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I'm not very experienced/knowledgeable in electronics so this may be a bit of a dumb question. I'm building a carbon fiber filament winder for which I need to be actively controlling the tension in the filament. The filament is pulled off the rotating reel and then goes over a pulley connected to load cells which measure the tension. Each reel of filament is connected to a brushless dc motor with an encoder, and the load cells and motors are in a feedback loop controlled by an Arduino Uno. The filament is being pulled by the winder itself, so the motors are just there to resist the rotation of each reel and provide tension. I'm not entirely sure how to control the resistance in these motors though, so if anyone has any ideas that would be much appreciated. I've been told that I can connect a large resistor across the motor's power leads and vary the PWM input to control resistance, would this work? Thanks!

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When the brushless DC motors are turned by the force of the fibers, they act as permanent-magnet synchronous AC generators. If they don't need to turn on their own, they can be connected to a rectifier loaded by a resistor with the DC load current modulated by PWM. If the PWM is controlled with feedback from the load cells, that should work.

If the motors need to be turning on their own without the pull of the filaments, the problem is more complex.

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