Every DC Machine block in Simulink comes with several preset models. Each of these models specify certain values for rated armature voltage, output power, rotational velocity and field voltage. Consider the image below:
This configuration suggests a DC Motor with output power of 5 HP and rotational velocity of 1750 rpm. It also considers armature voltage of 240 V and field voltage of 300 V. From these rated values we can calculate the rated load torque using the following script:
This is the full load torque. When we say that 20% of full load is available, this means the external load torque is 20% of the the calculated rated load torque T_L. We can introduce this torque to the Simulink model through a step block as illustrated below:
Here we can change the final value of the step function to the 20% of load torque and then run the simulation.
The attached Simulink(.slx) file might be helpful to understand the working of the DC shunt motor.
Hope this helps!