Introduction
The static or power converter is the electrical actuator of the drive. Four-quadrant converters are used in the industry as actuators for DC motors. By means of pulsing, these converters allow almost lossless control of connected motors. Schematically, a 4-quadrant converter can be compared to the four states of two switches turned on and off at specific times.
A four-quadrant converter operates in four quadrants, i.e. it permits reversal of the voltage and current, so that a connected DC motor can rotate in both directions. Braking of the motor is also possible as a result. Four IGBTS groups arranged in a bridge circuit enable multi- quadrant operation of the DC chopper, including simultaneous current and voltage reversal. Use of a resistive/inductive load allows generation of positive and negative voltages. Energy recovery is only possible in the presence of a feed source in the load circuit.
The ratio between the period and turn-on time is referred to as duty cycle. This cycle can therefore assume values from zero to one, and also be expressed as a corresponding percentage. A typical feature here is the rising and falling exponential function which characterizes the currents at mixed loads. At a sufficiently high degree of smoothing or high pulse frequencies, the currents assume triangular waveforms. A quadruple IGBT circuit allows voltage and current reversal, i.e. energy flow in both directions. The IGBTs are controlled by either simultaneous pulsing or alternate pulsing.
- Determine the current and voltage characteristics of a 1-quadrant converter, given a duty cycle of +50% using resistive load and calculate the average voltage and current.
- Determine the current and voltage characteristics of a 4-quadrant converter, given a duty cycle of +50% using resistive/inductive load and calculate the average voltage and current.
- Analyze the time responses of the output voltage and current in (1) and (2) above and state which quadrant will the controller operate when connected to DC motor.
- Investigate the influence of clock frequency on the current and voltage of the four- quadrant converter.
- Discuss your findings in (1 & 2) in relation to the armature voltage, motor speed and phase angle setting
- Determine the voltage current characteristic during amplitude modulation under varying clock frequency and amplitude.
- Using experimental set-up like that in fig. 2, examine output voltages and currents in the case of block commutation, sine modulation and space-vector modulation.
Fig.2 Four-quadrant 3-phase converter
Assessment
This assessment counts for 50% of the total module marks.
Focussing on the tasks highlighted, report the results in the form of a technical report along with an appropriate literature review, technical analysis of simulation results and their discussion.
Marks will be awarded based on the following scheme.
The report should describe the drives and compare the expected results or system response to the theoritical outcomes. In doing so a suitable understanding of how an electrical drive operates can be demonstrated.
Marks will be awarded based on:
- Structure: Clearly structured report with appropriate use of tables, graphs, diagrams and subsections. [10 marks]
- Literature Review: Effective review of literature resources such as textbooks and journal articles to build the foundation for the experiments presented in the report. In- text citation and referencing using appropriate referencing style. [20 marks]
- Simulation Results and their Technical Analysis: Appropriate summarisation of results from simulation organised into tables and figures without duplication of data. Identification and application of appropriate metrics for comparison/evaluation of results. [40 marks]
- Discussion/Conclusions: Clear discussion of findings with logical conclusions based on evidence and a competent critical analysis. Explanation of discrepancies or suggestions for improvement. [20 marks]
- Presentation: Excellent layout, conforms to all technical specifications with clear expression of ideas and appropriately presented [10 marks]
Answers to Above Questions on Power Electronics