The most widely used electric motors for railway traction are currently of three basic types, (Lozano, 2010):
1. Direct current electric motors with in-series excitation.
2. Direct current electric motors with independent excitation.
3. Alternating current electric motors.
Direct current electric motors usually work under a 3 kV supply and alternating current motors under 25 kV. Direct current motors are gradually becoming obsolescent in favour of alternating current motors. This is mainly due to maintenance problems with the direct current motor collectors and the better technological progress of alternating current motors.
This work does not aim to provide an exhaustive development of the behaviour of electric motors. It will only make a compilation of the equations and behaviour models of electric motors that have already been published and accepted, particularly those that apply the Bond-Graph Technique, (Karnopp, 2005; Esperilla, 2007, Lozano, 2010).
Direct current electric motors (hereafter DC), are mainly made up of two components: a stator or armature and a rotor or armature (see Figure 8). The stator's mission is to generate an electric field at the core of which the rotor is inserted. This magnetic field of the stator is generated by windings through which an electric current is made to flow. An electric current is also made to flow in the rotor. As this is immersed in a magnetic field generated by the stator, the electric current conductor undergoes mechanical forces that cause the rotor to rotate on its shaft. The outline shown in Figure 8 corresponds to the so-called DC motors with independent excitation, as the operating voltage of the stator and the rotor is independent. Also, the windings of the stator and the rotor can be interconnected giving rise to two other types of DC motors:
a. DC motors with in-series excitation, if the windings of the stator and rotor are connected in series.
b. DC motors with in-parallel excitation, when the windings of the stator and rotor are connected in parallel under the same operating voltage.
Firstly, we will deal with the modelling of DC motors with independent excitation and then go on to DC motors with in-series excitation, making some slight adaptations.
Fig. 8. Electromechanical circuit diagram of a DC electric motor.