Several factors must be taken into account when selecting the right electrical steel for motors.
Of course the magnetic and mechanical properties of the material are among the more decisive ones. The thinner the material is, the lower will be the eddy-current losses at high frequencies. The efficiency of the motor increases. This in turn means lower electricity consumption and thus higher distance ranges at the same level of battery capacity.
Yet what kind of effect does the thickness of electrical steel have on factors such as stampability, available stamping capacities, required stamping technologies, availability on the market and finally on the price of the material? Besides efficiency, are there any other technical factors for the design of the motor that require different thicknesses? Should the motor serve merely as a support for the combustion engine (mild hybrid) or should it independently drive the car up to a certain speed and cover shorter distances using electric power (plug-in hybrid) alone? Or should the electric motor take the car across far distances all by itself (electric vehicle)?
Is lower thickness really always the better choice from a holistic point of view? Join us while we think through some of the possibilities.
Thinner sheets, longer production time, reduced capacities
Let us take as an example a motor with an outer stator diameter of 250 mm and a stack height of 120 mm. The thinner the sheet thickness, the more laminations will be required to achieve the required overall height. The stamping speed varies between 220 strokes/min (for 0.25 mm thick sheet) and 250 strokes/min (for 0.35 mm) depending on the sheet thickness. Taking into account rejects, downtimes and availability of the system, the production volume would be between 32 stacks (0.35 mm) and 19 stacks per hour (0.25 mm), meaning that the stamping time increases by 1.7 times.
Impact on global stamping capacities
Efficiency maximization yes, but not independently of the application
Strong and powerful electric motors can be produced with a large number of electrical steel grades. When selecting the drive motor for purely electric operation, the primary question is how high the overall savings would be by using higher-quality, thinner and thus more expensive electrical steel. Even relatively small differences in efficiency can influence the range and thus the required (very expensive) battery capacity.
If the electric motor is only to support the combustion engine as in a mild hybrid car, or the vehicle is intended to operate purely electrically over short or medium distances as in a hybrid or plug-in hybrid, the demand on the performance profile is lower. In addition to material savings in the engine, the required battery capacity is significantly lower. This is why it can make sense to reduce the efficiency of the motor by using thicknesses in the range of 0.3 and 0.35 mm.
Additionally, very thin electrical steel leads to an increase in efficiency in high-speed motors, especially in the stator, but in the rotor often the achievement of very high strengths is required instead of less thickness. Special joining processes such as full-face bonding enable rotor designs that also have a positive effect on efficiency.
Conclusion
Thinner isn‘t always better. From a holistic perspective of processability, costs, areas of application, efficiency and other technical factors for the design of the motor the use of electrical steel must be viewed in a very differentiated way. Many conditions speak clearly in favor of using electrical steel in various thicknesses.
The technical specialists of voestalpine will be happy to help in the decision-making process when it comes to selecting materials and processes.
