Variable frequency drives are used with electric motors and generators in many systems, from HVAC to propulsion, on hybrid and electric marine vessels. But these drives create a voltage on the shaft of the rotating equipment, which can discharge through the equipment’s bearings or through coupled equipment like gearboxes. For long-term reliability, this equipment requires electrical bearing protection, like AEGIS® Shaft Grounding Rings. This post will look at a few particular drives applications in marine vessels.

Drive Applications in Marine Vessels

Main Powertrain

The shaft generator, along with the main engine, is coupled to the propeller shaft and controlled by a large VFD. Depending on demand, the shaft generator can generate power or act as a motor and provide additional power to the propeller. Because of the drive is subject to high-frequency shaft voltage and should be protected with an AEGIS Shaft Grounding Ring (image above) to prevent discharge through the generator bearings or gearbox. Discharge through the gearbox produces “tiger striping” on the teeth, shown below.

The main propeller shaft should also be grounded with a carbon brush. This connects the propeller to the cathodic protection system and prevents electrochemical corrosion. Carbon brushes are less effective than AEGIS rings at dissipating high-frequency voltage but better at conducting large DC and low-frequency current.

This double-shaft grounding scheme is similar to bearing and gearbox protection in wind turbine generators. Although these systems are very different, they both require AEGIS shaft grounding to remove high-frequency voltage and carbon brush grounding to handle low-frequency/DC voltage.

Thrusters

Both bow thruster and azipodal thruster motors are controlled by VFD and should be equipped with AEGIS rings to discharge high-frequency shaft voltage produced by the drive.

Heat Recovery System

The powertrain’s heat recovery system salvages waste heat from the engine and diesel generators. This system includes pumps and compressors, which are driven by VFD-controlled motors.

Other HVAC

Separate from the heat recovery system, a ship’s HVAC system includes fans, pumps, and compressors, all of which are controlled by VFD. These HVAC motors also require electrical bearing protection against high-frequency shaft voltage.