I have recently taken apart a few cassette players to get motors, and I have noticed there is always a metal sleeve around the main drive motor.
Here is an Ebay product photo showing this on a Mabuchi EG-530AD-2B motor: 
The sleeve is not attached to the motor and can be easily removed by sliding it off. I have not seen them on DC motors anywhere else. What is it there for?
The reason I chose an EG-530AD-2B motor is because one of the cassette players I took apart had a 'Mebochi' (not Mabuchi) brand EG-530AD-2B motor with a very similar label to the one shown below, except that the text was blue. I cannot seem to find any information about this online. Does anyone have any idea what it is?
Answer
It's a mu metal shield. This is a special material that 'conducts' low frequency and static magnetic fields very well (but saturates at a low field strength). The basic idea is that any small residual field generated by the motor magnets gets shunted through the mu metal rather than propagating out into the air around the motor and interfering with the tape reader head.
Mu metal has a relative permeability around 25x greater than soft iron, so if they wanted provide the same amount of shielding using the motor body, it would have to be roughly 25x thicker than the thin strip of mu metal.
No comments:
Post a Comment