Induction vs. Synchronous Generators

A question frequently asked about steam turbine generator packages is "What is the difference between an induction and a synchronous generator package?".

An induction or asynchronous generator is, in very simple terms, an electric induction motor driven at speeds above its synchronous speed. It has a solid armature, or squirrel cage, that is an electrical short circuit. When the current is connected, the machine will start turning like a motor at a speed that is just slightly below the synchronous speed of the rotating magnetic field from the stator. If we manually drive this rotor at exactly the synchronous speed of the generator (1800, 3600 RPM etc.), the magnetic field rotates at exactly the same speed as the rotor, we see no induction phenomena in the rotor, and it will not interact with the stator. If we increase speed above the synchronous speed of the generator, the rotor moves faster than the rotating magnetic field of the stator, and the stator induces a current in the rotor. The more mechanical power that is delivered to the rotor, the more power will be transferred as an electromagnetic force to the stator, and in turn converted to electricity, which is fed into the electric grid.

Because most single stage steam turbines develop their peak efficiencies between 3600 and 5500 RPM, the 2-pole (3600 RPM) induction generator allows for a direct drive application, eliminating the need for a speed reduction gear and a costly lubrication system, as the turbine is usually ring lubricated and the generator has grease packed ball bearings. Although induction motors / generators are available in very large sizes, they are typically utilized in smaller (<1000 kw) applications. There are several reasons for