480v motor, running like a bat out of hell.
If the motor does not already have an electronic speed controller, the only way to reduce the torque and speed is to add one. Assuming the motor is a three-phase induction motor, you need a variable frequency drive (VFD). The cost of a VFD will be significantly more than the cost of a motor.

Before you buy a VFD, you should determine if there is something else that needs attention.


Determine if the augur filling up is a normal or abnormal condition.
Determine if something other than just filling up is contributing to the problem and whether or not that is a normal condition.
Determine if the augur is suited to the material and conditions.
Determine if the motor and mechanical drive train is suitable for the augur, material and conditions.


If the condition that caused the shaft to break is something that happens only occasionally under abnormal conditions, a mechanical or electrical means of shutting down the motor and or disconnecting the drive train may be suitable.

Make a sketch of the equipment, motor, drive train, augur, infeed system, outfield system etc. Note down all of the relevant dimensions and ratings.

If there is a VFD, it should be simple to adjust the current or torque limit.

Note that limiting the motor torque will likely result in the motor stopping when the problematic condition occurs. Limiting the torque will reduce the speed, but it is not the nature of an auger to require significantly less torque to operate at a lower speed.