DIAGNOSIS A hot base and burning smell point to motor overload. You're either running it too long on high speeds with a thick blend, or there's a physical obstruction making the motor work way too hard.
FIX Stop using it immediately and unplug it. Let it cool completely for at least an hour. 1) Once cool, inspect the pitcher. Remove the blade assembly (twist counter-clockwise) and check for anything wrapped around the shaft or damaged blades. 2) Check that the pitcher is seated perfectly on the base—if it's crooked, the coupling isn't fully engaged, causing slippage and heat. 3) For your next blend, use shorter bursts with the Pulse function for thick tasks, and give the motor a 30-second rest between cycles.
IF THAT DIDN'T WORK If the burning smell happens with an empty pitcher or on a low speed, the motor's bearings or windings might be failing. This is a safety hazard. Do not continue to use it. You'll need to contact Ninja customer service for a repair or replacement assessment.
HEADS UP The motor is powerful, but it's air-cooled. It needs airflow. Never run it for extended periods (like multiple minutes continuously) on max speed, especially with dense ingredients. And never cover the vents on the sides of the base.