So here is an update: We have a few of the bad motors back from customers and they display the same symptom. I will try to explain briefly.
There are 7 commutators on the BAG motor and two brushes. For those not familiar with how a DC motor works, the commutators are the things that spin with the rotor (shaft) and contact the brushes to generate the magnetic field.
In the bad BAG motors, there is an open connection between the two wires that make up the winding. This results in one set of 3 commutators disconnected from a set of 4 commutators. When one brush is contacting only within the set of 3 and the other brush is contacting only within the set of 4 the motor will receive no current and not spin. This symptom will only show if the motor stops at a spot described above.
We are inspecting all of our BAG motors in inventory to find any motors with this condition. We have inspected about 500 so far and have not found any bad motors.
What we don't know is if this is a condition caused by use or a problem right out of the box. Our theory is that the problem is right out of the box so our inspections should eliminate this issue.
You can inspect your own motors by using a ohm meter on the two leads and spinning the motor slowly. If the resistance goes into the k Ohm or M Ohm range, then you have the problem.
We will replace all motors with this condition if you contact our customer support at
support@vexrobotics.com.