Electronic – Why is there often a metal clip around a dc motor

dc motormetalmotor

I have seen many DC motors with a piece of sheet metal that is bent and snaps onto the motor casing.

What is its purpose?
Is it an additional cooling body?
Or maybe to concentrate the magnetic field better? It covers the whole area where the permanent magnets are on the inside.
picture of such a motor with clip

Best Answer

The metal has a very high relative permeability. This means that most of the stray magnetic field will go through the metal and not through the surrounding air. The advantage of this is less magnetic field radiation because most of the magnetic flux is confined to the metal. Mu metal is expensive but performs well in this job. These metal covers were common well before EMC regs became an issue. Some circuits get mucked up by stray magnetism; CRTs and Tapes are examples.