A d.c. PM commutator (brush) motor can be compared with a d.c. separately excited motor. The only difference is in the excitation flux Φg in the air gap: for a PM motor Φg = const whilst for a separately excited motor Φg can be controlled. This means that the speed of a standard d.c. PM commutator motor can normally be controlled only by changing the armature input voltage or armature current. A typical d.c. PM commutator motor is shown in Fig. 4.1. By adding an additional field excitation winding, the flux Φg as well as the speed can be changed in a certain limited range.