TFDE Propulsion Layout
The name TFDE originates mainly due to the power generation engines being able to use three different types of fuel, thus the name "tri-fuel diesel electric propulsion."
With respect to propulsion, the most modern trend is to have electrical motors instead of diesel engines. Power generated in the alternators is in the range of 10 to 20 megawatts, which then feeds the main propulsion motors.
Again we have an option of selecting synchronous motors or induction motors for propulsion. The picture specifies various propulsion motor options with fixed and controllable pitch propellers.
Electric propulsion offers various advantages like layout flexibility, low running cost and maintenance, better efficiency, and good maneuverability. A typical electrical propulsion system has 4 generators each generating 8 to 12 mW of power at 6600 or 11000 Volts, 60 Hz. This is fed to the main bus bar from where the propulsion motors are fed. The propulsion motors are two in number coupled together with a reduction gear, driving the propeller.
Each propulsion motor is connected from the bus bar through propulsion transformers. The motor has two stator windings for redundancy reasons. When one stator winding fails, the motor can still run at 50% redundancy.
Even though the generated voltage is 6.6 kV, the motors run on 3000 V or lesser. This is because of the limitation in the thyristor firing circuits. The motors can even run as low as 1 or 2 Revolutions which offer better control for maneuvering. The number of generators connected to the bus bar depends on the load on the electrical motor.
The main propulsion electric motors can be started in many ways. The most common two types are
- Starting normally as induction motor and then making it a synchronous motor, and
- Pony motor driving the motor making it as synchronous motor.
On all these ships, which have electrical motors for propulsion, kVAr load is more important that the conventional kW load. When the propulsion motor starts, the kVAr load is more than the kW load. As the motor picks up speed, gradually kW load increases to a greater extent as the kVAr starts to stabilize.