In the last 30 years railway traction motors have ceased to be direct current motors controlled by the connection and disconnection of resistances to become asynchronous alternating current motors controlled by IGBT transistors for medium powers, or GTO thyristors through the use of pulse width techniques for high powers (Faure, 2004).
As explained in the previous sections, the first controls using rheostats for direct current motors gave way to thyristor-based control. DC motors can be better controlled by this technology by avoiding the transitory "jerk", effects caused by connecting and disconnecting the starting and shunting resistances. So, thyristors led to a much better and uniform functioning of DC motors. Notwithstanding, the drawbacks derived from the collector still existed, which required large-size motors that needed frequent maintenance. These requirements became much less with the appearance of synchronous alternating current motors and were practically eliminated with the asynchronous motors.
The development of traction control systems has led to the use of simpler, more robust and cheaper asynchronous motors. They are more complicated to regulate than synchronous motors but this complexity became much less with the development of IGBT transistors. Nowadays, the major research and development projects are focussing on the technologies related to asynchronous alternating current motors (hereafter asynchronous AC motors).
The three-phase asynchronous motor is an induction motor based on the generation of rotating magnetic fields by means of the stator windings, which induce electric currents in the rotor windings. Due to the interaction between these induced currents and the magnetic fields, forces are generated in the conductors that produce a motor torque. If the rotor rotates at the same speed as the magnetic fields, there is no variation of flux passing through the turns of the rotor and the induced current is zero. For a motor torque to be produced there needs to be a difference (slip) between the angular velocities of the magnetic field and the rotor.
For a low frequency alternating electric current, the torque generated by the motor "t" satisfies an expression that is similar to the expression for direct current motors:
t = K 0 lr=K lelr (30)
Where, "Ki" and "K" are behaviour constants of the motor; "O" is the magnetic flux generated by the stator, and "Ir" is the intensity of the current flowing through the rotor. It may be deduced from this expression that in DC motors, the torque increases along with the intensity of the current in the rotor.
However, it is also known that the current in the rotor is proportional to the magnetic flux "O" generated by the stator and to the slip "s" between the rotor and the magnetic field. Moreover, the magnetic flux "O" is proportional to the quotient between the feed voltage "U" and the current frequency "f", which means the torque can also be expressed as:
t = Kf g)2s (31)
Where "Kf" is a new constant of the motor function. The law expressed by equation (31) is valid for small slips, but for larger slips the ratio ceases to be linear. Figure 23 shows the motor torque curves "t" compared to the rotational velocity "m", for different values of the current frequency "f".
Fig. 23. Torque curves for an AC asynchronous squirrel cage motor.
By regulating AC motors and fitting their working to the theoretical torque-speed curves "t-that is to say, starting under constant torque and constant power hyperbole (remember Figure 14), the feed voltage "U" and the current frequency "f" can be varied simultaneously. To achieve this, IGBT, GTO thyristor-based technologies are used for high powers, which modulate the width of the electric pulses through the superposition of a triangular wave and a voltage signal that is proportionate to the signal required. Doing this will ensure that the motor describes the constant torque curve and constant power hyperbole without any steps.
Figure 23 shows the torque-speed curves for an asynchronous squirrel cage AC motor with a power of 1200 kW, simulated using the Bond-Graph model dealt with in Figure 13, for different frequency values of "f" and the operating voltage "U". Obviously, by a continuous regulation of the frequency and voltage, the real working of the motor will exactly fit the constant torque straight line during starting and the constant power hyperbole.