Facts

Students of electrical engineering at the first days of their first electric machines course learn that rotating electric machines are classified into DC, induction and synchronous. They might get familiarized with switched reluctance (SR), stepping, PM brushless and axial-flux machines too.
In an utmost classic effort so far, the electric machines have been categorized into DC and AC; then DC divided into brushed and brushless, AC into synchronous and asynchronous and each into single and three phase ones. Except DC and induction machines, all the other above mentioned types are classified as synchronous machines.
Actually, from geometrical point of view, all except axial-flux machines belong to the category of cylindrical/drum or radial-flux types.
From electromagnetic point of view, all except switched-reluctance and stepping motors (singly-excited types), belong to the same category: the first type of doubly-excited machines.

The classic criterions of classification do not establish any logical basis to help us shape our accumulated knowledge of electric machines; it doesn’t present the big picture to know what is already found and what is missing, so direct us with a logical approach for inventing new ones.
We need a novel classification to understand their fundamental differences and similarities in the entire perspective. So we can build a methodological approach to help invent new species of electric machines.
First, some agreements: consider two solid elements, one stationary and one capable of moving or rotating around an axis, (called stator and rotor respectively). Without missing generality of the discussion, we assume that in rotating case, the stator is the outside element and the rotor is the inside one; and for any kind of motion, the moving element is called rotor.
Second, the electromagnetic energy conversion is the result of interaction of the following three fundamental concepts (variables) that are perpendicular to each other:
♦  Current
♦  Flux
♦  Torque

Classification Types

The classification can be done from two general points of views:
♦  Geometric Viewpoint
Depending on the geometric preferences and the axis that we prefer the torque to be developed along, the number and perpendicularity of these three variables, make them each be appropriately assigned to one of the axes in one of the coordinate system types. This leads to diverse topologies, one in Cartesian, six in cylindrical and three in spherical coordinate systems.

♦  Electromagnetic Viewpoint
Where and how to setup the sources of the above three variables to properly create interacting magnetic fields for developing a continuous and unidirectional torque (or force) leads to three major types. All familiar electric machines belong to any of these three types.

Geometric Classification

Depending on the geometrical features of an electric machine, it could fit in one of the three coordinate systems:

1. Cartesian coordinate system: When flux is along Z axis and current along Y, the moving part of the device will have a motion along X. As all three axes have the same properties, the axes could be exchanged without any change in the topology of the machine. Therefore only one combination is possible. Linear machines (like linear induction motors and MHD DC machines) fit within this system.
2. Cylindrical coordinate system: The three axes (R, θ and Z) have different properties and the exchange of the axes of the coordinate systems leads to six different kinds classified under radial, axial and toroidal flux machines. Drum type machines (like DC, synchronous, induction and so called linear generators) and axial-flux motors fit within this system.
3. Spherical coordinate system: Among three axes (R, θ and φ), θ and φ have the same properties; so the exchange of the axes of the coordinate systems leads to three different kinds. I’m not aware of any real machine in this category, but it’s quite possible to design and build a spherical electric machine.

For now, only those machines are considered that fit within cylindrical coordinate system, as the majority of electric machines have been built in this category.
As mentioned, different assignments of the three fundamental variables to the three axes, lead to six different configurations (Table 1). The configurations in green color are the ones that have been developed so far.

To be able to visualize the transformation of different types to each other, we have to keep in mind the following facts:
1. The active component of flux crosses the air gap between the stator and the rotor.
2. The currents close along their distribution direction.
3. The developed torque acts along the currents distribution.

It’s important to note that in this classification only the effective path of each variable has to be considered.
For example, in the first column, the flux flows through laminations teeth (R component) using the stator and rotor yokes (ɵ component), but only the R component cuts the wires. Currents flow through the wires along Z axis (Z component) and close using the end windings (ɵ component), but only the Z component is cut by the flux lines.

In Table 1, the type of machine is related to the direction of the flux; therefore drum machines could be called radial-flux machines too. The diligent reader could find out if any more configurations other than the first three can be built and how.

In next posts, before classifying the electric machines from electromagnetic point of view, the necessary and sufficient conditions for electromagnetic energy conversion will briefly be explained.
Then we choose one configuration (Drum type) and see how differently we can set up the three main variables sources within that configuration for a successful unidirectional and continuous electromagnetic energy conversion.