Equipment and Materials

 EXPERIMENT 1 

Safety Precautions:

a) Wear appropriate attire: slippers, sandals, and open-toe footwear are not allowed. Ensure your hands are dry.

General instructions

a) Please read the lab instructions and gather the necessary information for completing the simulation before attending the lab.

b) Wear appropriate attire: slippers, sandals, and open-toe footwear are not allowed.

c) Attend the lab only on your assigned day; switching days is not permitted.

d) On-the-spot evaluations will be conducted during or at the end of the lab session.

e) Your lab marks will reflect your performance, teamwork (if applicable), and learning attitude.

f) Submit your lab report by the deadline to avoid penalties for late submission

Design of DC motor speed control techniques using MATLAB/SIMULINK

Aim

This experiment aims to design, simulate, and compare various speed control techniques for a DC motor.

Objectives

The main objectives are:

a) To design and simulate various speed control techniques for a DC motor, including the three most common methods: field resistance control, armature voltage control, and armature resistance control. This also involves using power electronic converters (half converter, semi-converter, and full converter) for thyristorized control of the voltages.

b) To perform. a comparative analysis of the different speed control techniques.

1. Introduction to Speed Control of DC Motor

DC motors are widely utilized in industry due to their straightforward control structure and ability to operate across a wide range of speeds and torques. They offer high starting torque, making them suitable for traction applications. Speed control for DC motors is achieved by adjusting the field flux, armature resistance, or terminal voltage applied to the armature circuit. The three primary methods for speed control are field resistance control, armature voltage control, and armature resistance control. This section presents Simulink models for these methods, along with a feedback control method, for dynamic analysis of DC motor drives.

Field Resistance Control Method (Fig. 1):

This method involves inserting a series resistance into the shunt-field circuit of the motor to regulate the field current and adjust the motor's flux. Increasing the field resistance is expected to increase the motor's no-load speed and modify the torque-speed curve accordingly.

Armature Voltage Control Method:

In this method, the voltage applied to the armature circuit (Va) is adjusted independently of the motor's field circuit voltage. This approach requires the motor to be separately excited. Increasing the armature voltage results in a higher no-load speed of the motor, while the slope of the torque-speed curve remains unchanged due to the constant flux level.

Armature Resistance Control Method (Fig.2): 

In this method, the resistance in the armature circuit is varied to control the motor's speed. By adjusting the armature resistance, the voltage drops, and consequently the speed of the motor can be controlled. While armature resistance control is effective for basic speed regulation in DC motors, it has limitations in terms of efficiency, speed range, and complexity of control compared to more advanced methods like field flux or electronic speed control techniques.

Fig:1 Fig.2

2. Mathematical Modelling of a Separately Excited DC Motor

Understanding the torque-speed characteristics of a separately excited DC motor requires establishing both dynamic and steady-state models. Figure 3 illustrates the schematic representation of the motor model, featuring the following parameters:

Terminal voltage (e_a)

Armature resistance (R_a) and inductance (L_a)

Field resistance (R_f) and inductance (L_f)

Generated back emf (e_b)

Electromagnetic torque (T_m).

Fig. 3: Equivalent circuit of separately excited DC motor

The torque is produced as a result of the interaction of field flux with current in armature conductors and is given by Eq. (1)

Here,  K_t is a constant depending on motor windings and geometry and f is the flux per pole due to the field winding.

The direction of the torque produced depends on the direction of the armature current. When the armature rotates, the flux linking the armature winding will vary with time and therefore according to Faraday’s law, an emf will be induced across the winding. This generated emf, known as the back emf, depends on the speed of rotation as well as on the flux produced by the field and given by Eq. (2)

By applying KVL at the input side of Fig. 3,  

In terms of torque and speed, the steady state equation will be given by Eq. (5)

Thus, from the above equations, speed can be controlled by varying three parameters, namely E_a, R_a, and f. The three methods of speed control are as follows:

i. Armature voltage controlled ( E_a).

ii. Armature resistance controlled ( R_a).

iii. Flux controlled (f).

Speed control using armature resistance by adding an external resistor R_ext is not used very widely because of the large energy losses due to the R_ext. Armature voltage control is normally used for speed up to rated speed (base speed). Flux control is used for speed beyond rated speed but at the same time the maximum torque capability of the motor is reduced since for a given maximum armature current, the flux is less than the rated value and so as the maximum torque produced is less than the maximum rated torque. Here the main attention is given to the armature voltage control method. In the armature voltage control method, the voltage applied across the armature e_a is varied keeping the field voltage constant. As equation (6) indicates, the torque-speed characteristic is represented by a straight line with a negative slope when the applied armature voltage is ideal, that ideal torque-speed characteristic is illustrated in Fig. 4.

