Running an electric motor backwards is less efficient?

Maybe. Maybe not. There, isn't that nice?

I'm assuming that you mean permanent-magnet commutated DC motors (the kind that just have two wires coming out).

The armature current in a DC motor lags the voltage on the armature coil a bit, because of coil inductance. You can tune a commutated DC motor to run more efficiently by advancing the brushes a bit on the commutator -- this is rarely done, but there is a large body of lore associated with it among slot car racers.

However, most run of the mill small DC motors are designed to run the same in either direction. You may get motors that run a bit differently in one direction than the other due to manufacturing variations, but that would be a consequence of chance, not design.

So, probably not.

That depends hugely on how the motor is driven. The same effect with lagging armature current exists, and you can make things a bit better by advancing the commutation, but first the brushless driver designer would have to care, and then he'd have to dare.

If you had a brushless drive sophisticated enough to give a bit of lead to the voltage then it would probably be easy-peasy to extend this to do so in both directions. But I very much doubt that a cordless drill, if it's brushless at all, would have such a feature.

Greetings Tim, I bet that cordless, brushless, DC motors in drills do advance the timing a little to emulate advancing the brushes. It is so easy to do. And many of the hobby brushless motor drivers allow the user to cahnge the timing. I don't remember how much of a difference it makes in motor effeciency to optimize brush placement but I do know that often times the brushes are rotated just for the lowest sparking and RF, which wouldn't apply to brushless motors. Still, with modern electronics controlled by a microcontroller it would be fairly easy to optimize commutation timing to get the longest battery life. I sure would like to know. Eric

Since the majority of drill bits are designed to run clockwise, running it backwards is very likely to be inefficient whilst drilling anyway. Running it backwards to remove swarf etc isn't likely to load the drill up too much. Electric screwdrivers might be run backwards to remove screws or bolts but the biggest issue is not having enough torque to undo the screw but not shearing the head of the bolt/screw or snapping the bit.

IMNSHO

Ron

Beam me up, Scotty.

Heh! mebbe he thought it might fill the hole in if it went backwards.

I suspect that the gears are cut to optimise running in the forwards direction

I removed the controller from an impact wrench and stuck it into a same brand hammerdrill. Now apparently it runs just as fast backwards as forwards. The motor metal gets hot within ten seconds on high-speed, but I guess that's normal.

I wrote:

The DIY electric car builders are into adjusting brush position in their motors also.

Mikek