How a VFD Works

Electric motors are found in a number of applications and industries, and work by converting electrical energy into mechanical energy. VFDs, or variable-frequency drives, help control AC (alternating current) electric motors.

Function

• A VFD is used to control the electrical power supplied to an electric motor. VFDs are a type of adjustable-speed drive, or device used to control machinery speed by allowing that speed to be selected from preset ranges.

Process

• A VFD manages the power supplied to an AC motor by changing frequency, or the ratio of applied voltage to the applied frequency. When starting a motor, the VFD supplies low frequency and voltage in order to avoid a current rush to the motor. After the motor starts, frequency and voltage are added at a controlled rate to increase the load without drawing excessive current. This enables the AC motor to develop optimum torque while drawing less than 50 percent of its rated current.

Stopping Sequence

• VFDs stop an electric AC motor by ramping down voltage and frequency to the motor at a controlled rate. The motor is shut off when the frequency nears zero.