Motor for brake shaft

Good afternoon everyone, first of all thank you for opening this topic.
I am looking for a motor to brake a shaft, that is to say that most of the time it will be turning in the opposite direction.
I would like to control the motor torque by means of a signal, and the torque would be to oppose resistance to the rotation since it will rotate in the opposite direction (generating).
The motor should be able to generate a torque of between 1 to 6 Nm of resistance with an rpm close to 20. And the size of the motor should be small, to fit in one hand at most.

I am writing in this forum and I would like advice to find a solution. I have thought of using “Dynamic Braking” through some kind of digital resistor but I don’t know if I can control the torque.

With DC motors the current is proportional to the torque, so you want a variable constant current sink circuit to do programmable braking. This can be completely passive and analog, or powered and switch-mode using a MOSFET and a dump resistor to dissipate the heat.

For a motor without gearing torque is closely related to the size (volume) of the rotor. You can't indepedently choose torque and size unless you can have gears.

20rpm is very slow for a motor (4000 rpm is typical), which also suggests gearing would be a good idea, however a reduction ratio of 200:1 which is back-drivable isn't a viable option, so that's completely out. Just how little drag torque do you want when not actively braking?

20rpm is ~2 rad/s and at 6Nm that's 2 x 6 = 12 watts to dissipate, which isn't too problematical, a medium sized heatsink should be adequate.

BTW with only a small gear ratio (to allow low resistance backdriving), the motor will be spinning slowly (compared to its ratings), and the winding resistance may limit the current too much - you need to know the motor constant (volts per rpm) and the winding resistance to figure out if this might be the case.

Thank you very much for your reply. I want to use the motor in a project I am developing. The project aims to help recover muscles through exercise, the project wants to mimic a fishing rod.
The motor should make it look like it is a fish resisting the spinning shaft.
Then the motor will be most of the time spinning in the opposite direction, as it will be rotated manually.
As the motor will be integrated in the rod, what do you recommend me to use for such a project in terms of the motor and its torque control.

About the rpm I made a mistake when writing, I meant between 20 and 60 (max 100).
And i had thought about buying a geared motor.

Thank you very much

You seem to be describing TWO identical motors with the armature shafts coupled together. One motor turns one direction and the second ATTEMPTS to turn the opposite.

So you are using the motor to both power the shaft and to brake it - that takes a 4-quadrant motor controller with programmable torque limits and modes - more than a regular motor controller, something more like a servomotor controller in fact.

If you'd said all this straight up I would not have answered as I did above. Check out

This is quite advanced motor control and finding a controller that's at hobby prices rather than industrial prices is going to be a bit tricky I think.

I am sorry I did not provide all the information from the beginning. I understand what you mean.

Could you advise me an idea or possibility where to look for solutions. I've been stuck looking for an engine for quite a while now as I'm not sure. I will try to look for a 4 quadrant controller with torque control.

In case of using a servomotor would it be simpler ?
Thank you very much and sorry for the inconvenience

A servomotor is typically driven from a dedicated controller supporting 4-quadrants, torque, speed and position control modes. But expensive as they are normally industrial (designed for high reliability in harsh environments).

Then if I use motors I will have to look for a fairly complex controller to be able to carry it out. Couldn't there be a solution using motor only in one of the quadrants?

The other option I had considered is to use electromagnetic brakes, but the ones I found were quite heavy so I tried to look for motors.
Thank you.

Hi ,a question, if I were to make a circuit with a geared dc motor which I would protect with a diode and then limit the maximum current. If I were to rotate the motor in the opposite direction manually what would happen ?