Variable Frequency Drive (VFD) / Frequency Converter

What defines this type

A variable frequency drive rectifies incoming AC supply to DC, smooths it on a DC bus, then uses an inverter stage to synthesise a new AC waveform at whatever frequency and voltage the motor needs, so motor speed follows the drive's output frequency instead of being fixed to the supply frequency. That sets it apart from a soft-starter, which only ramps voltage during starting while the motor still runs at line frequency once started, and from mechanical speed control by valve, damper or fluid coupling, which wastes the excess energy as heat or pressure loss rather than avoiding drawing it in the first place.

Main components

• Input rectifier and filter — converts incoming AC to DC and limits the harmonic disturbance fed back onto the supply.
• DC bus capacitors — smooth the rectified supply and store energy for the inverter stage.
• Inverter (IGBT) module — switches the DC bus to synthesise the output AC waveform at the required frequency and voltage.
• Output filter — a sine-wave or dU/dt filter, important when motor cable runs are long, protecting motor winding insulation from switching voltage stress.
• Control board and interface — sets ramp rates, protection limits and communicates with the vessel's alarm and monitoring system.
• Cooling arrangement — air-cooled heatsinks on smaller units, liquid cooling on higher-power installations.

Selection / sizing

Sizing starts from motor power and voltage, but the load type matters as much as the number on the nameplate: a centrifugal pump or fan is a variable-torque load that needs relatively little torque at low speed, while a winch or thruster is a constant or high-torque load at low speed and needs a drive with a correspondingly higher overload margin. Cable length between drive and motor decides whether an output filter is needed to protect motor insulation from voltage reflection. Where several drives share one switchboard busbar, the combined harmonic distortion may exceed what the installation can tolerate, requiring input filtering or a multi-pulse rectifier on the larger units.

Regulations / Class

Class rules set a limit on harmonic distortion at the switchboard, which can force filtering when several drives are installed on the same bus. A drive feeding one of a duty/standby pair of essential pumps or fans does not remove the redundancy requirement — the standby unit still has to be independently available if the drive itself fails. EMC performance matters on a steel hull carrying sensitive navigation and communication electronics close to the switching noise a drive generates.