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. Stuart Harvey of SoftStart UK comments

Electrical engineers have had a
common thread running through
their work for the last five or 10
years – reducing energy consumption to cut
carbon emissions. This has the 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 replaces the voluntary
CEMEP scheme.

Under MEPS the current EFF1 and EFF2
efficiency classification will be discontinued.

Indeed EFF2 rated motors will become
illegal to buy. 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% and these
motors may have been driving equipment via
equally oversized – and therefore
inefficient – gearboxes, leadscrews, 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% full load, and many will not be
running within the range.

The new MEPS address this by measuring
the efficiency level of the whole drive system
rather than just the motor. 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 are from 2013. There are by far
the two biggest users 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 be a simple matter of
changing habits and
assumptions, but it may also
mean getting company policies
and design procedures changed.

OEMs may need to inform their regular
customers of the changes.

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 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; 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 if a motor
is expected to run for less than 2000h/year,
the total running cost of a high efficiency
motor won’t be significantly below those of a
lower category motor.

New for old It is natural to question whether it is worth
swapping old motors for new. Another rule
of thumb would be that a new motor is 25-
50% more efficient that one built before the
turn of the Millennium. (As medium and
large motors often have an installed life of
20+ years, 1999 isn’t 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.You could also add in
engineering costs of the swap out, but you
should offset this against the cost of routine
maintenance and improved reliability.

Before MEPS is fully implemented the EU
intends to introduce a similar scheme for VS
drives. Hopefully the two schemes will mesh
together seamlessly rather than burdening
engineers with even more paperwork.