Engineers should no longer oversize motors

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Engineers have traditionally oversized electric motors to allow for worst-case operating conditions and to provide an extra margin for 'peace of mind.' However, Stuart Harvey of Softstart UK explains why this is no longer wise, given today's need to reduce energy consumption.

Engineers should no longer oversize motors Electrical engineers have had a common thread running through their work for the last five or ten years: reducing energy consumption to cut carbon emissions. This has the very useful additional benefit of cutting electricity bills, so achieves the engineering impossibility of making the company accountant happy.

Not surprisingly, new regulations and guidelines have emerged to encourage this drive towards a greener world, the latest of which is the EU's Minimum Energy Performance Standard (MEPS). This is being phased in, with the first part already a legal requirement and the last part due to be enacted in 2017, and it replaces the voluntary CEMEP scheme.

Under MEPS the current EFF1 and EFF2 efficiency classification will be discontinued. Indeed, EFF2-rated motors will become illegal. Also a new testing procedure that gives highly accurate efficiency figures will be introduced. It is expected that the increased purchase price of the new high-efficiency motors will be recouped many times over by the energy savings.

In the past, however, designers routinely oversized motors by 10–20 per cent. And these motors may have been driving equipment via equally oversized and therefore inefficient gearboxes, lead screws, belt or chain drives, clutches or couplings. Added to this is the fact that motors have a fairly narrow optimal efficiency band of typically 80–85 per cent of full load, and many will not be running within that range.

The new MEPS address this by measuring the efficiency level of the whole drive system rather than just the motor on its own. This means every installation has to be assessed individually, whereas in the past motors could be bought with an efficiency rating.

Fan systems are already subject to MEPS, and pumps will be from 2013. These are by far the two biggest user groups for motors, so OEMs need to make sure they are familiar with and compliant with the requirements. Over the subsequent four years all motor drive systems will be drawn into the system, but it is likely that end users will start specifying compliance to their OEMs rather sooner. Engineers of a certain age who were brought up over-specifying motors will need to move to a new paradigm. This may be a simply matter of changing habits and assumptions, but it may also mean getting company policies and design procedures changed. Established OEMs may need to inform their regular customers of the changes.

End users are not obliged to change existing equipment, but it may be worth modernising equipment in line with MEPS in order to improve energy efficiency, enhance green credentials and save money.

Efficiency calculations

Significantly, if a motor is used in conjunction with a soft start or variable-speed drive, the electronic component is also considered to be part of the system for which overall efficiency is calculated. This means mains-borne harmonics and power factor correction (PFC) will have to be brought into the efficiency calculations, as will filters and other associated line equipment.

It is also notable that a drive may make a significant contribution to overall efficiency by enabling a smaller motor to be specified. The motor would be sized for 'normal' duty, but the drive can push through a bit more power when required for short-term exceptional circumstances. The net result could be a significant energy saving, with a payback period of just a few months, leading to a positive contribution to the overall bottom line within the first financial year.

It should also be understood that a variable-speed drive (VSD) can be a good investment for a fixed-speed application. If the load is to run at a different speed than the output speed of the motor, the speed shift can be achieved with a gearbox, belt/chain drive, mechanical variator or an electronic VSD. Each of these will cost something, and the advantage of a drive is that it can easily be reset if requirements change.

However, it must be made clear that a drive will not improve the efficiency of the motor it is feeding; if a low-efficiency motor is specified, it cannot be made more efficient by the addition of a drive.

The advantage of lower-efficiency motors is that they are cheaper to buy, so may be suitable for duty cycles that have low overall running times. As a rule of thumb there are 9000 hours in a year, and if a motor is expected to run for less than 2000 hours a year, the total cost of ownership of a high-efficiency motor will not be significantly below those of a lower-efficiency motor.

New motors for old

It is natural to question whether it is worth swapping out old motors for new ones. Another rule of thumb would be that a new motor is 25–50 per cent more efficient than one built before the turn of the millennium. (Note that medium-sized and large motors often have an installed life of 20 years or more, so 1999 is not really that long ago.)

The efficiency improvement gives the basic data for payback calculations – the simplest of which would be based on capital cost plus running costs, but you could also add in engineering costs of the swap out (but this should be offset against the cost of routine maintenance and improved reliability).

And finally, before MEPS is fully implemented the EU intends to introduce a similar scheme for VS drives. Hopefully the two schemes will be so similar that they will mesh together seamlessly rather than burdening engineers with even more paperwork.

To find out more about motors under the new Minimum Energy Performance Standard (MEPS), go to

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