Why a variable frequency drive can't just keep raising frequency forever and expect the same full torque at every speed.
A VFD controls motor speed by changing the supply frequency, and it's tempting to assume that's the whole story: want the motor faster, command a higher frequency, done. Below a certain speed that's essentially true. Above it, the drive runs into a hard physical limit — its output voltage can only go so high — and once that ceiling is hit, something has to give. What gives is the motor's magnetic flux, and with it, the torque the motor is capable of producing. That transition point is called base speed, and understanding why it exists is the difference between correctly evaluating a high-speed VFD application and assuming a motor will simply deliver full torque at any commanded speed.