Energy efficient motors

Saving money or costing the Earth?

Ways of improving motor efficiency
Material

Average electric motor ‘designers’ mill tell

you that the only way to increase efficiency is to
increase the active material content of the
motor. They say this because they accept
without challenge too many features of existing
designs which have their origins 30 years or
more ago. They ignore improvements which
have been made to lamination steels, can be
made in motor design and manufacturing
processes and should be made to conductor
insulation.

So, let’s be a little imaginative and start with

the lamination steel. Over the year:; steel
manufacturers have reduced losses of their
standard products so that a typical non-silicon
steel with a guaranteed minimum loss of 8
W/kg will have an average loss of about 6.5
W/kg. The most recent introduction, Polycor
420, has a mean loss of under 4.0 W/kg better
than some low-loss silicon steels but with a
much higher permeability.

These improvements have been largely ignored by motor designers because whatever the loss figure quoted by the
steel manufacturers this figure has to be multiplied by anything between 1.2 and 2.0 in order to make the calculated
motor iron loss anywhere near the values measured on motor test. This is to account for winding harmonics – or
high-frequency harmonics - or something. And hitherto, while motor designers could quote you the loss figures for
the steel they used, many of them remained remarkably indifferent to its permeability. Of course there
was a ‘good’ reason for this. About 70% of the magnetizing current is required to drive the flux across the motor air
gap. True permeability of the steel would have an effect on the remaining 30%, but this was a pretty low order
consideration and not worthy of much attention.

Motor losses
Motor losses can be conveniently divided into five groups:
 stator copper loss
 rotor loss (mostly conductor)
 iron loss
 wind age and friction
 stray loss

Stray loss is important and will become even more so if, as seems likely, the current IEC 34-2 test standard is revised to make direct
input/output testing the preferred method and substantially increase the fixed stray loss allocation for motors tested by the existing
Summation of loss method. However, in the author’s view stray loss has as much to do with motor manufacturing methods and quality as
it has to do with basic design, and in the early stages of establishing design principles it can be ignored.
The division of losses between the four
Remaining elements differs widely with motor size. This gives the first clue as to the relative importance of loss and permeability in
electrical steels. In typical 1.5kW and 15kW motors, copper losses (and particularly the stator copper loss) dominate.
On the other hand iron losses form the single highest loss element for a typical 150kW motor. Thus low-loss steel is not in itself of much
value in the smaller motors but is of considerable value in the 150kW machine.

it becomes clear that in the 1.5kW motor the magnetizing current forms a high proportion of the total full load current whereas in the 15
kW and 150 kW motors it forms relatively smaller proportions.

Designing motors to make best use of steel characteristics

If steel permeability is increased the magnetizing current will fall considerably in the 1.5kW motor and will help reduce the dominant
copper losses. This is true even though the air gap ampere turns will remain sensibly constant. But this benefit can be used in more
profitable ways.

With higher permeability the same flux can be driven through a smaller cross section of steel for a given magnetization. This means that
Tooth widths and back iron thickness can be reduced and slot areas increased allowing more copper and then the choice of either lower
Copper losses (increased efficiency) or increased output. Alternatively the efficiency and output can remain the same and core
length reduced to reduce motor cost. Clearly if permeability is the only steel characteristic that is altered, iron losses will rise as flux
density is increased, and this design philosophy is only applicable to smaller motors where iron losses are a low proportion of the total
loss. However, if a steel such as ‘Polycor 420 is used which combines low loss and good permeability then the benefits of this design
philosophy are immediately available (although to differing degrees) across the whole motor range.

Motor building factors associated with steel which affect motor performance
Having low-loss high-permeability steel delivered from the steel supplier does not guarantee a low loss or high performance motor. The
50Hz iron losses can increase significantly as the motor passes through its various manufacturing stages’ and this can account for much
of the iron loss multiplication or ‘fudge’ factor used in motor design calculations. A number of factors can contribute to this increase
including:
 lamination thickness
 inter laminar insulation
 mechanical stress in the laminations
 methods of core building
 rotor die casting
 rotor turning
 Conductor insulation.
Lamination thickness: The thinner the lamination the less the eddy current losses but the higher the costs to both steel supplier and motor
manufacturer.

Inter laminar insulation: UN insulated laminations give rise to unacceptably high eddy current losses when clamped together to form the
motor core. hybrid organic/inorganic coatings of the L3 type which are not only very consistent but can also withstand annealing
temperatures in excess of 700°C and can be used on semi finished steels.

(c) Mechanical stress

Mechanical stress in laminations can increase losses. The stator frame exerts a radial force on the stator core. If this radial force is
increased beyond a certain point the iron loss increases sharply, so ways have to be found of anchoring the core pack to the frame with
minimal radial force.

(d) Fastening the laminations to form a core Many methods including bolting, welding, cleating and semi-piercing the laminations
have been used over the years and all have the effect of increasing loss. on larger motors this method is not generally used and
motor designers must design their laminations to allow for the fastening method. It is not acceptable for the electrical designer to design
the lamination and then for the mechanical designer to add some cleating notices or welding runs later!

