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I am using a very tiny motor (GM15, to be precise) for an application where optimizing weight is the biggest challenge. The motor weights around 1.1 g. I opened up the motor and weighed all the different parts and found that the outer metal casing of the motor (along with the bronze bushing) alone is around 650mg. For those who are curious about weights of different parts: copper coils - only 120mg, permanent magnets ~ 60mg, gearbox - 220 mg.

So my question is on the exact utility of this casing. I know iron is used inside motor rotors because of their high magnetic permeability but for the purpose of outer casing, does the use of metal offer any advantages other than structural robustness? If I were to take out all the motor parts and assemble it back in a plastic casing with half the weight, would the performance of the motor be the same in terms of torque production and current cunsumption?

I couldn't find much info on the subject with a quick google search. I would appreciate if someone can point to any standard literature on this.

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The casing is the flux return path for the magnets. If you replace it with plastic, the strength of the flux through the coils will be significantly reduced which will reduce the torque production efficiency of the motor. This will mean you'll need a bigger motor for the same amount of power output, so you'll probably gain nothing. Of course, if the motor is already too powerful, then this might be an option. It is hard to estimate exactly how much the torque production will be affected, but it would be easy to measure using the back EMF.

If you need to reduce weight a better approach would be to use stronger magnets (so they can be smaller) and replace the iron paths with magnetic material (such as a Halbach array pattern). Iron paths are sort of like plumbing for flux, so replacing them with active magnetic material essentially reduces the proportion of the motor mass that is 'piping'. The ultimate embodiment of these principles is the coreless motor which is a copper coil suspended between two magnets. These motors have very high power density.

However, I would imagine any modification other than removing the case is going to be too difficult for your situation, so you might have to look at alternative motors.

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  • \$\begingroup\$ Thanks Jon for your answer. Wondering if you would you have any pointers to reliable sources where I can read more about this? I was contemplating design of an electromagnetic actuator that is less than half a gram in weight and was wondering as to how you would go about calculating the minimum thickess of outer metal shell for maximum torque efficiency. \$\endgroup\$ – user1628671 May 29 '18 at 16:24
  • \$\begingroup\$ There is a good explanation of coreless motors here: robotpark.com/Coreless-Motors \$\endgroup\$ – Jack Creasey May 29 '18 at 21:14
  • \$\begingroup\$ @user1628671 I actually would recommend Duane Hanselman's book (Brushless Permanent Magnet Motor Design) as an excellent introduction to electromagnetic actuators. The first few chapters go over the basics and magnetic circuit analysis in a well structured format, and it keeps away from complicated maths until later on. It is quite easy to follow compared to the many of the jargon/math heavy texts on the subject. You can also find some good pdfs on magnetic circuit design, but they can be quite confusing if you don't already have a general idea of what is going on. \$\endgroup\$ – Jon May 29 '18 at 21:25
  • \$\begingroup\$ BTW do you have a picture of the disassembled GM15? I might be able to suggest areas of the casing you can machine down. \$\endgroup\$ – Jon May 29 '18 at 21:28

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