Mixer motor shut down

It sounds like there might be a switch in the arm, but I don't see one. It appears to be an Australian model so results found in North America may be skewed..

https://www.ereplacementparts.com/breville-bem800xl-5quart-diecast-stand-mixer-parts-c-116052_116058_116074.html

Phone Breville and ask. You are the only person to report this issue, and to me it sounds like normal operation as everything works with the arm down.

https://www.google.com/search?q=breville+scraper+mixer+pro+bem800xl+display+cuts+ou

According to this there have been modification to the mainboard and shows a 'safety switch'.

https://www.manualslib.com/manual/1771651/Breville-Bem800-Series.html#manual

....

There are several potential reasons why your KitchenAid dough mixer might be shutting off. Here are a few possible causes:
Overheating: If the mixer has been running for an extended period of time, the motor may overheat and shut off as a safety precaution. In this case, you should allow the mixer to cool down for a while before using it again.
Faulty wiring: If the mixer is shutting off when you turn it on or when you change the speed, there may be a problem with the wiring. You should check the wiring to see if there are any frayed or damaged wires that could be causing the issue.
Clogged air vents: The mixer may shut off if the air vents are clogged, as this can cause the motor to overheat. You should check the air vents to make sure they are clean and unobstructed.
Failed motor: If none of the above issues are present, the problem could be with the motor itself. A failed motor can cause the mixer to shut off, and it may need to be replaced.
If you are not comfortable troubleshooting the mixer yourself, you should consider contacting KitchenAid customer service or a professional appliance repair service for assistance.

In the days before electronic speed controls, speed reduction was by a number of series resistors and at low speed there was a lack of torque. Professional mixers overcame the problem by using a multiratio gearbox that wouldn't disgrace a motorcycle or a small car.

Electronic speed control allowed speed feedback so when a load is applied and the motor slows, more current would be supplied tending to maintain the speed - not perfect by any means but usually much more efficient than the old system. I don't know the precise details how the system works but I expect that is where the fault is.

Motor or brushes to motor are weak and not making a good connection inside motor or motor armeture failure.

If it slows down when mixing, it is most likely that the mixer is being asked to do more than it is capable of. If it slows down when nothing is in the bowl, then the motor/gears/lubrication needs looking after.

The Service Manual is at:

http://www.kitchenaidparts.eu/documents/pdf/service-manual/classic-svc-manual-new-email-version.pdf

You may have to adjust the mixer speeds. Obtain a copy of the KSM150 Repair Part List from Kitchenaid.com. Remove the screw holding the End Cover and locate the three screws from the Control Plate. All three (2 top left and right, and 1 bottom) must all be adjusted. 1. Loosen the lock nuts enough & Turn the 2 top screws clockwise until they cannot be turned
anymore. Leave the bottom screw alone for now.
2. Use a dab of white paint or paper whiteout to mark the positions of the Phillips screws.
3. Plug in your mixer and Set the speed selector to "Stir".
4. Turn each top screw One full turn couterclockwise (alternating between Left and Right)
Keep making one full turn at the time (for both left and right) until the motor starts running.
5. Put your hand near the Agitator shaft and allow it to brush your finger slightly. Use your
watch or clock and start counting the number of times the shaft touches your finger in
60 seconds. The correct count should be 60 RPM (Rounds Per Minute). If it is less
or more than 60, give each screw 1/4 turn counterclockwise or clockwise and start counting
again. Now tighten both lock nuts.
6. Set the speed selector to "6" and turn the bottom screw either clockwise or counterclockwise
to achieve a speed of 180 RPM.

For your reference, the planetary RPM's for the 5-qt. tilt-head Artisan Stand Mixer are as follows:
Stir speed - 60
Speed 2 - 95
Speed 4 - 135
Speed 6 - 180
Speed 8 - 225
Speed 10 - 2

Make sure the plug is securely plugged into the wall.
Next, remove and reattach any mixing arms etc that may be attached to the mixer. Having one not attached properly can result in a start, start.
If problem persistes make sure the speed is reset.

If problem continues, here is an excerpt from the user manual found here.

http://www.cuisinart.com/share/pdf/manuals/sm-55.pdf

Problem: Your stand mixer has an
overload protection device and if
overloaded, it will shut down to protect
the motor.
Solution: In the unlikely even that this
happens:
• Turn off and unplug the unit.
• Reduce the load by removing some of
the ingredients, and allow the mixer to
stand for a few minutes.
Plug in and reset the speed. If the
stand mixer does not start when you
press the on button, allow the unit to
stand for additional time.

If you are still having trouble, the product comes with a 3 year limited warranty, so if you're bought it within the last 3 years it may be covered.

Good Luck!

Your motor needs to be replaced. The speed setting should be able to be set from the start, and if it will not slow down or change speeds, the motor has been over-heated and will need to be replaced. The only quick fix for this is to bypass the speed setting, and change the brushes on the motor. If you do not know how to do this, ask a serviceman to change the brushes only. Brushes usually run about $30 to repair. Although, you will only have one speed.

your kitchen aid has a little device inside called the hall effect sensor. These often come dislodged or just stop functioning. If you are mechanically inclined you can replace this yourself.

The likely problem is the Hall Effect sensor located on top of the motor. This sensor is used to determine the speed of the motor. Since it does not sense a speed, the motor revs to top speed and shuts down. You have to shut the switch to Off/Reset to try again. Replace the sensor which costs under $10 before replacing the speed control which costs $50.