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Engineers: you no longer need to oversize motors

04 January 2013

For generations engineers happily oversized electric motors to allow for worse case working conditions, plus an extra margin of comfort. But what made good sense then is something of an anathema to today’s energy conscious engineers, claims Stuart Harvey.

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. Not surprisingly, new regulations and guidelines have emerged to encourage this drive, 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.

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.

However, in the past designers routinely oversized motors by 10-20 percent. 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 full load, and many will not be running within the range.

The new MEPS addresses 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.

All systems go
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 up to speed and comply with 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.

Thus engineers of a certain age who were brought up over-specifying motors, need to move to a new paradigm. This may simply be a 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 their own energy efficiency and enhance their green credentials.

Significantly if a motor is used in conjunction with a soft start or variable speed (VS) 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 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 allowing 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-lived unusual 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 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 VS drive. 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. Thus 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 9,000 hours in a year and if a motor is expected to run for less than 2,000 hours a year, the total running cost of a high efficiency motor won’t be significantly below those of a lower category motor.

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!

Stuart Harvey is managing director of Softstart UK

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