(e) Rotor die casting

The circumference of the punched rotor slot presents an un insulated surface and when the rotor is die cast the rotor conductor:; form
conducting bridges between adjacent laminations. These have the effect of increasing stray loss and eddy curren t loss. Worse, the
conductivity of the bridges is highly variable and this gives rise to inconsistencies in product performance. For many years the method
used to minim se this problem has been to dip the die cast rotors into cold water immediately after casting and hope that the differential
contraction rates of the steel and aluminum will cause the two to separate. To some extent this works More recently one Japanese
manufacturer has published details of a rotor slot insulation method2 which makes use of the resistive residue left by a phosphate loaded
EP lubricant oil. Very consistent results are claimed. Theoretical and practical research in this field is continuing.
(f) Rotor turning
To ensure air gap concentricity it is common practice to turn the rotor to size from the motor shaft centers. This can burr over the
Laminations and turn the surface of the rotor into a conducting sheet. For many years this was considered unimportant because the rotor
Operated at slip frequency and this would limit the loss. More recent research work by Mueller et has shown, however, that the losses on
the rotor surface are predominantly high frequency losses caused by the stator slotting. This results in a high-frequency ripple flux
Which can be 10% of the fundamental flux, and the iron loss arising from a 23rd harmonic (one commonly found) can be more than
twice that of the fundamental frequency iron loss. Combine this with a conducting surface on the rotor and you have a recipe for disaster
(or at least inconsistency). The best solution is to punch laminations to size, but this is not usually practical on larger sizes. If rotor
turning is necessary then means have to be developed for separating the laminations at the rotor surface during or after the turning
operation.

Conductor insulation
Conductor insulation is one area which is surely overdue for improvement. Sixty five years ago Class E PVA (poly vinyl acetate) coating
replaced Class A double cotton covered insulation on conductors for low-voltage induction motor. These were in turn fairly
quickly replaced by Class F polyester insulated conductors - usually dual coated because the polyester, whilst being able to withstand
higher temperatures than PVA was less abrasion resistant and less suitable for use with automatic winding machinery. Recent
developments have included ‘filled polyester’ coatings where the polyester contains an insulating material in fine particulate form
which it is claimed enables the insulation to better withstand the voltage spikes sometimes associated with inverters.
None of these developments has done anything to reduce the volume of the insulation, which on a 0.8mm conductor represents 12% of
the cross-sectional area (more on smaller conductors). Its thickness is not regular and it tends to suffer pinhole occlusions in each coat.
Materials scientists working in different fields are well advanced with so called ‘surface modification’ techniques, which depend on
molecular bonds being formed between two dissimilar materials.

The benefits of getting things right

Even without improvements in conductor insulation much can be achieved. The motor designer’s iron loss multiplication or fudge
factor has been referred to earlier. increases in 50 Hz ‘manufacturing’ iron loss caused by manufacture can be largely avoided if the
motor is designed and built correctly. The results are sensibly independent of the type of steel used and, whilst there is still some increase
in the manufacturing iron loss’, the increase is nowhere near that in older motors. More recent work has resulted in almost no loss
increase on some designs. Redesign of the laminations and winding geometry together with improved thermal design and attention to
detail in manufacturing can give significant efficiency increases. One European manufacturer has achieved increases in efficiency
averaging slightly over 3% for four pole standard motors in the range 2.2-22kW.this represents an almost 30% reduction in losses. A
similar reduction in losses is expected to be achieved in all ratings above 1.0kW.

The cost of improving efficiency

There is no simple answer to the question ‘How much does increased efficiency cost?’. Different motor manufacturers will give different
answers which will be influenced both by their investment policies and by the creative thinking (or otherwise) of their design and
manufacturing teams. ‘Energy efficient’ motor ranges have been offered by a number of manufacturers since the early 1980s. Whilst
there was no clear definition of what constituted an ‘energy efficient’ motor they have typically offered efficiencies some 3% higher than
the manufacturers’ standard ranges and certainly originally this increase was achieved simply by the use of more and better active
materials - usually a lower loss (but lower permeability) silicon steel and considerably more copper, particularly in the stator winding.
This approach is costly, and energy efficient motors have typically been sold with price premiums of between 20-30%. Although it can
be shown quite easily in most cases that this extra cost is recouped in 1-2 years, through reduced running costs, most motors in the
range up to 300 kW are sold by the manufacturers to OEMs not users. Since the OEM only fits the motor to his machinery and
sells it on his main interest is lowest first cost (or price), and in Europe at least this has proved to be a major barrier to premium priced
‘energy efficient’ motor sales - and thus to their potential for improving the environment. Against this background one or two
European motor manufacturers with active investment policies and creative designers have set about the task of reducing, by design, the
cost of the active materials required for improved efficiency. They have been very successful. The additional active material as a
percentage of net selling price in the new higher efficiency motors made by one manufacturer is now only 2% (for an average efficiency
gain of just over 3%) compared with over 10% extra material for the same efficiency gain in their older ‘energy efficient’ range, which
was based simply on adding active material to existing motors. This small additional material cost has been offset by reductions in other
costs and the new improved or higher efficiency motors (HEMS) are sold atstandard prices, thus overcoming the ‘OEM barrier’.

However, such a policy does Inquire investment both in tooling and training; for the pioneers it also required considerable investment in
RQD. Many argue that this investment should be recouped through price premiums, even though it has been shown that because of the
structure of the market this would be self defeating. If these improved efficiency motors are to be sold without price premiums and no
other financial -incentives are available their introduction must be !Jhased over the normal replacement cycle of the key tooling. For
larger manufacturers this will be a relatively short period, say, 4-5 vears, but for some small manufacturers it could be longer.
To summarise:

 Improved efficiency motors can be made by adding active material to existing designs. The policy is unimaginative and
costly and will result in a premium priced product with limited customer appeal.
 A holistic approach embracing creative design, new materials and improved manufacturing techniques can reduce the
increased material cost almost to zero in many cases, but will require investment in both tooling and training.
  If improved efficiency motors are to be produced by such an approach in the absence of other financial incentives
their introduction needs to be phased over the normal replacement cycle for key tooling.

In last I thank David Walter for his guidance in completion of this paper