# Regulation of the DC motor speed of independent excitation with...

## Regulation of the DC motor speed of independent excitation with the help of resistors in the armature winding circuit

The circuit of the power circuits of the DC motor of independent excitation when the speed is controlled by changing the resistance in the armature winding circuit is shown in Fig. 3.7. The circuit contains the motor armature winding M , two series-connected additional resistors and , which are shunted by the closing contacts KM and KM2. The motor excitation winding LM is powered by a separate voltage source

The motor speed is controlled at the nominal excitation flux Fn and the nominal armature winding voltage . Consequently, the electromagnetic moment of the motor is proportional to the armature current and the mechanical and electromechanical characteristics will coincide in relative units, therefore the analysis of the artificial characteristics will be performed using the example of electromechanical characteristics.

Fig. 3.7. The circuit of the power circuits of the engine when speed is controlled by changing the resistance in the armature winding circuit

The analysis of artificial electromechanical characteristics will be performed with respect to the basic natural characteristics (Figure 3.8). Since the equation of the electromechanical characteristic is a straight line equation, it is sufficient to know the behavior of two points for its analysis. These points can be arbitrary, belonging to equation (3.3).

however, the simplest analysis is done if these two points lie on the coordinate axes.

With the armature winding current the motor rotates at the ideal idle speed

Fig. 3.8. Rheostatic characteristics of a DC motor of independent excitation

The ideal idling speed is independent of the additional resistance of the anchor chain and remains constant for this speed control method. Consequently, all the artificial electromechanical characteristics of the independent excitation motor go to the ordinate axis from one point with the coordinates .

At a speed of a short-circuit current is flowing along the winding of the motor armature . The short-circuit current is inversely proportional to the resistance of the armature winding circuit and decreases with increasing this resistance. This property is used to limit the armature current spikes during engine start-ups of independent excitation in relay-contactor control circuits.

The family of rheostatic electromechanical characteristics is shown in Fig. 3.8.

Rheostatic speed control, usually stepped and only with low-power engines, smooth speed control is possible when the variable speed rheostat is connected to the motor armature circuit.

Speed ​​control is performed under load. The direction of speed control is down from the natural characteristic. The steady-state values ​​of the velocity under load decrease with an increase in the additional resistance in the armature winding circuit.

Speed ​​control range at rated load .

The error of speed control increases with the increase of the additional resistance in the armature winding circuit.

Speed ​​control is accompanied by power losses in the additional resistance of the armature winding circuit.

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