An advantage of having an alternator with a rotating field is that the rotating mass is:

Having an alternator with a rotating field design offers the advantage of reducing the rotating mass. In this setup, only the field windings are carried on the rotor, while the heavier components, such as the armature winding and the core, are stationary and located on the stator. This results in a lighter rotor compared to designs where the armature is on the rotor, which minimizes inertia and allows for smoother acceleration and operation.

This reduced mass is significant because it enhances the efficiency of the alternator by reducing the energy required to spin the rotor. The lighter rotor means that less power is needed to reach operating speeds and to respond to changes in load, leading to improved overall performance. Additionally, the reduced rotational mass helps decrease wear on bearings and other mechanical components, extending the life of the alternator.

In contrast, a heavier rotating mass would require more energy to maintain its operation and could lead to increased mechanical stress on the components, making the design less efficient and more prone to mechanical failure. Consequently, the benefit of having a rotating field in alternators primarily revolves around the reduction of the rotating mass, thus enhancing the overall efficiency and reliability of the system.