The choice of lamination material for rotors or stators has a fundamental and far-reaching impact on motor design in terms of output, heat generation, weight, and cost (the term "motor" is used loosely here to include generators, tachometers, rotary transformers, and AC generators). Unfortunately, few engineering schools spend time delving into this material selection, as the materials come from multiple suppliers, making it difficult to find a summary of all materials in one place.

First and foremost, it should be clear that no single material is best for every application (or even available), and a trade-off between cost, weight, size, and other factors must be made. Additionally, the post-manufacturing lamination processing of the material significantly impacts the performance of the designed core. The material selection criteria include cost, permeability, electromagnetic loss, and saturation flux density. The magnetic permeability and core loss change with the magnetic reversal frequency (in Hertz) and magnetic flux density, and the shape of the hysteresis loop becomes important in certain applications. Each available material is optimized for one or more of these characteristics, while not being perfect in other areas.

Factors

When selecting steel for a motor core, several factors must be considered:

Currently, the most commonly used material for motor cores is cold-rolled grain-oriented steel, which is the lowest cost bulk-applied material, easy to press, and low tool wear helps reduce the cost of lamination. In applications where high stacking losses (DC pole pieces, low air gaps, etc.) and low cost are acceptable, carbon steel should be considered. However, the magnetic and mechanical performance of the material is a more decisive factor. The thinner the material, the lower the high-frequency eddy current loss, and the higher the efficiency of the motor. This means lower power consumption and longer transmission distances within the same power capacity. According to the power formula below, the best material thickness is as thin as possible.

Thinner plates mean longer production times and lower productivity

For example, in a motor with a stator outer diameter of 250 millimeters and a stack height of 120 millimeters, the thinner the plate, the more layers are needed to achieve the required total height. The pressing speed varies between 220 presses per minute (for 0.25 mm thick plates) and 250 presses per minute (for 0.35 mm thick plates), depending on the plate thickness. Considering scrap, downtime, and system availability, productivity will be between 32 stacks per hour (0.35 mm) and 19 stacks per hour (0.25 mm), meaning pressing time increases by 1.7 times.

Consider efficiency maximization, but not independently of the application

High-power motors can be produced using a variety of steel grades, and when choosing a motor for pure electric drive operation, the primary concern is how much cost can be saved by using higher quality, thinner, and thus more expensive electrical steel. Even relatively small efficiency differences can affect battery range and thus the required (very expensive) battery capacity.

If the motor is only used to support an internal combustion engine in a light hybrid vehicle or if the vehicle is only intended for short or medium distances (e.g., in hybrid or plug-in hybrid vehicles) with purely electric operation, the requirements for the performance curve are lower. In addition to saving material in the engine, the required battery capacity is also significantly reduced, which is why reducing the efficiency of the motor in the thickness range of 0.3 to 0.35 mm makes sense.

Furthermore, very thin lamination steel can increase the efficiency of high-speed motors, especially the stator's efficiency. However, in the rotor, very high strength is often required instead of thinner thickness, and special connection techniques, such as full surface bonding, also have a positive impact on the efficiency of the rotor design.

In conclusion, thin is not always better. From the perspective of machinability, cost, application area, efficiency, and other technical factors in motor design, the use of lamination steel must be viewed very differently. Many conditions clearly indicate that different thicknesses of steel are advantageous.