There are many topologies of electric machines, which create various classifications. Traditionally, they were classified into two groups DC and AC. With the advent of new machine types, this classification becomes ill-suited. Figure 3 shows the proposed classification of electric machines in which the bold types are those that have been applied to renewable power generation; meanwhile, the branches that are not viable for renewable power generation have been pruned. Basically, they are classified into two main groups’ commutator and commutator less. The former simply denotes that they have a commutator and carbon brushes, while the latter have neither commutator nor carbon brushes. It should be noted that the trend is focused on developing new types of commutatorless machines. Electric machines for renewable energy harvesting should not be considered as a subset of electric machines for industrial application since they have fundamental difference in requirements. The renewable energy machine should form an individual class of electric machines, which possesses the following features:
· High efficiency over wide torque and speed ranges so as to increase the utilization of extractable energy;
· High power density so as to reduce the overall size and weight;
· Wide speed range so as to harness the energy at different speeds;
· High reliability so as to reduce the operational failure or fault;
· Maintenance free so as to eliminate the maintenance cost and possible outage for maintenance;
· High robustness so as to withstand harsh operating conditions and natural environment;
· Good voltage regulation so as to maintain the system voltage;
· High power factor so as to enhance the power transfer;
· Low cost so as to reduce the system cost.
