Theoretical calculations
The characteristics of the d.c. machine you will be investigating are given on the last page of this part. Use 200V DC supply for both armature and field windings. Follow the instructions carefully as you must use the parameters given.
Refer to the lecture notes covering this topic. Use these to predict the shaft speed in revolutions per minute (r.p.m.) over a range of mechanical loads. The range depends on the characteristics of your machine but should include torque values from zero to the torque that will reduce the speed of your machine to approximately 80% of its no-load speed. Calculate sufficient values to enable you to produce a torque/speed graph. Present the results in table form and construct a graph as Shaft Speed (y-axis) verses Load Torque (x-axis. Comment on your results.
Simulation
Start MATLAB then Simulink. Draw the test circuit for the DC machine as shown below. Refer to Electrical Systems Laboratory 4 for guidance on the use of MATLAB/Simulink and the SimPower (Power Systems) Blockset.
The parameters of the DC machine are given on the last page of this document.
In some versions of Simulink, it is necessary to add a powergui block. This can be found in the SimPower blockset (Power System Blockset) folder.
Investigate the Simulink DC Machine model by clicking 'Help' from its parameters window. Note the four outputs at 'm'.
Determine the speed/torque characteristics of the machine by simulation. The range of torque applied should be the same as the range of load torques you used in the theoretical calculations. Simulate for a time that enables the shaft speed to reach a stable value. Draw the simulated Speed/Torque characteristics on the same graph as the calculated Speed/Torque characteristics and compare them. Comment on the results.
Efficiency
For each of the simulated points on the torque/speed graph of section 2, determine the mechanical output power, the electrical input power to the armature and the efficiency of the machine. Draw a graph of efficiency against mechanical load. Comment on your results.
Armature speed control using variable field resistance
Use the DC Machine Test Circuit of Figure 1. Select a fixed load that causes a rotational speed of around 7.5% less than the unloaded speed. Insert a resistor in series with the field winding and simulate several times with a range of different resistor values. Plot a graph of speed against resistance value. Repeat the above with a fixed load of twice the value. Comment on your results.
Armature speed control using variable armature voltage
Use the DC Machine Test Circuit of Figure 1. Select a fixed load that causes a rotational speed of around 7.5% less than the unloaded speed. Vary the armature voltage from 0 to 250V and plot a graph of speed against armature voltage.
Repeat the above with a fixed load of twice the value.
Comment on your results.