savings and carbon footprint reductions. It has been calculated, for example, that with an 11 kW motor running for 8000 hours per year, the reduction in energy costs by using an IE2 motor instead of an IE1 motor would amount to around €2,700 per annum, and that CO2 emissions over the same period would be cut by 15 tonnes. The benefits of using an IE3 motor instead of an IE1 motor are even more impressive – a cost reduction of €4,600 and a 25 tonne reduction in CO2. These figures are taken from EuP (Energy using Products) Lot 11, one of the documents that makes up the Ecodesign Directive (2009/125/EC).

Because of the large benefits achievable with high efficiency motors, the Ecodesign Directive incorporates a mandatory timetable for their adoption in new equipment and in existing equipment that is being substantially modified or upgraded. This timetable requires that from June 2011, all motors with ratings from 0.75 kW to 375 kW shall be class IE2 or better, with fixed or variable speed control. From January 2015, motors from 7.5 kW to 375 kW have to be class IE3 with fixed or variable speed control, or IE2 with variable speed control. Finally, from January 2017, all motors from 0.75 kW to 375 kW have to be class IE3 with fixed or variable speed control, or IE2 with variable speed control.

These requirements show that it’s acceptable to use a lower efficiency motor (IE2) rather than an IE3 motor provided that the motor is controlled by a variable speed drive (VSD). The implication is that using a VSD will automatically result in increased efficiency and lower energy usage. Unfortunately, things are not quite that simple. Let’s be absolutely clear about this – it is perfectly true that there are very many applications where using a VSD will produce energy savings and, in a lot of cases, these savings will be substantial.

These are typically applications like fans or pumps where the amount of power required falls rapidly as speed decreases and, crucially, where full output is not always required, so the motor can operate at reduced speed for at least some of the time. The benefits of using VSDs in these applications are beyond question. There are, however, applications where the motor is required to operate at full speed all or almost all of the time – examples are escalators and some types of conveyor. Contrary to what is usually believed, using a VSD in applications of this type will actually reduce rather than improve energy efficiency!
Here’s why. Even the best of VSDs have some inherent losses. These losses are small – around 2% for small to medium sized drives – but they are significant. With fixed speed motor starters using modern components, however, the inherent losses are so small as to be negligible. Let’s see what this means in practice.

If a VSD is used in an appropriate application the energy savings that result from being able to run the motor at reduced speed will far outweigh the losses in the drive, so the result will be a worthwhile reduction in overall energy usage. If, however, a VSD is used in an application where the motor has to run at full speed most of the time, there will be no energy savings to offset the losses in the VSD so overall energy efficiency will actually be reduced. In a lot of applications where the motor operates only at full speed, a simple contactor starter is the most economical and most energy efficient choice. However, in cases where controlled starting and stopping are required or where the motor is likely to operate lightly loaded for much of the time, a soft starter may be a better choice. Soft starters provide controlled acceleration and deceleration, and usually incorporate a bypass contactor, which means that they are as efficient as a contactor starter once the motor is up to speed. Alternatively, for motors that are running with light loads, they can be left in circuit to improve power factor and overall efficiency.

But what of the timetable for adopting efficient motors that is laid down in the Ecodesign Directive? Doesn’t that make the use of VSDs mandatory after certain dates if IE2 motors are used? On the face of it, the answer is yes, but it is to be hoped that the requirements will be interpreted with a degree of common sense to allow the continued use of fixed speed control with IE2 motors in cases where this can be shown to be more energy efficient. The intent of the Directive, after all, is to save energy, not to force the adoption of practices that waste it!

In summary, it’s never a good idea to jump to conclusions about the type of drive that will deliver the best energy efficiency in a given application; each application must be individually examined and evaluated. If necessary, advice should be sought from an expert in drive technology. However, be sure to choose an expert that, like Eaton’s Electrical Sector, is able to offer a full range of fixed and variable speed solutions, and will therefore be able to take an impartial view of the application than being constrained by commercial pressures to sell one particular type of solution.


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