# What happens if you manually turn a moving DC motor the opposite way

If I have a brushed DC motor moving at a slow speed, and I apply a force to it in the opposite direction, enough for it to rotate the other way, does this damage the motor?

I have a steering wheel connected to the shaft of the motor.

I'm trying add resistance to the steering wheel when it turns, and this was one idea.

As you are trying to add a torque to your steering wheel, to resist the motion, this means you should consider current-feeding the motor. You can control the torque by controlling the current.

A motor (a DC brushed motor at least) is a sort of transformer between electrical and mechanical domains. Voltage maps to speed, and current maps to torque (both with losses of course).

If you supply a voltage from a battery, then the motor will try to run at constant speed, more or less independantly of the torque needed for that. If you try to stop or reverse it manually, then it will try to draw whatever current is needed to continue running at that speed. The high current could heat the motor to the point of damage, unless the power supply limits the current.

If you supply a current from a current source, then the motor will try to deliver a constant torque, at any speed, including standstill. If that torque is enough to overcome friction, then the motor will accelerate to a speed which is in practice limited by the current source's maximum voltage, or the increase in friction and air-resistance of the motor.

Because you are controlling the current, you are controlling the power dissipation in the motor windings, the $I^2R$. Choose a low enough current, and the motor can sit stalled or be turned backwards without damage or overheating, while still delivering the constant torque determined by the current. That's always assuming there's no mechanical reason why the motor can't be turned backwards, like a worm gear on the shaft.

This mode of operation is called using it as a torque motor. There are special types of motor optimised for this operation, with better cooling when stationary (no air being stirred around) and lower cogging, but all DC brushed motors can work like this.

• Another poster suggested a torque motor, too, but I think it's a little fancier for my simple application, so I'll try your tip with a standard brushed DC motor. Thanks! Sep 19, 2018 at 12:19

Not necessarily, but it greatly increases the current drawn by the motor and/or induces a current drawn in the opposite direction. It's possible for this to cause the motor to heat up causing damage, or overload the driving circuitry.

• Thanks for your response. I'm trying add resistance to the steering wheel when it turns, and this was one idea. Do you suggest something else instead? Sep 19, 2018 at 10:47
• I think it's fine provided that you take those factors into account; limit the current through the system to the motor's steady state maximum. A bit of googling finds a patent talking about this: patents.google.com/patent/US8554408 ; that suggests a couple of other techniques for limiting the maximum torque such as a slipping drive belt or drive wheel, and a suggested gearing ratio of not more than 4. Sep 19, 2018 at 10:55

What you really need for your application is torque motor. They are specifically designed to provide torque on a shaft while being stalled or rotated in opposite direction without the risk of damage or very high current. You had the right idea, now you just need a right part for it.