Question

Do the experimental results consistent with the expected results for dc motor? Discuss any discrepancies and explain possible causes. Suggest or recommend how the error can be reduced

Answer 1

The experimental results are not consistent with the expected results in the case of a DC-Motor. Ideally, we expect a linear relationship between armature current and the output speed (rotational) of the DC-Motor without any distortion, oscillation about the setpoint, or any steady-state error. But all the previously stated errors are observed in the case of Practical DC-Motor output response. A typical response curve of a DC-Motor is shown in the figure below:

This is because of many reasons like inertia, switching noise, Frictional losses, core-loss, Back EMF, and the other non-ideal dynamics of the DC-Motor. After attaining certain rotational speed, due to inertia, it faces further increment in speed. Due to insufficient input, it tries to come down to a lower speed and again it deteriorates down from its desired speed. This process continues until the steady-state is reached.

Due to all core-loss, Back EMF, and Frictional losses, the steady-state output of the DC-Motor is reduced further down to a certain value according to the amount of all the losses.

For less oscillation, we suggest a closed-loop control system using a PID controller, that eliminates steady-state error, with less rise time, settling time, overshoot, and less oscillating nature. For the demonstration purpose for better understanding, a simulation of an arbitrary DC motor model is done in open-loop and closed-loop with the PID controller to describe the difference in error practically.

Note: The subsystem block is the DC-Motor Model and all the parameters of the DC motor are taken arbitrarily for the demonstration purpose only. The setpoint is a step input of 150 RPS

You can clearly see that the armature current for both the case is the same but the error for that specified armature current, the error in the Speed (RPS) is very much less (almost negligible) in case of Closed-Loop control with tuned PID controller.