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Worn steel-cutting tools can blunt motor efficiencies

20 February, 2018

German researchers have discovered that the sharpness of the tools used to cut the electrical steels used in the cores of electric motors, can have a profound effect on the motor’s efficiency. The researchers, from the Technical University of Munich (TUM), found that using blunt tools could quadruple the amount of electricity needed to magnetise the cores of the finished machines.

The sheets of electrical steel at the heart of motors (as well as transformers) are vital because the magnetic fields that drive the motor using attractive and repulsive forces are generated within the sheets. Holes need to be cut in the sheets – for example, to make room for the motor’s copper coils. These holes are created by stamping the sheets in presses. Special cutting tools ensure that the required geometries are created in the steel sheets. After cutting, the sheets are assembled to form the motor core.

The TUM researchers have found that the sharpness of the cutting tools has a “very significant” impact on the sheets’ magnetic properties. They liken the effect to a pair of scissors that gets blunt over time, requiring more energy to produce a cut. Worn cutting edges bend the sheet material rather than cutting it, increasing internal mechanical stresses and affecting its magnetic properties.

“In some cases, as much as four times the amount of electricity is needed to achieve the same degree of magnetisation,” reports the TUM project director, Hannes Weiss.

Another factor that has a major influence is the cutting clearance – the distance between the cutting edges. Returning to the scissors analogy, when the screw that holds them together becomes loose, the distance between the blades widens and they will fray, rather than cut, paper. “Sharp cutting edges and a very small cutting clearance are optimum for achieving the best magnetic properties and thus a high level of efficiency,” Weiss explains.

The TUM's Hannes Weiss with a punching machine
Photo: Andreas Heddergott / TUM

The TUM researchers have drawn up recommendations on how to minimise the losses that can be created when processing electrical steels. However, they acknowledge that this might make the motors more expensive. “When the cutting tools and their maintenance incur additional costs, the final price of the electric machines produced rises as well,” says Weiss.

But, he points out, applying the lessons learned to the production of transformers, for example, could have significant economic benefits. “If we think of the enormous number of transformers in use, then even a small increase in efficiency can save a large amount of energy.”

The project, carried out by the TUM’s chair of metal forming and casting, has been supported by the German Research Foundation, DFG.

Motor cores are formed from stacks of sheets of electrical steel which have been cut to shape individually
Photo: Andreas Heddergott / TUM

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