26-04-2012, 12:27 PM
MATLAB/SIMULINK Modeling of Speed Control Methods for DC Motors
MATLABSIMULINK Modeling of Speed Control Methods for DC Motors.pdf (Size: 207.82 KB / Downloads: 630)
I. Introduction
The speed of a DC motor can be varied by controlling the field flux, the armature resistance or the terminal voltage applied to the armature circuit. The three most common speed control methods are field resistance control, armature voltage control, and armature resistance control [1]. In this section, modeling procedure of these three methods and feedback control method [2] for DC motor drives for dynamic analysis are presented.
A. Field Resistance Control
In the field resistance control method, a series resistance is inserted in the shunt-field circuit of the motor in order to change the flux by controlling the field current. It is theoretically expected that an increase in the field resistance will result in an increase in the no-load speed of the motor and in the slope of the torque-speed curve [1]. Figure 1 shows the Simulink implementation of the field resistance control method. A DC motor block of SimPowerSystems toolbox is used. The DC motor block implements a separately excited DC motor. An access is provided to the field connections (F+, F-) so that the motor model can be used as a shunt-connected. The field circuit is represented by an RL circuit (Rf and Lf in series) and is connected between the ports (F+, F-). The armature circuit consists of an inductor La and resistor Ra in series with an electromotive force EA and is connected between the ports (A+, A-). The load torque is specified by the input port TL. The electrical and mechanical parameters of the motor could be specified using its dialog box. Observe that 240 V DC source is applied to the armature and field circuits. An external resistance Rf1 is inserted in series with the field circuit to realize the field resistance speed control. The output port (port m) allows for the measurement of several variables, such as rotor speed, armature and field currents, and electromechanical torque developed by the motor. Through the scope and display block, the waveform and steady-state value of the rotor speed can be easily measured in radian per second (rad/s), or the corresponding data can be written to MATLAB/workspace using the data box to make use of other graphical tools available in MATLAB.
B. Armature Voltage Control
In the armature voltage control method, the voltage applied to the armature circuit, Va is varied without changing the voltage applied to the field circuit of the motor. Therefore, the motor must be separately excited to use armature voltage control. When the armature voltage is increased, the no-load speed of the motor increases while the slope of the torque-speed curve remains unchanged since the flux is kept constant [1]. Figure 2 shows the Simulink realization of the armature voltage speed control method. This simulation model is similar to that of the field resistance control method shown in Figure 1. The main difference is that the armature and field circuit are supplied from two different DC sources to have a separately excited connection. Moreover, the external resistance Rf1 in Figure 1 is removed in this model.
C. Armature Resistance Control
The armature resistance control is the less commonly used method for speed control in which an external resistance is inserted in series with the armature circuit. An increase in the armature resistance results in a significant increase in the slope of the torque-speed characteristic of the motor while the no-load speed remains constant [1]. Simulink model of this method is not shown here since it is almost the same as that of the field resistance control method shown in Figure 1. The only difference is that Rf1 resistance in Figure 1 is removed and an external resistance Ra1 is inserted in series with the armature circuit between the ports (A+, A-) to vary the armature resistance.