During the simulations carried out on the behaviour of the motors, the previous sections have taken account of the discrete values for the starting resistances or for the shunting resistances. This has enabled us to display the corresponding torque curves. However, the real operating curve, as Figures 16, 18 and 21 may recall, jumps up and down as it attempts to firstly fit to the continuous torque straight line and then to the constant torque hyperbole. This was how the operation of a real motor was originally regulated. It can currently be highly valid during a computer simulation for understanding the phenomena that occur. However, with the development of power electronics, these proceedings for regulating the operation of motors are not the most appropriate.
The electronic regulation of direct current motors is essentially based on regulating the feed voltage. The devices used to do this are called Choppers.
When dealing with a motor with independent excitation a complete regulation can be achieved by controlling the stator and rotor voltage separately. If the motor has in-series excitation, the feed voltage to the rotor and the stator Shunting must be controlled simultaneously.
The chopper's mission is to act as a switch to open and close the motor feed circuit, as Figure 22 illustrates. In this way, by starting with a fixed continuous feed voltage "U" in the catenary, a variable voltage applied to the motor is obtained which manages to regulate its speed. Since the switch is off (driving the chopper) the catenary voltage is applied to the motor to make an electric current flow through the motor. When the switch is opened the voltage applied to the motor is cancelled, cutting off the current to the switch. During the time the switch is on, the motor current flows through the diode placed in parallel. By varying the times the switch is on and off, the mean voltage in the motor can be varied and therefore, the current intensity flowing through it. This method perfectly manages to regulate the motor torque and perfectly follows the theoretical continuous torque curve (starting), and the constant power hyperbole without any steps appearing in the real torque operating curve mentioned at the beginning of this section. To achieve this phenomenon, devices called Thyristors are used.
Fig. 22. Principle of how a Chopper-controlled DC motor functions.