Variable Frequency Drives (VFD's) are used to vary the speed of an electric motor. They do this by changing the frequency of the electric power going to the motor which they work only with three-phase power. Today they are very economical and a VFD will save energy in addition to providing a desirable control function. VFD's have a built-in circuit breaker that shuts down the motor if the amps get too high for the speed at which the motor is being ran on. This provides excellent electrical protection for a motor and the machine it is driving.