Electric-powered propulsion is becoming an increasingly popular alternative to traditional diesel systems in certain applications due to their sustainability credentials, efficiency and reliability. However, many people may not be familiar with the inner workings of an electric motor and why a reduction gearbox is normally required. In this article, we will explore the basics of electric propulsion motors and explain why a reduction gearbox is normally necessary for optimal performance.
First, let’s look at an electric motor’s components. An electric motor consists of two primary parts: the stator and the rotor. The stator is the stationary part of the motor and contains the windings that create a magnetic field. The rotor is the rotating part of the motor that contains the conductors that interact with the magnetic field to produce torque.
A magnetic field is created when an electric current is applied to the stator windings. This magnetic field interacts with the conductors in the rotor, causing it to rotate. The speed of the rotor is determined by the frequency of the applied current and the number of poles in the stator.
So, why is a reduction gearbox required for an electric motor? The answer lies in the torque-speed characteristics of the motor and the rotational speed-to-efficiency relationship of the chosen propeller. Electric motors produce peak power at a given rpm and this is often much faster than the rotational speed of an efficient propeller design for a vessel.
For example, if you connect an electric inboard propulsion system directly to the vessel’s propeller and the propeller is designed to produce the required thrust at the high rotational speed to meet the vessel’s speed requirement, then it is likely the efficiency of the propeller will be low. If the motor is coupled directly to an efficient propeller design with a lower design rotational speed then the motor will not be able to turn fast enough to deliver the required power. In this case a higher power motor can be fitted so that the required power is available at a lower rotational speed however, this adds unnecessary cost and weight.
The best way to overcome these issues is to select a reduction gearbox with a ratio that allows a match between the peak power rotational speed of the motor and the most efficient practical propeller designs required speed. The reduction gearbox works by using a system of gears to reduce the speed of the motor output. This reduction in speed increases the torque output, making it possible to optimise the motor and propeller for peak efficiency.
If you are considering electric propulsion for your vessel, contact our technical team, who can offer support and advise on the best set up for you and your vessel.
Did you know the Bellmarine Torquemaster has a built-in mechanical reducer with various gear ratios for every operational need? Find out more about its performance and responsiveness in The Tulip case study.
