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Is it possible to strengthen permanent magnet DC motors by simply attaching extra magnets on the outside of the motor casing - adding to the magnetic field?

If this is not possible, the question becomes; what happens if I replace the magnets inside the motor with better magnets?

I know that the coils will not handle more current than they currently do, so what will the net effect on the motor be?

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  • $\begingroup$ I'd guess this wouldn't work that great. They'd be too far and you can't really control the shape of the magnetic field. Sounds like a fun experiment- but if all you want is a stronger motor just buy one? $\endgroup$ – Guy Sirton Jun 30 '14 at 6:07
  • $\begingroup$ @GuySirton Yes, but I'm just curious as to what effect magnet strength has on the performance characteristics. $\endgroup$ – frodeborli Jun 30 '14 at 12:21
  • $\begingroup$ Are you doing this for an inrunner motor or outrunner? $\endgroup$ – Ian Jun 30 '14 at 17:02
  • $\begingroup$ @Ian Inrunner, permanent magnet brushed DC motor. $\endgroup$ – frodeborli Jun 30 '14 at 18:40
  • $\begingroup$ @frodeborli I believe the torque will go up linearly to the magnetic field though you won't really have good control of that field due to the location of your magnets... Why don't you go ahead an do the experiment. For the physics look here: hyperphysics.phy-astr.gsu.edu/hbase/magnetic/elemot.html $\endgroup$ – Guy Sirton Jun 30 '14 at 18:41
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First, consider that the purpose of the magnets in a motor are to create a permanent magnetic field. The magnetic flux associated with this magnetic field follows a path from one pole of the magnet, through the air gap to the armature, then through the opposite air gap, then through the other magnet, then around the housing of the motor back to the opposite pole of the first magnet. The magnets are embedded in the motor's magnetic circuit. If you place magnets on the outside of the motor, they will not be in the magnetic circuit of the motor. Rather, one pole of the magnets will be on the housing that the other will be facing out away from the motor. Little to no flux will be added to the motor's magnetic circuit.

If you took the motor apart and replaced the standard magnets with new magnets, then that is a different question. This is a complicated subject and there is no definite answer to your question other than "it depends." Most motors are designed so that the steel in the motor is near magnetic saturation. Increasing the strength of the magnet will drive more of the steel into saturation. How much and what affect it will have depends on the motor and how much you change your magnet by. But you will increase Kt, which means for a given torque, your current will be lower. And your speed will be lower. Your ability to dissipate heat will not change. However, your magnet may be more susceptible to demagnetization. You'd want to make sure your magnet's coercivity (Hc) is high enough.

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