What is the definition of a variable speed drive vs variable frequency drive? The two terms are often confused, but they refer to two distinct technologies. A variable speed drive (VSD) is an electronic system that adjusts motor speed in response to changing conditions, such as load or environment. By controlling motor speed, a VSD can improve energy efficiency, reduce downtime and extend the life of the motor. In contrast, a variable frequency drive (VFD) is an electronic system that modulates the electrical power supply to a motor to control the motor’s speed and torque. VFDs adjust the frequency of the electrical supply to the motor, allowing it to run at different speeds and increase or decrease torque. VFDs are especially useful for motor-driven systems that require frequent starting and stopping, such as pumps and compressors. VFDs are also less expensive and simpler to install than VSDs, making them a popular choice for many applications.