Although their basic design remains unchanged, today’s electric motors are more sophisticated when it comes to numerous technical details. This includes the electrical steel strip used, which largely determines the motor’s performance and efficiency. Extremely thin, high-performance grades are in high demand for this application. In terms of efficient use in the stator of the electric drive of a battery-powered car, the NO 20 grade has proven to be an excellent option for electric vehicles.
There are many arguments in favor of using particularly thin electrical steel strip grades in the drive systems of electrified automobiles – a compact design, for example. There is usually only limited space available for the electric motor, and this is particularly true in hybrid vehicles, where the unit is located between the combustion engine and the transmission or is integrated into the belt drive instead of the starter.
At the same time, however, the thin steel strip must be extremely strong in order to withstand the centrifugal forces at speeds of up to 20,000 rpm. Norbert Brachthäuser, responsible for the special product electrical steel strip within Waelzholz’s Materials Technology Department, explains: “Even our thin NO 20 electrical steel strip grade, with a nominal thickness of only 0.20 millimeters (0.008 in), is produced in a high-strength version with a yield strength Rp 0.20 of 500 MPa (73 ksi) – while retaining its excellent electromagnetic properties. As a result, we can offer our customers the assurance that the material will reliably withstand the forces applied to it, including in high-stress applications such as thin bars or rotor teeth.”
And most importantly, the thickness of the electrical steel strip has a major effect on the electric motor’s performance and efficiency. This is primarily related to an effect known as core loss. When an electric motor is in operation, the polarity of the electric steel strip in the rotor changes at a very high frequency. According to electrical steel strip expert Brachthäuser: “When the magnetic field direction alternates, part of the supplied power is lost in the form of heat. These losses increase with increasing thickness of the electrical steel strip. So every hundredth of a millimeter that the material is thinner makes a difference.” As such, thin grades such as NO 20 consume less power, which in turn extends the range of battery electric vehicles.
Figure 1: Effect of electrical steel strip sheet thickness on core loss
Another factor that affects the performance of the electric motor is the magnetic flux density of the electrical steel strip grade used. In this case, as well, Waelzholz NO grades deliver exceptional performance, measurable as polarization at various field strengths.
Expertise in thin electrical steel strip
Thanks to its special know-how, Waelzholz can reliably produce thin grades even down to NO 10, i.e. down to a strip thickness of 0.10 mm (0.004 in). According to Brachthäuser: “We supply these extremely thin electrical steel strip grades primarily for special high-end applications such as pacemakers, drones, or other special drive systems. In the automotive sector, strip thicknesses of 0.20 mm (0.008 in) to 0.30 mm (0.012 in) (NO 20 – NO 30 grades) have become the standard for both technical and business reasons.” Producing an NO 20 is a challenge for manufacturers of cold rolled steel strip – a challenge that only some companies can reliably master. “Conventional five-stand rolling mills are only designed for electrical steel strip with a thickness of down to 0.30 millimeters (0.012 in). For thinner strip, special reversing stands with small working roll diameters are required. And the continuous annealing furnace also has to be specially designed for the thin strip, because the material in the furnace is as soft as butter. Thin grades quickly run the risk of tearing or warping here. We have developed special process expertise in this area over the course of many years,” explains Brachthäuser. In this context, Waelzholz ensures that the dimensional tolerance of the materials remains within a few thousandths of a millimeter. This, in turn, ensures that the rotor and stator stacks exhibit a high degree of dimensional accuracy. The reason that this is so important quickly becomes clear when you consider an electric motor’s high rotational speeds – the motor components still have to run perfectly smoothly and steadily, even at high frequencies.
Efficiently bonding electrical steel strip stacks with Backlack
Typically, lamination stacks made of electrical steel strip are joined together by means of stamping, welding, or riveting. This has a significant effect on the motor’s performance, however, because these bonding techniques cause mechanical damage to the material – disturbing the magnetic flux and negatively impacting performance. Waelzholz’s Backlack technology is the perfect solution to this problem (see figure 2). In this method, the electrical steel strip is supplied with a special coating that is still adhesive.
During further processing, the steel laminations punched by the customer are baked in the furnace with the Backlack to form a compact stack. The process homogeneously bonds the individual steel laminations without negatively affecting the material properties.
Figure 2: Coating and bonding process of electrical steel laminations
This process also offers additional benefits. For example, it creates compact stacks that are impermeable to liquids. There is also no risk of stack humming, which often occurs when other bonding methods are used. Furthermore, Backlack also makes the material easier to punch and makes it possible to create complex shapes. This means that with its thin NO electrical steel strip grades and Backlack technology, Waelzholz puts manufacturers in the perfect position to produce efficient and high-performance electric drive systems.
Would you like to learn more about our special product electrical steel strip? Our brochure is available for download.