LABORATORY WORK № 4

TECHNIQUE OF FULFILLING THE LABORATORY WORK №3. MATLAB

4.1.    Objectives

Analyze properties and obtain torque-speed characteristics of separately excited direct current motor using Matlab-Simulink.

 

4.2.    Task

Use simulation to obtain characteristics of separately ex-cited direct current (DC) motor:

·          without additional resistance in the armature;

·          with additional resistance in the armature;

·          at reduced supply voltage.

 

4.3.         Direct current motors. MATLAB

MATLAB software allows to model and simulate almost any linear or nonlinear system. The model of the system is composed of Simulink library blocks using Drag-and-Dropp technology, allowing selecting neccessary block in the library and dragging it by mouse into the model window and leaving it here. Afterwards all blocks are connected in the required manner. Similar system is called S-model. Elaborated model is saved and has extension *.mdl.

The model used to simulate separate excited DC motor is opened in this way:

·          double click on icon «Matlab» to open Matlab;

·          double click on Simulink icon in the horizontal toolbar which looks like this:

 

 

·          open file separately_excited_motor.mdl;

·          in the opened window separately_excited_motor.mdl the model of separately excited motor should appear, which will be used to investigate characteristics of the sepatarately excited motor.

·          The model allows analyzing dynamic and steady-state modes of direct current motor. The model of separately excited DC motor is presented in Fig. 4.1. Elements enterring the model and their purpose are not analyzed. Here only elements of the model parameters of which will be changed are discussed.

Given or calculated parameters are entered in the blocks of the model. The appointed armature resistance value is substituded to block «Gain». Block «Gain» should be selected and opened by double click with the left mouse buton on the block. The dialog window presented in Fig. 4.2 appears.

In the square Parameters the required value is entered. For ex-ample, if we need to change the resistance of armature, we need to make double click on the corresponding element, then the dialog window Block Parameters: Ra opens. In the Parameters window square Gain the required value of armature resistance is entered and the button OK is pressed.

 

 

Fig. 4.1. Simulink model of separately excited DC motor

 

Motor supply voltage, coefficient cF, load torque are entred into blocks Constant.

This block is opened and values are entered in the same way as to block Gain.

 

 

 

Fig. 4.2. Graphical view of block Gain and window to enter armature

resistance

 

 

 

Fig. 4.3. Graphical view of block Gain and window to enter armature

resistance

 

Simulation results (field and armature current, motor speed) are observed in the block Display.

 

 

Fig. 4.5. Block Display

 

The model of separately excited DC motor is elaborated on the base of differential equations that are replaced by equations in frequency domain. The elaboration of the model is not considered.

When all parameters are entered, simulation is started. It can be started by some different commands: choosing in Simulation menu option Start; using button combination Ctrl + Tor pressing special icon in Toolbar.

Initial data for modeling are given in Table 4.1.

Table 4.1.

Initial data

 

4.4.         Testing of direct current motors

1.       Speed –torque characteristic , at  and  

2.       Enter the indicated or calculated values of motor parame-ters to the model blocks. The torque-speed characteristic is obtained by changing load torque by the indicated step and measuring rotation speed of armature.

3.       Speed-torque characteristic , at the reduced armature voltage:  and

4.       Enter to model blocks calculated or indicated values of pa-rameters.

5.       Change the motor load by the indicated step and measure torque and armature speed.

6.       Put the obtained results in Table 4.2.

Table 4.2.

Speed torque characteristics at reduced voltage

	0	0.2	0.4	0.6	0.8	1	1.2
T, N
w, rad/s

 

7.       Speed-torque characteristic  at the rated armature voltage:  and

8.       Enter to the model blocks calculated or indicated values.

9.       This characteristic is simulated by changing motor load torque by appropriate step and measuring speed of armature.

10.  Put the obtained results to Table 4.3.

 

Table 4.3

Data of speed-torque characteristic at the increased armature resistance

x	0	0.2	0.4	0.6	0.8	1	1.2
T, N
w, rad/s

 

Plot speed-torque characteristics   in one reference frame according to the data of Table 4.1, Table 4.2 and Table 4.3.

 

4.5.           Content of Report

1.       Objective, task and model of the laboratory work.

2.       Work results (tables of simulation results and characteristics).

3.       Conclusions.

 

4.6.    Control questions

1.       Graphical and alphabetical notation of motor windings.

2.       What speed is called no-load speed of rotation?

3.       What dependence is called speed-torque characteristic?

4.       What dependence is called speed regulation characteristic

5.       On what parameters does the motor starting current depend?

6.       What technique can be used to change the motor rotation speed?