So, for most small, high Flux DC motors there is this phenomenon called magnetic field saturation. It happens when the permanent magnet doesn’t have enough field strength to drive the high amount of Flux through the moving parts of the motor. I am simplifying here to try to keep it short.
The bottom line is that at high current (aka, near stall), the magnetic field saturates producing less actual torque than ideal and this is one of the reasons for the difference between Kt and Ke.
However, it is not the only difference. This can really only be found with motor testing.
If Ke = Kt (which would be a perfect world scenario)
Then the max torque the motor could output would be:
Tstall = Ke*Istall
Which for a bag motor comes out to:
Tstall =~ .32 Nm
This is less than the .4 Nm in the motor specs, and given we have come to the conclusion that the real world Kt is always less than Ke, wouldn’t the true stall torque of the bag motor be even less than .32 Nm?
(I also don’t understand why our calculated Kt is larger than Ke or is that a result of the curve actually being nonlinear?)