A reader writes:
When I release the trigger on my old AEG power drill (so old that it's from when a power tool was an INVESTMENT), the motor takes at least a second to spin down to stationary.
When I release the trigger on my Black and Decker cordless drill, though, the chuck stops spinning instantly.
Why the difference? Is there a mechanical brake in there? Is this some sort of regenerative braking to keep the battery charged for longer? Is there just a lot of friction in the drill because they don't make them like they used to?
Old-style power drills have a simple design, in which the trigger connects mains power to the drill motor, releasing the trigger disconnects the power, and if you want more than one speed you can maybe move a clunky slider to change between two gear ratios and count yourself lucky because in my day laddie we used to have a bit and brace made frae whalebone wi' a sandstone chuck, et cetera.
Most modern corded power drills have a proportional speed-control system, where the further you pull the trigger, the faster the motor spins. When you release the trigger with the drill spinning, though, the motor will still take its time spinning down, unless of course there's a source of outside friction like a bit in the chuck that's still sticking through a piece of wood.
This spin-down behaviour is natural for almost all rotary electric motors. If you don't count certain odd birds like stepper motors, any spinning electric motor will, when you disconnect the power, coast down to a halt.
Except, as you say, cordless drills always stop at pretty much the exact moment you release the trigger, as long as you're not spinning some large object with the drill, like a hole-saw or sanding drum. And even then, they stop pretty quickly.
The reason for this is that cordless drills use simple, inexpensive brushed DC motors. (Actually, brushless motors are starting to show up in fancy cordless tools, but I'll shamelessly handwave that awkward fact for the purposes of the current conversation.)
Brush motors are really easy to stop dead: You just short out the input terminals.
If you've got a bare DC motor sitting around somewhere - I'll wait, while you dig up your box of old radio-controlled car parts or smash open that useless bloody $5 electric screwdriver that the batteries never even properly fit into - you can demonstrate this for yourself. Spin the motor's spindle by hand, and then short the terminals on the back of the motor with a paper clip or something and spin the spindle again. In the second situation, the faster the spindle spins, the greater the braking power on it.
The reason for this is "back EMF", a special case of the counter-electromotive force which, in brief, causes the currents induced in a piece of metal by a magnetic field to create another magnetic field opposing the first one. You can make an "eddy current brake" that employs this force to convert motive power directly into heat in the brake assembly, without any friction; this is useful in everything from heavy industrial applications to the delicate aluminium-paddle magnetic brake that sticks out of the side of a laboratory balance, whose purpose is to stop the darn scales from swinging back and forth around the correct reading until the research project runs out of funding.
In brushed DC motors, back-EMF braking works really well, which is why it is, for instance, the normal braking system for the abovementioned electric radio-controlled cars. A fast-stopping drill is a desirable thing to have, too, so releasing the trigger disconnects the power from the motor, and shorts the terminals to each other. There's no simple way to do the same thing in an AC motor, so you don't get this feature in corded drills.
Back-EMF braking won't instantly stop a motor if it's turning fast enough. I've got a Dremel Stylus, for instance, which is a brilliant little tool for all of those jobs for which my old mains-powered Dremel is a bit too powerful and clumsy, but for which a cheap AA-powered Dremel or similar suspiciously inexpensive rotary tool would be too feeble. I think the Stylus has a simple brush motor in there (as you change its speed, it sings the distinctive song of a brush motor vibrating because of audio-frequency pulse-width modulated speed adjustment), but its top speed, as with all rotary multi-tools, is much higher than the top speed of a cordless drill. So when you turn the Stylus off, it stops pretty darn quickly, and quite a bit quicker than the mains-powered Dremel, but it still takes about a second to run down.
And then commenters will, I hope, correct at least the most obvious flaws in my answer.