I think you are using DC motor with ordinary switching circuit. Send me complete diagram for your electronic circuit. I will send you the soluction.
It was long time ago I designed something with electronics, so I hope someone can help me.
What I need is an electric braker-pusher. It will not need to come to an absolute stop, it should just add or release resistance on a rolling device. It's very much like the electric braker on a bicycle trainer, but with the option to give energy too.
A little spec would be:
A small motor, maybe 12 V and 30 W. Not sure how much needed yet.
Should take a value, for example -100 to +100 from a computer, controlling the output. Minus values for braking and positive for pushing.
Returns RPM from a magnet switch.
The motor will always be rotated, even when braking at maximum, some kind of heat sink is definitely needed.