Fig.4: Torque speed characteristics of separately excited DC motors at different armature voltages

3. Thyristor-based techniques of DC motor speed control

A separately excited DC motor fed through a single-phase half-wave converter is shown in Fig. 5. A single-phase half-wave converter feeding a DC motor offers only one quadrant drive. Such a type of drive is used up to about 0.5 kW rating of the DC motor.  

Fig. 5: Single phase half wave converter drive

For single phase half wave converter, the average output voltage of the converter can be calculated using Eq. (7)

A half-wave converter in the field circuit will increase the magnetic losses of the motor due to the high ripple content on the field excitation current, so an ideal DC source is preferred over a half-wave converter for the field circuit. A separately excited DC motor fed through a single-phase semi-converter is shown in Fig. 6. This converter also offers only one quadrant drive and is used up to 15 kW DC drives.

Fig.6: Single phase semi converter drive

With a single-phase semi-converter in the armature circuit, equation (8) gives the average armature voltage as,

The armature voltage varies by single-phase full-wave converter as shown in Fig.7. It is a two-quadrant drive and is limited to applications up to 15kW. The armature converter gives +V₀ or –V₀ and allows operation in the first and fourth quadrant. The converter in the field circuit could be semi, full or even dual converter. The reversal of the armature or field voltage allows operation in the second and third quadrant.

Fig.7: Single phase full converter drive

The average armature voltage in armature circuit for single phase full converter drive is given by Eq. (9)

To realize single phase dual converter, two single phase full converters are connected as shown in Fig. 8.

Fig.8: Single phase dual converter drive

In Fig. 8, there are two single phase full wave converters either converters 1 operates to supply a positive armature voltage V₀, or converter 2 operates to supply negative armature voltage –V₀. Converter 1 provides operation in first and fourth quadrants, and converter 2 provides operation in second and third quadrants. It is four quadrant drive and provides four modes of operation: forward powering, forward braking (regeneration), reverse powering, and reverse breaking (regeneration). The field converter could be a full wave converter, a semi converter, or a dual converter.

If converter 1 operates at a firing angle of α₁ then equation (10) gives the armature voltage as,

And similarly, if converter 2 operates at a firing angle of α₂ then equation (11) gives the armature voltage as,

Procedure

1. Select a DC shunt motor of appropriate rating and suitable parameters.

2. Tabulate the ratings and parameters of the motor chosen in step 1.

3. Develop SIMULINK models for the motor under consideration in step 1 for the following scenarios:

3.1 Field control of DC shunt motor

3.2 Armature voltage control of DC shunt motor

3.3 Armature resistance control of DC shunt motor

4. Field control of the DC shunt motor using simulation

4.1 Set the nominal value of the field resistance.

4.2 Perform. simulations across a wide range of load torques.

4.3 Record the steady-state speed at each load level and compile the data in a table.

4.4 Increase the field resistance and repeat the simulations at the same load levels.

4.5 Record the steady-state speed for each load level and compile the data in a table (Table 1).

4.6 Plot torque-speed curves for both resistance values and evaluate the impact of increased field resistance on torque-speed characteristics.

5. Armature voltage control of the DC shunt motor using simulation

5.1 Select three different values of armature voltage while maintaining the field circuit at its nominal voltage.

5.2 Conduct simulations across a wide range of load torques.

5.3 Record the steady-state speed at each load level and compile the data in a table (Table 2).

5.4 Plot torque-speed curves for all armature voltage values and analyze the effect on torque-speed characteristics.

6. Armature resistance control of the DC shunt motor using simulation

6.1 Select three different values of armature resistance while maintaining the field circuit at its nominal voltage.

6.2 Perform. simulations across a wide range of load torques.

6.3 Record the steady-state speed at each load level and compile the data in a table (Table 3).

6.4 Plot torque-speed curves for all armature resistance values and analyze the effect on torque-speed characteristics.

7. Thyristor-based control of the DC shunt motor using simulation (Open-ended section)

7.1 Apply the knowledge gained from this experiment to control the armature voltage of a separately excited DC motor using an appropriate converter drive system.

7.2 Perform. simulations across a wide range of firing angles for the rated load condition.

7.3 Record the steady-state voltage and speed at each firing angle and compile the data in a table (Table 4).

7.4 Plot the voltage, and speed curves for all firing angle values and analyze the impact of the firing angle on the motor's performance.

8. Discussion

8.1 Examine how field resistance influences the torque-speed characteristics.

8.2 Analyze the impact of armature resistance on the torque-speed characteristics.

8.3 Evaluate the effect of armature voltage on the torque-speed characteristics.

8.4 Investigate how the firing angle affects the voltage and speed characteristics.

8.5 Compare the various speed control techniques explored in this practical session.

8.6 Summarise what you have learned from this simulation assessment.

8.7 Identify and discuss the problems encountered during the experiment.

9. Conclusion

10. References