Variable Frequency Drives (VFDs) control AC motor speed and torque by adjusting power frequency and voltage. They consist of a rectifier (AC to DC), DC bus (filters/stores power), and inverter (DC to variable AC). VFDs save energy, enable soft starts, and improve process control in pumps, fans, conveyors, and HVAC systems. Key features include V/f control, vector control, and dynamic braking. Challenges include harmonics, motor heating, and electrical noise. Choosing the right VFD depends on motor power, load type, and control needs.