Unusual uneven brush wear in blender motor (Optimum 9400)

[fzabkar]

Some time ago a friend asked me to help him troubleshoot a commercial grade blender, model Optimum 9400. The symptom was that the blender's motor either wouldn't start, or it would start intermittently. It turned out that the armature circuit was open. The stator was OK.

When my friend removed the brush holders, he found this:

https://web.archive.org/web/20230522151510/www.users.on.net/~fzabkar/Blender/worn_brush.jpg

The short brush wasn't stuck -- it really had worn down that low. We couldn't find any reason for the dissimilar wear. All we were able to determine from the owner was that the unit had previously been returned to the supplier for service after it had failed a routine insulation test (why?). New brushes restored the unit to fully working condition, with no hint of arcing or vibration.


BOM for semis:

PIC16F1503, Microchip, 8-bit uC, 14-pin, 2.3V - 5.5V, 16MHz:
http://ww1.microchip.com/downloads/en/DeviceDoc/41607A.pdf

26T12BI, NELL Semiconductor, 25A, 1200V TRIAC, TO-3P:
http://nellsemi.com/nell-cht/certi/9c3e7f1f6191ff20c800af7842994fff.pdf

MOC3023, Fairchild, optocoupler, 400V Phototriac Driver Output:
http://www.ti.com/lit/ds/symlink/moc3022.pdf

PT-1540P, Mallory Sonalert Products Inc, buzzer, 4kHz, 5Vpp, 1.5mA:
https://www.state-elec.com/store/PDF/PT-1540P.pdf

HT7044A-1, Holtek, TinyPower Voltage Detector, 4.4V:
http://www.sycelectronica.com.ar/semiconductores/HT7044A.pdf

Internal photos:

"a comparison of the parts that make up the blender":
https://www.froothie.com.au/Documents/Optimumv.sLowerQualityBlenders_20may2014.pdf
https://www.froothie.com.au/optimum-vs-lower-quality-blenders

User Manual:

http://www.froothie.si/Documents/Optimum%209400%20User%20Manual%20RO.pdf
(Romanian)
https://www.medirando.com/images/medirando/media/Optimum_9400_UserManual.pdf
(German)

My CCD scans of the control PCB:

https://web.archive.org/web/20230522151507/http://www.users.on.net/~fzabkar/Blender/Blender_PCB_bot_resize.JPG
https://web.archive.org/web/20230522151504/http://www.users.on.net/~fzabkar/Blender/Blender_PCB_top3_resize.JPG
https://web.archive.org/web/20230522151511/http://www.users.on.net/~fzabkar/Blender/Blender_PCB_top2_resize.JPG

[Armadillo]

High Load, High Current, Positive Brush has been shown to wear faster by embrittlement process of the graphite.

[fzabkar]

High Load, High Current, Positive Brush has been shown to wear faster by embrittlement process of the graphite.

It's an AC commutator motor.