How to prevent external magnet rotate a motor

Question

A small dc motor working and moving a disc. When device/motor is power off, a hacker with a strong magnet came and rotate the motor; without touching the motor, just moving the magnet near the motor is sufficient to cause the motor-nucleus to rotate, and hence the shaft rotate and the disc likewise. A friend told me this happens also with dc stepper motors, but not sure (Currently, no stepper motors available to test.)

Can you suggest a measure/solution to prevent magnet to rotate a motor?

Thanks

Answer 1

Two solutions that come to my mind:

1. Shielding around the motor. Encase the entire motor, or its vulnerable side with a ferrous metal shield that will prevent outside interference.

2. Braking. Add a mechanical brake to prevent movement when it's not desired.

A stepper motor can be left energized and will hold its position but DC motors don't offer this.

Answer 2

Can you suggest a measure/solution to prevent magnet to rotate a motor?

• Distance. Motors in typical vending machines are far enough from the surface of the enclosure that magnetic fields generated by hand-held magnets are insufficient to rotate the motors.

• Orientation. Instead of facing the side of the motor to the enclosure surface, face the back face of the motor.

And even if you think that you can use a magnet to rotate the motor, put the motor into an enclosure first - even very close to the surface, but make the rest of the enclosure large enough to represent whatever machine the motor will be used in. You may well find out that the limited surface are that's close to one side of the motor may be not enough to actually rotate it a full turn. It may be possible to deflect the motor a bit from its neutral position, but it won't be making full rotations, just small deflections in either direction from the "neutral".