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I was using my cordless drill today. I was trying to drive a bolt that was stuck with my drill in low gear and it wasn't really budging. I tried a few times and then noticed that smoke was coming out of my drill's cooling slots.

My drill has a brushless motor and 725in-lb of torque and it was stalling out. Does that mean that something inside the drill fried, or is it common to see smoke come from a drill while under a heavy load?

EDIT: The drill is a Milwaukee M18 Fuel.

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I'd say the drill is gonna die and it is likely a warranty-covered death. This is specific drill is professional, so it almost surely has overload protection that should limit the current when there's serious overload. I guess this protection simply didn't kick in and this is a warranty-covered incident. – sharptooth Feb 4 '13 at 8:16
I'm surprised this happened given that Milwaukee's REDLINK PLUS Intelligence electronics should have stopped the drill well before it would get hot enough to generate smoke. I would definitely send it in to Milwaukee under warranty, telling them the drill overheated and smoked when the drill failed to stop on an overload. – bwDraco Jun 14 '14 at 14:35
up vote 7 down vote accepted

Don't Stall Electric Motors. Ever.

Stalled motor = no back EMF = High Current.

It burns the windings out.

I'd think the technology in more expensive drills with brushless motors would sense a stall condition and go into shutdown, but yours seems to be willing to sacrifice its life, cooking the insulation off the windings and attempting to burn out the MOSFET array that drives the motor.

An Electric Impact wrench is more appropriate for this kind of abuse. It has a mechanism that allows the motor to spin up, deliver a sharp shock to turn the bolt and release immedately so the motor never stalls.

If an electric or pneumatic impact wrench driving the appropriate impact socket won't loosen it, the time has come to apply some Mouse Milk and a breaker bar.

Here's a Dewalt Planetary Set with attached Brushless Motor. Your windings will be dark brown instead of that gold, green or red color they usually color the enamel. The next step for it is to turn black and the windings then start to short.

Dewalt Drill Brushless Motor & Planetary Gearset

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My drill does have protection circuitry but I tried a few times and that's when it started smoking. I did grab my impact wrench when I saw the smoke from my drill and it got it out. But where was the smoke coming from? – Sponge Bob Feb 4 '13 at 5:04
Enamel covering on the motor windings starts to smoke from the heat, the epoxy cases on the Mosfet drivers starts to get too hot before thermal shutdown starts becoming active. Basically you're cooking insulation attached to the conductors in the motor and the circuitry in the motor driver. – Fiasco Labs Feb 4 '13 at 5:12
Thanks a lot, that's good to know. – Sponge Bob Feb 4 '13 at 5:36
To add one last bit to Fiasco Labs well written answer: cooking that insulating coating on the wires within the windings breaks down the insulation, and it loses effectiveness. This creates short circuits within those stacked up wire bundles you see. – Tim B Feb 5 '13 at 13:28

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