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Turning losses into gains

12 January 2015

Standard induction motors use about 30% of all the electricity generated in the world. Armed with that statistic, it is not hard to realise why increasing efficiency is a crucial R&D strategy among manufacturers. Sebastião L. Nau and Daniel Schmitz of WEG consider what can be done


So what are motor manufacturers doing to reduce losses in induction electric motors to meet the level of efficiency required by IE4? Key to designing a more efficient motor is to understand where the losses occur. When compared to IE3 motors, those that reach IE4 will need to reduce their losses by between 10 and 24%.


Joule losses in the stator winding

Top of the list of losses is usually Joule losses in the stator winding. To reduce these it is essential to reduce the resistance of the winding. There are two main ways that this can be achieved – enlarging the wire diameter or increasing the stator length.


Enlarging the wire diameters can be done at the same time as maintaining or decreasing the number of turns. The disadvantage with maintaining the number of turns is that it requires improvements in the winding manufacturing process because with a higher fill factor it is difficult to insert the windings into the stator slots. 


By enlarging the stator length, the cost of conductors and steel lamination is increased. The stator resistance becomes higher, increasing Joule losses, but it is compensated by the reduction of iron losses due to the magnetic flux density reduction. The performance characteristics of the motor remain the same because the magnetic flux was not modified. Additionally, the area for heat transfer from stator surface to the frame becomes larger and hence the temperature rise decreases, meaning less energy is required for ventilation. 


Joule losses in the rotor

Joule losses in the rotor are a matter of rotor bars conductivity, shape of rotor bars and short-circuit rings. To reduce losses of cast aluminum rotor cage, simulation software should be used to analyse the quality of rotor slots and short-circuit rings filling and its correlation to the cast aluminum process parameters. 


When considering the conductivity of the rotor bars and short-circuit rings, the advantages and disadvantages of using cast copper or aluminum must be reviewed carefully 


Mechanical losses

Good ventilation is key to reducing the mechanical losses, particularly for two pole motors. Because of its high efficiency, an IE4 motor has inherently less demand for heat removal, so the ventilating system can be optimised to reduce mechanical losses and consequently acoustic noise. The geometry of the fins is also important. There is a good relationship between the fin height and width between two adjacent fins. They have to be designed in order to provide the best thermal dissipation. 


The positioning of terminal box is important so as not to interfere with the air flow and the design of the end shields, which should have fins to remove the heat generated by the bearing’s friction. When designing the fan and end cover consideration must be given both to efficiency and noise. 


Iron losses

A major cause of inefficiency comes from iron losses. Using higher quality, silicon steel lamination (usually thinner lamination) than those currently used would appear to be the answer, however as with most improvements there is a question of costs against efficiency gained. Reducing magnetic flux density is another possible solution.


The problems, however, come with software used to calculate the iron losses. Software usually considers the magnetic saturation (BH) curve of the steel lamination material obtained from the Epstein Frame Test. However, this procedure is not accurate because the magnetic flux behaviour in an Epstein Frame Test sample is different from reality.


To evaluate the steel lamination properties properly, a three-phase electromagnetic device was developed at WEG that truly represents the magnetic flux of a motor. It evaluates the magnetic losses generated in the stator, taking into account the lamination’s geometry and the magnetic field rotating component. This allows evaluation of the total magnetic losses taking place in the stator, which comprises those generated in the yoke and in the teeth.  


Future path

To reach IE4 efficiency, motor designers and manufacturer have an abundance of choice.  Each step down the path to improve efficiency has an effect on the motor’s performance, cost and ease of manipulation, so care must be taken to understand the required parameters before embarking on any change. Induction motors, when properly designed, can reach IE4.

 

Sebastião L. Nau and Daniel Schmitz are from the Research and Technological Innovation Department of WEG Jaraguá do Sul - SC, Brazil


 
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