Electrical Power Converters & Machine Operation
Power Converters: Regulation & Control
Introduction to Power Converters
Converter: An element that changes the form of electrical power, alternating between direct current (DC) and alternating current (AC), or between different frequencies of alternating current. They are formed by devices made from semiconductor materials and operate essentially as switches, allowing or blocking the passage of current under specific conditions.
Figure 1: Symbols for various converters: AC-DC (rectifier), DC-AC (inverter), and AC-AC converters.
Rectifiers: An element or circuit that converts alternating current (AC) to direct current (DC).
Controlled Rectifiers: These devices allow the average value of current and voltage supplied to the load to be regulated.
Inverter: An element or circuit that converts direct current (DC) into alternating current (AC).
Voltage Regulators: To regulate the voltage applied to a load from a fixed voltage source, voltage dividers can be used (for both DC and AC sources). Auto-adjustable regulators are also an option, but only for AC sources.
Automatic Control of Processes: In all construction and manufacturing processes, the following functions can be identified:
- Definition of the process’s objective function.
- Data collection from the process.
- Data processing.
- Action on the elements of the process.
A process is controlled automatically when all these functions, instead of being performed manually, are implemented within a system that can operate without human intervention.
Operating Characteristics of Electrical Devices
Understanding Device Operation
The normal operation of these devices is determined by the behavior of their dielectric insulation and the thermal characteristics of their windings.
The operation of electrical machines involves transient states during startup. These include:
- Electromagnetic Transient: Occurs until the currents in the coils and the voltages in the capacitors reach their steady-state values.
- Thermal Transient: Occurs until the temperature in all parts of the machinery achieves its steady-state value.
- Mechanical Transient: (Applicable to generators or motors) Occurs until the steady-state speed is reached.
Once in a steady state, the electrical characteristics (such as voltage, current, and power ratios) and, in rotating machines, the mechanical characteristics (relationship between torque and speed) determine the most appropriate application for each machine. For elements like lines, transformers, or any electrical energy converter, only electrical parameters and quantities (voltage, current, frequency, power, resistance, and reactance of internal circuitry) are involved. In rotating machines, speed must also be considered.
Class of Service and Overload Conditions
Class of Service: The duty cycle for which a machine is designed, ensuring it operates within its thermal limits.
The power rating is assigned by the manufacturer to the machine. It ensures that, for the specified duty cycle, the machine does not exceed its maximum allowable temperature. Rated power usually refers to baseline environmental conditions: 40 °C and altitudes below 1000m.
Overload: Electrical machines can operate for a period above their rated power (e.g., for S1 service) without exceeding the maximum allowable heating.