If we speak about electric motor regulation and locomotive traction control, then we also have to speak about power electronics. Even for the three motors dealt with in the preceding section, the technology used in each case varies a great deal. For the reasons already stated in the previous section, at present, research conducted in this field is focused on three-phase traction technology with asynchronous motors and a cage rotor, converters with thyristors, and, of course, control technology based on IGBT transistors and microprocessors. However, let us begin by reviewing the regulation of the two DC motors studied to then go on to the regulation of asynchronous three-phase AC motors. So as to reach a proper understanding of the logic of the regulation techniques used in each case, we will continuously refer to the behaviour equations of the motors that were dealt with in the previous section.
As already stated towards the end of Section 2.2, the ideal traction curve is a constant power hyperbole. It is said to be optimum for traction because if the motors had this operating curve their power could be put to full use, with large torques at low speeds and small torques at high speeds, as Figure 5 shows. But at very low speed, that is when starting, the motor's rotational speed is very low and the torque that it is able to produce tends towards infinity. This causes the adhesion forces to exceed their limit in the wheel-rail contact. In addition, as will be seen further on, the intensity of the current flowing in the motors takes on very high values that will lead to the destruction of the electrical circuits. For these two reasons, during the starting process the torque supplied by the motor needs to be limited so as not to exceed the adhesion limits. At the same time the intensity of the current flowing in the motors is limited so as not burn out the electrical circuits.
On the other hand, as we shall demonstrate further on, the characteristic operational curves of the motors (motor torque curves "t", dependent on the rotational speed "o"), do not fit exactly with the constant power hyperbole, while at high speeds, the torque supplied by the motor is less.
From the above two paragraphs two very important conclusions can be reached that affect the operational regulation of the motors:
a. During starting at very low speed, we must regulate the intensity of the current and the motor torque so that the motor will supply the maximum possible torque without exceeding the current limits or the adhesion limits in the wheel-rail contact, until the conditions of the constant power hyperbole are reached in the shortest possible time. This operation is called “starting regulation" or “constant torque regulation".
b. Once the constant torque hyperbole has been reached, we must check how to regulate the motor function so that its characteristic "t-o" curve fits the constant power hyperbole. This is called “constant power regulation".
The following sections will deal mainly with regulating the motors during starting and then their regulation under constant power. The conclusion to be drawn from all this is that by applying these regulation procedures, the electric traction motors are made to operate in three stages: initially under constant torque during starting: then under constant power at moderate and high speeds, and finally, following the characteristic curve of the motor at very high speeds. (see Figure 14).