Friction Losses in Motors

Friction losses in motors (AC or DC) are pretty simple when you get right down to it. Basically, any book on the relevant bearing type (sleeve or rolling element) will cover almost everything you need to know.

Induction motors (both squirrel cage and wound rotor) are pretty easy, as they're essentially cylindrical. This means the "friction" developed from moving the rotor through a medium (typically air) is minimal. As the rotor geometry deviates from a true cylinder toward a "fan" design (think of the separate salient poles in a synchronous machine), there will be more disturbances and therefore more friction generated.

There will also be "windage", which is the effort required to pass objects through the air (think fan blade). Most of the induction machine windage is due to the end winding, where the rotor bars (or coils) act as fan blades. For synchronous machines, the whole salient pole acts like a huge blade. Both types may have ACTUAL fan blades as part of the rotor.