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Overall driveline efficiency and the Ecodesign standard

13 November 2015

Does using the highest efficiency motor that money can buy together with an inverter really give you the lowest running costs? Andrew Stephenson takes a closer look at the practicalities.

Cutaway showing the internal structure of an inline helical geared, directly mounted motor

When we consider the motor; as of January 1st 2015 we now have a minimum efficiency level of IE2 for 4 pole motor powers from 0.75kW up to 7.5kW and IE3 for higher powers. In January 2017 it will be IE3 across the complete range from 0.75kW up to 375kW. And looking further ahead motor efficiency levels of IE4 have been defined and many manufacturers can offer this technology today, but as yet no planned introductory date has been announced.

The question arises, does it really make sense to use these higher efficiency level motors? Well, in many cases where the machine has a constant load and it runs for long periods of time without starting or stopping, yes it does. But again in many cases no it doesn’t. For instance, if you have applications where the load constantly varies or the motor has to start and stop frequently then probably the answer is no.

The main reasons for this are that the starting current for the higher efficiency motors is increased and also the rotor inertia is greater. In the standard it allows for exemptions exactly for this reason. The normal rating for a motor is defined as S1 mode which is continuous operation but a motor can be rated for short term operation S2 mode or intermittent operation S3 mode, and so on, and then is exempt from the standard efficiency regulations and alternate efficiency motors can be utilised.

Most motor manufacturers still produce IE1 and IE2 level motors for areas of the world that have no mandatory requirements of minimum efficiency specifications, and therefore these motors should be considered for short term and intermittent duty rated applications. In addition, the initial price is generally more cost effective. As an example of the efficiency levels when looking at the difference between IE2 and IE3, if we take a 7.5kW motor the respective efficiency levels are 88.7 percent and 90.4 percent.

The use of inverter drives to achieve energy efficiency savings has been prolific and in a huge amount of cases - especially fan and pump applications - energy savings as high as 50 percent can be achieved. For applications where the load varies or where external mechanical baffles or valves/chokes are used to maintain pressure/flow then it’s ideal for an inverter to be incorporated. The inverter typically can reduce the energy used by automatically reducing the torque/speed to suit the application requirements derived from the inverter’s own current monitoring and/or functions such as internal PI control.

The inverter can also provide basic functions such as a soft start which again aids energy efficiency where the current is limited to typically 200 percent of the motor current rather than the possible 600 or 700 percent in the case of direct on line starting. There’s no doubt that in many cases inverters are good for energy saving where varying loads are present. However, it should be remembered that inverters are, of themselves, 97 percent efficient on average. 

As part of the minimum efficiency standard, for IE3 motor powers 7.5kW up to 375kW, it is allowed to install an IE2 motor and an inverter and meet the standard; but this can create an anomaly. If the application doesn’t require variable speed and the load is constant then in reality there is a deterioration in efficiency - a lower efficiency motor and an inverter that’s 97 percent efficient.

Readers may or may not be aware that there is a new Ecodesign Standard EN 50598 which looks at the efficiency levels of the Power Drive System (PDS) and takes both the efficiency of the Complete Drive Module (CDM – inverter/starter and all necessary components) and the motor into consideration. This then gives a resulting overall efficiency level and a new designation of IES0, IES1, IES2 etc. As part of this new standard, extended products such as mechanical transmission are to be considered, which does make complete sense.

Not many applications run at the typical running speeds of electric motors. They will in general be required to run at a slower speed and therefore a reduction device will be required. Typically when reduction ratios of up to 8:1 are required, flat, V-belt, toothed belt or chain and sprocket solutions could be used. These are typically quite low cost but their efficiency is generally between 92 and 97percent.

Mechanical gearboxes are very often utilised and typical designs are categorised when referencing input drive direction to output drive direction, and are therefore inline or co-axial, parallel and right-angled. The efficiency of gearboxes does vary depending on the number of reduction stages and also the type of gears used within the gearbox. Regarding inline and parallel gearboxes, helical gears are used where reduction ratios of on-average up to 9:1 can be achieved per stage and typical efficiencies of 98 percent per stage are common. For higher ratio requirements multiple stages may be used and total reductions of over 14,000:1 can be achieved with very good efficiency levels.

For right-angled drives there are generally two technologies utilised, worm/wheel and bevel gears. With worm/wheel, a single reduction is seen with ratios of 5:1 - 100:1 and corresponding efficiency levels of typically 90 - 40 percent respectively. Obviously at 100:1 and 40 percent efficiency this solution is, for most applications, not ideal and therefore many manufacturers produce a two stage helical first stage and then select the worm and wheel with better efficiency levels to give a better overall efficiency. 

For many years, Worm boxes as they are typically called, have been the common solution for right-angled drives. That said, more recently there has been a huge increase in demand for the use of helical bevel gearboxes where for the same ratio reduction efficiency levels of 94 percent can be easily achieved. With this increase in efficiency, over 50 percent in many cases, much smaller kW motors and inverters can be used making a dramatic reduction in running cost. Historically helical bevel gear boxes have been 3-stages (helical, hevel, helical), but recently 2-stage (bevel, helical) models with higher efficiency and better cost competitiveness are attacking the worm box market.

In general there are two methods of connecting a motor to a gearbox; a standard IEC motor or a so-called direct mount motor. With the IEC motor - typically with a B14 face or B5 flange and a corresponding flange on the gearbox – a jaw type or similar coupling connects the motor shaft and gearbox input shaft. It should be noted that wherever another driving element is introduced such as the coupling, it can never be 100 percent efficient. Typically a coupling will only be 98 percent efficient and therefore is potentially negating any benefit that you’ve gained by fitting an IE3 motor over the IE2.

With the direct mount design, the motor is directly mounted to the gearbox housing and the motor shaft is machined to have the first gear pinion directly mounted on it. This then forms part of the first reduction stage and is therefore more efficient. There are other benefits of being a more compact solution with the motor closer to the gearbox and hence cooler running.

This same principle applies to the output of the gearbox and wherever possible this should be directly connected to the machine without any coupling or belt drives. With parallel and right-angle drive solutions, hollow shaft options can be utilised where the gearbox is connected on the actual machine shaft and the gearbox is prevented from rotation by an output flange or torque arm.

Your drive supplier should be able to help make an appropriate decision; provide them with the machine’s performance requirements and rely on them for best advice and solutions. Request breakdown and documented analysis for the selection - and if you’re not convinced then get a second opinion.

Andrew Stephenson is managing director, NORD Drivesystems UK

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