The effects of varying the frequency of a switched mode power supply controlled DC motor

In industry DC machines are still used today due to their torque-speed characteristics [1]. The DC motors speed characteristics can be further enhanced by the addition of external electronic controls, such as buck converters. [2]. However, the addition of these external electronic controls to the DC motor also adds a complexity to these type of systems and problems can also arise from these buck converter/DC motor configurations [2]. This thesis examines the complexities that arise from these type of systems, in particular how these complexities effect that the speed-torque characteristics of a separately excited DC Motor may have during Discontinuous Conduction Mode (DCM) and Continuous Conduction Mode (CCM). To demonstrate the affects that Discontinuous Conduction Mode and Continuous Conduction Mode has on the speed-torque characteristics of a DC motor, a setup was designed and constructed with the following design objectives: • Convert AC power from mains to regulated DC power, using a power supply that must: o Supply power to a DC motor o Operate with a variable switching frequency from a few Hz – 2 kHz o Operate with a variable voltage from 0 – 240 V o And have a maximum current output of 1.5 A • The setup must be able to measure the speed, load torque, the armature current and voltage waveforms must also be observable and produce speed-torque curves of the DC motor Once completed, the setup was used to examine the speed-torque characteristics of the DC motor setup. The setup was able to meet all the design objectives. The results from the setup demonstrated a nonlinear region during DCM operation and a linear region CCM operation, in speed-torque curves of the setup. The main observations from the setup showed that the torque-speed curves experienced a larger nonlinear region at lower switching frequencies when compared to the torque-speed curves of systems that were operating at higher frequencies. Additionally, it was found that at higher voltages, the speed-torque curves experienced a smaller linear region when compared with the speed-torque curves of systems at lower voltages. Further analysis revealed the speed characteristic of the system was mainly caused by dramatic voltage drops over the armature of the motor, and that the dramatic voltage drops occurred during DCM, when load was applied to the motor. A simulation of the final setup was produced, although the simulation model could benefit from further improvements and modifications. The model was able to demonstrate some general trend that was observed in the results section. The setup was also adapted for future use as teaching aid that was aimed at 3rd year engineering students at Murdoch University. The purpose of the setup was to contribute to the student’s basic understandings of motor operation during DCM and CCM. A student laboratory exercise was successfully completed as part of project objectives. The project setup can be replicated in a laboratory environment and used to demonstrate the effects that DCM and CCM, have speed-torque characteristics of a DC Motor Overall, this project was considered to be successful, as all the objectives were completed during the course of the thesis project.