Abstract
DC motors are used in rotor excitation of synchronous motor and also to provide mechanical work in industries and even homes. As transients constitute major source of energy loss in an electrical circuit which DC motor is a component part, there is a need to study its responses in DC machines. The aim of this paper, therefore, is to analyse the DC machine dynamic responses when certain variables are varied. This work involves producing Simulink block diagram of a DC separately excited motor from where the system responses are generated. And it involves studying the output current and speed transient duration at each parameter variation using the block diagram. This is done under three scenarios; varying the load torque, varying the armature resistance at different values of load torque and varying the moment of inertia at different values of load torque. From the system responses, it is observed that when the load torque is varied, there is no significant difference in transient duration of output current and speed at the different load torques. But it can be seen that the output current at steady state is about 2A at zero load torque, 18A at load torque of 20Nm and 38A at load torque of 50Nm. The speed at steady state is observed to be 155rad/s at zero load torque, 135rad/s at load torque of 20Nm and 105rad/s at load torque of 50Nm. When armature resistance is varied at 20 percent, the output current attains steady state at about 0.05 second while at 50 percent, steady state is attained at about 0.23 second. Also, when armature resistance is varied at 20 percent, the speed attains steady state at about 0.05 second while at 50 percent, steady state is attained at about 0.15 second. More transient oscillation is observed both in the output current and speed when the armature resistance is varied at 20Nm than when it was varied at 50Nm. In the third scenario, when the moment of inertia is zero, the output current was at steady state right from start. But at zero load torque, it achieved steady state at 0.07 sec when the moment of inertia is varied at 20 percent while when the moment of inertia is varied at 50 percent, it achieved average steady state at about 0.24 sec. At 20Nm, it achieved steady state at 0.07 sec at 20 percent and 0.23 sec 50 percent. Then at 50Nm, it achieved steady state at 0.07 sec at 20 percent and 0.22 sec at 50 percent. For speed, at zero load torque, the motor experienced steady state at 0.05 sec when the moment of inertia is varied at 20 percent and 0.23 sec when the moment of inertia is varied at 50 percent. For load torque of 20Nm, it experience steady state at 0.05 sec when the moment of inertia is varied at 20 percent and 0.23 sec when the moment of inertia is varied at 50 percent. And for load torque of 50Nm, it experience steady state at about 0.04 sec when the moment of inertia is varied at 20 percent and 0.19 sec when the moment of inertia is varied at 50 percent. From the analysis, it is observed that for a DC machine to work at a shorter transient time, (a) the armature resistance can be increased considerably at about 50 percent, (b) the moment of inertia can be reduced considerably to about 0.02kg.m2, and (c) the load torque can be reduced considerably to about 20Nm. This study can help in understanding the effect each parameter has on transient duration of the DC motor.
Keywords: transient response, MATLAB/SIMULINK, separately excited DC motor, armature resistance, load torque, moment of inertia.

Key words: dynamic response, MATLAB/SIMULINK, separately excited DC motor, armature resistance, load torque, moment of inertia.