It must be emphasised that the standard electric motor is already a very efficient device with efficiencies above 80% over most of the working range, rising to over 90% at full load. However, because of the high energy consumption, and the very large number of installed units, even a small increase in motor efficiency can have a major impact on costs.
The efficiency of an electric motor depends on the choice of materials used for the core and windings, their physical arrangement and the care and precision with which they are handled and assembled.
Losses can be categorised into two groups. Those that are relatively independent of load (constant losses), and those that increase with load (load dependent losses).
The factors that affect efficiency are:
- Conductor content – load dependent
- Magnetic steel -mainly constant
- Thermal design – mainly load dependent
- Aerodynamic design – constant
- Manufacture and quality control – constant
Resistive losses in the windings increase with the square of the current (which increases with the load) and normally account for around 35% of the total losses.
Since more copper requires more space, both for the end windings and in the stator slots, the volume of material in the magnetic circuit would be reduced, leading to earlier saturation and increased iron losses. Consequently, it is necessary to increase the length of the magnetic core, and sometimes the diameter as well.
Normally, the increased length is accommodated by increasing the overhang at the non-drive end of the unit. Because copper losses are load-dependent, the benefit of increasing the copper content is most apparent at high loading. Since the coefficient of resistance of copper is positive, the losses increase as temperature rises.
Magnetic steel is the most expensive component of the motor, so any increase in the total amount used is undesirable on cost grounds. The iron losses are of two types – hysteresis loss and eddy current loss.
Hysteresis loss is due to the non-linearity of the flux density/magnetising force curve and is a property of the steel itself and to minimise it two properties are required – a low energy loss and good high field permeability, i.e. the steel must be easy to magnetise and must not saturate at high flux densities of up to 1.8 Tesla.
This is the subject of on-going research that is making promising progress. Eddy-current losses are due to induced current in the stator laminations and are reduced by reducing the thickness of the laminations and by ensuring good insulation between adjacent laminations.
Thinner laminations are, naturally much more expensive to produce and more difficult to handle, so the chosen thickness is always a compromise. Magnetic losses are particularly important when the supply is distorted by harmonics because eddy current losses increase with the square of the frequency while hysteresis losses are proportional to frequency.
The benefit of using improved magnetic steel is a reduction in loss across the whole of the working range, but, because it is not load dependent, it is particularly apparent at low loadings.
New modelling techniques have allowed the production of motors with optimised cooling flow, reduced clearances (increasing the efficiency of the magnetic circuit) and lower copper losses. Lower losses and good thermal design result in lower operating temperatures and hence a longer service life.
Most electric motors are cooled by drawing air through the windings by an integral fan and exhausting it over the externally ribbed casing. The airflow is complex and computer modeling has been used to optimize the design of the fan and cowling to produce more efficient cooling with a lower noise level.
Windage losses can be reduced by careful design of the rotor.
The introduction of stresses in the magnetic steel during motor assembly can increase iron loss by up to 50%. By considering assembly techniques at the design stage and by paying attention to handling techniques, this increase in iron loss during manufacture has been reduced to negligible proportions. Eccentricity between the stator and rotor generates harmonic fluxes with consequently higher losses.
The overall result of these improvements is an increase in efficiency of 3% (corresponding to a reduction in loss of about 30%) at full load and a halving of losses at low loads. Figure 4 shows the comparison between the efficiency of 75 kW standard and high efficiency motors against actual load.
Because many motors spend considerable time running at low loading or idling, designers of high efficiency units have paid great attention to reduction of the constant losses.
The result is a halving of losses at loadings less than 25% load and an efficiency improvement of 3 to 5% at full load, a reduction in losses of about 28%. This represents an impressive achievement.
Reference // A Good Practice Guide to Electrical Design – Copper Development Association