Diagnosis
The motor is overheating. You are either running it too long on high speed or blending a very thick substance without enough liquid.
How to Fix It
Stop using it and unplug it. Let the base cool completely for at least 30-45 minutes. For your next blend, use more liquid and alternate between high speed and lower speeds. Do not exceed the recommended maximum run time on high.
If That Didn't Work
The thermal fuse or protector may be failing. If the smell happens quickly even with easy blends, the unit likely needs professional service.
Heads Up
That smell is the insulation on the motor windings. Repeated overheating causes permanent damage. It's a safety feature, not a normal operating condition.