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, VSDs 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
have a myriad other applications. Achieving
the highest possible energy efficiency from
motors is key both to cutting energy bills
and to minimising environmental impact.

The International Electrotechnical
Commission 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 later.

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 11kW motor running for 8000h/year, the
reduction in energy costs by using an IE2
motor instead of an IE1 motor would
amount to about ¤2700 per annum, and that
CO2 emissions over the same period would
be cut by 15t. The benefits of using an IE3
motor instead of an IE1 motor are even
greater – a cost reduction of ¤4600 and a 25t
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).

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

These requirements show that it’s alright
to use a lower efficiency motor (IE2) rather
than an IE3 motor provided the motor is
controlled by a VSD. The implication is that
using a VSD will automatically result in
increased efficiency and lower energy usage.

Things are not quite that simple. It is true
that there are many applications where using
a VSD will produce energy savings and, in a
lot of cases, savings will be substantial.

Examples include 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 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 include
escalators and some types of conveyor. Using
a VSD in such applications will reduce rather
than improve energy efficiency. Here’s why:
Even the best of VSDs have some inherent
losses. These are small – around 2% for small
to medium sized drives – but they are
significant.With fixed speed motor starters
using modern components, the inherent
losses are so small as to be negligible.What
does this means in practice?
If a VSD is used in an appropriate
application the energy savings from being
able to run the motor at reduced speed will
outweigh the losses in the drive, so the result
will be a reduction in overall energy use. 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 efficiency will be reduced.

In a lot of applications where the motor
operates only at full speed, a simple
contactor starter is an economical and
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 have a bypass
contactor, which means that they are as
efficient as a contactor starter once the motor
is up to speed. For motors that are running
with light loads, they can be left in circuit to
improve power factor and overall efficiency.

It is to be hoped that the requirements of
the Ecodesign Directive 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.