Brushless speed controller teardown and hacking a rotary tool
Couple of kW inrunner brushless motor in Proxxon dremel.

Couple of kW inrunner brushless motor in Proxxon dremel.

The stack build of the ESC with mosfet leads showing.

The stack build of the ESC with mosfet leads showing.

Friend of my friend broke another rotary tool with daily grinding so I got another one to take apart. This time a Proxxon D-54518. Initial probing showed that there was voltage on the big 100V DC motor leads so I ruled that the motor was busted. Just for fun I tried to fit my 100A 1.5kW big brushless inrunner in its shell. It fit perfectly! The screw holes aligned and coupling to toolhead only needed a little drilling to make it fit perfectly. Later I found out that the original motor was working too.. so.. No idea why it didn't work.

To control the big brushless I used a Red Brick 100A ESC (Electric Speed Controller). But the model I had didn't have BEC (battery elimination circuit), so it didn't feed out any voltage from signal cable. This means - teardown time!

Half of the mosfets in speed controller

Half of the mosfets in speed controller

The build of the thing is quite surprising.The input capacitors are two 35V 1000uF electrolytic ones, nothing weird in there. But the thing itself is stacked up from three PCBs. The top one is two layer one and used only for controlling logic. Two bottom ones are both 3 or 4 layer PCBs that hold 18 d-pack mosfets each. The inner layers are needed for high current carrying capability and thermal reasons. All mosfets are N channel, 30V, 70A P0603BD. The total output current could be 420A but they have branded it as 100A, probably because they don't have active cooling and super awesome gate driving. All gates are charged through 33Ω resistors, probably to account differences in gate capacitances or to protect gate drivers.

The main board looks the same like in every Chinese brushless ESC. Half of the board is unpopulated, that was the reason for no BEC - the switching regulator was not in place. The main ingredients that make such ESC are:

Mainboard of electric speed controller

Mainboard of electric speed controller

Atmels ATmega48 (or ATmega88 for smarter ones) that does all the work. For some weird reason Chinese manufacturers LOVE ATmega48, it is in every design. Like always it is clocked at 16MHz with resonator. AVR in this design is in QFN package, in most of the cases TQFP is more common.5V voltage regulator in D-pack and some ceramic capacitors.Maxim MAX662 12V 30mA charge pump for high side gate driving. The output of the charge pump is stored on the ceramic capacitor on the right.

International Rectifier IR2101S high and low side mosfet driver. One for every channel. On the right of every driver there is a diode and a capacitor. This is probably bootstrap circuit that collects higher voltage for gate driving from inductance spikes of outputs.

In between of gate drivers are resistors for feedback.

Adjustable switching regulator from ebay. Currently used as constant voltage solar charger.

Adjustable switching regulator from ebay. Currently used as constant voltage solar charger.

The top part is for BEC. It looks identical to very common switching regulator block used in ebay products. Texas Instruments LM2575, originally a 1A step-down but now apparently a 5A one.

I did couple of tests with the rotary tool after installing the new motor and it is SCARY. It feels like there is a angle grinder in your tiny rotary tool's body. We tested with speeds up to 30% and didn't dare more.

3 thoughts on “Brushless speed controller teardown and hacking a rotary tool

  1. hello buddy.

    i had a bosch rotary tool which has been through hell and back
    bottom line burned the motor. specs for the motor are 90w 230v 50hz 0.4a and it can go 26k rpm. been searching the web all day for a motor . u see trusty tools are like loyal friends i never let them down. i wanna fix it . where can i find a similar god damn motor?

  2. I think normally they use custom made "universal motor". You can try searching one that matches.

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