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.

Psycho Science is a regular feature here. Ask me your science questions, and I'll answer them. Probably.

And then commenters will, I hope, correct at least the most obvious flaws in my answer.

3 Responses to “Short-stopping”

  1. gearloose Says:

    Thanks, this is a thing I've been curious about for quite some time!
    Is it in any way bad for the motor? I always feel guilty when I fo this, part of it is the clunking sound it makes and part of it is the spark I can see in the ventilation holes. Am I just being sensitive or did I ruin my tool buying sense by forcing myself to (successfully I might add) do SMD soldering with a €3 soldering pen? :)

    Sorry about my crappy English, I blame the goings out of style of IRC and point-and-click adventures. ;(

  2. dan Says:

    In theory, that sparking - from the very high current created by the shorted motor, which is essentially working like a generator for that brief moment - will wear the brushes out faster. In practice, it won't make much difference; the brushes and commutators in the Mabuchi-type closed-endbell motors that power almost all power tools are pretty durable.

    (Radio-controlled models can use these kinds of motors too, but often have a fancier design with an endbell you can rotate to change the motor timing, and big fat brushes you can replace when they wear out. The reason why such enormous brushes DO wear out, though, is that they're deliberately made with a soft compound and strong springs pushing them onto the commutator, to get absolute maximum power out of the motor. Normal DC motors have more durable brushes and commutators, and don't push them nearly as hard; if you need more power from one of those motors to run, for instance, a drill, you just use a larger size of motor. Most R/C models are restricted to the "540" size, which is a size or three down from a drill motor.)

  3. TwoHedWlf Says:

    The same thing works for brushless motors. It's a bit less obvious because obviously it's run by a speed controller usually.

    I don't know about cars, but in RC planes all but the cheapest crappiest ESCs have programmable braking. At the very least on and off. With it off the prop will windmill, with it on the prop stops almost instantly. You can feel the drag if you try to spin the prop by hand(Disclaimer: Don't use your fingers, electrics can and will eat fingers for lunch)

    Dan, I usually only see brushed motors rated in sizes like 540. Some inrunner brushed, but not many. Brushed motors have been obsolete for a LONG time anyway. They're extremely heavy and weak compared to brushless motors. A quick browse of specs it looks like you'll be somewhere around 4 times the power at similar(Or even less) weight

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