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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

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  • \$\begingroup\$ Oh snap.. I tried searching for simealar questions but couldn't find one, you did. The one in your link has a nice detailed answer so thanks for that! \$\endgroup\$
    – Bruce
    Jun 17, 2017 at 12:36

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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.

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  • \$\begingroup\$ If you leave it off will it give performance difference? Or just more stray magnetism leaking into the surroundings? Thanks for the answer \$\endgroup\$
    – Bruce
    Jun 17, 2017 at 11:10
  • \$\begingroup\$ @ Bruce .These clips have been around for decades .When I was a teenager I did not think that motor performance was affected .I did not have means to detect small differences .The thinness of the strap compared to armature diameter gives a theory basis that performance wont change much either way . \$\endgroup\$
    – Autistic
    Jun 17, 2017 at 11:17
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    \$\begingroup\$ It depends a on the magnets. If there is a magnetic field outside the motor, you are losing efficiency. \$\endgroup\$
    – amwalker707
    Jun 17, 2017 at 12:13
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The purpose of this ring is to concentrate the magnetic field produced by the permanent magnets inside the motor. It is made from normal steel, unlike the thin Mumetal that is wrapped around the outside of tape drive motors to prevent EMI.

Increasing the magnetic field strength inside the motor increases torque and reduces rpm, and makes it more efficient at high power. The difference is quite small (~5% in my tests) and the ring increases the diameter of the motor so it won't fit in some devices.

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  • \$\begingroup\$ How did you perform those tests? Just curious on how I can test my own motors. \$\endgroup\$
    – Bruce
    Jun 22, 2017 at 13:13
  • \$\begingroup\$ Measure Volts, Amps, and rpm with different loads. Since I was testing motors for RC model airplanes I used propellers. I also built a dynamometer to measure torque, but this is not needed for comparative tests because propeller power and torque variations can be calculated reasonably accurately over a small rpm range. Drive calculator, some of my test data check out the Mega MIG 400 6V (no flux ring) and MIG 400 7.2V (with flux ring). \$\endgroup\$
    – Bruce Abbott
    Jun 22, 2017 at 18:33
  • \$\begingroup\$ Any comments or experience on how the ring tends to cover the openings in the motor casing and how this may impact motor temperature? \$\endgroup\$
    – jrive
    Jan 7, 2019 at 16:44
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    \$\begingroup\$ @jrive If the flux ring is the correct size for the motor it should not cover the cooling holes. It only needs to go where the magnets are. \$\endgroup\$
    – Bruce Abbott
    Jan 8, 2019 at 6:34

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