Frequency converter (VFD)

A frequency converter, or variable-frequency drive (VFD), controls the speed of an AC motor by converting fixed-frequency ship’s supply to a variable voltage and frequency. It rectifies the 50 or 60 Hz supply to a DC link, then an IGBT inverter synthesizes a variable output by pulse-width modulation, holding the voltage-to-frequency ratio roughly constant to keep motor flux and torque. VFDs cut energy on pump and fan loads that would otherwise run at full speed, soften starting inrush, and drive thrusters and cargo pumps. The switching generates harmonics, so filters and a clean-earth design protect the network.