Crash Course on Power Electronics
For mastering the Power Electronics Construction Set component of OSE work - step one is an overview tutorial on power electronics. This means basic explanations of multi-purpose controllers and their connection to power-handling elements. This includes:
- How to control current output from any AC or DC input
- How to control voltage level coming out of any AC or DC input
- Managing conversion from AC to DC for various purposes.
Explanations should include wiring for stock components, such as PWM signal generators, and corresponding power handling elements.
The above should address full modularity, and scalability. By modularity - we mean plugging in additional units for added power, or added voltage.
The practical tools should also be explained, in terms of open source software for:
- Designing and modifying circuits
- Generating fabrication files
- Producing them/getting them produced
Thus, the above should cover conceptual and practical understanding of building, at all scales from watts to 500kW (the scale we deem sufficient on all counts for a full, resilient community):
- Charge controllers for various generators feeding a microgrid: Inverters & Grid Intertie
- Inverter welder power supplies
- Welder power supplies for the Open Source Welder
- Plasma cutter power supplies for the Plasma Cutter
- Induction furnace power supplies: Category:Induction Furnace
- DC, AC, and stepper motor controllers: CNC and Electric Motor Controls
- Battery chargers
The point is to start with conceptual understanding of an integrated framework for handling applied power electronics, and then moving to particular applications. If presented conceptually, in a modular, open source design fashion - with modules for functionality and for scalability - I don't see why a non-expert like myself cannot pick up on the material to be very proficient in creating/building real-world applications.
Specifically, here are some examples of the needs of OSE:
- a low-cost, high-power windmill cannot be designed properly without mastery of power electronics for handling power that fluctuates widely.
- Mastery of power electronics is essential for building a robust charge controller for a steam engine feeding a microgrid or a battery bank - especially if the steam engine is powered via fluctuating solar concentrator power
The end point of this is a Crash Course on Power Electronics, in the general framework of our approach to creating resilient communities.
- 1 Common Types of Power Circuits
Common Types of Power Circuits
Power converters are the most common type of power electronic circuits. Converters take electrical power in one form and reshape it into another. The following table shows the different conversions and the common circuits used.
|AC to AC
|Transformers are used when the output frequency is the same as the input frequency. If the frequency needs to be changed, then a Rectifier-Inverter cascade needs to be used.
|AC to DC
|A rectifier alone will produce an unregulated DC voltage. Rectifiers are often followed by a DC to DC converter or other voltage regulator.
|DC to DC
|Switching Mode Converter
|There are many ways of converting DC voltage, but Switching power supplies tend to be the most versatile and efficient.
|DC to AC
Efficiency is a key consideration with all converters. Power lost to inefficiency results in heat. This means that more expensive parts may be needed, more parts added for cooling systems, and an overall higher stress on the whole unit. Converter circuits will have a sweet spot where they are most efficient, and then efficiency will roll off as the operating point moves away from that spot. This can make it difficult to make a single unit that is capable of a wide range of operations.
Transformers work with AC power. They have two (or more for multi-phase AC) coils that are inductively coupled. Electricity flowing through one of them with produce an electricity flow through the other. By using a different number of turns in the coils, the voltage can be stepped up or down. Transformers are pretty simple to operate, requiring no external power or controller, and can be quite robust. However they tend to use a lot metal, making them large and heavy.
The most useful rectifier is the diode bridge. It uses four diodes to convert AC power, which is negative half of the time, into all positive power. The voltage will still vary, however, so it becomes most useful when a capacitor is added to the DC output to smooth the ripples. The maximum DC voltage that a diode bridge will have is the peak AC voltage minus twice the voltage drop of the diodes.
Switching Mode Converters
Switching mode converters use Pulse Width Modulation to controller a transistor. This means that the transistor is either fully on, or completely off. Both of these state ideally result in no power loss in the transistor. Two of the basic examples of this type of converter are the Buck and the Boost. The Buck converter can convert DC voltages to a lower voltage, while the Boost can convert to higher voltages. Different configurations of these circuits can be made that combine these two types or adjust them to produce a large range of DC outputs, including outputs that are negative from the inputs.
Inverters take DC voltage and create a square or sine wave. They often include a transformer to bring the voltage to the desired output voltage.
Controllers adjust the voltage, current, or total power going to a load.
Pulse Width Modulation (PWM)
Pulse Width Modulation is a method of controlling current to a load by using short bursts of full power of varying duty cycles. The advantage of PWM is that the controller operates in a fully switch mode, giving very good efficiency, and the load will receive full power during the 'on' portions. PWM can be used in all kinds of applications from controlling the brightness of an LED, to adjusting the speed of an electric motor.
Constant Current / Constant Voltage
Constant Current and Constant Voltage supplies will try to keep the current going through a load, or the voltage across a load, constant even if load changes.
There are many specialized devices that perform a combination of functions and monitoring.
Battery Chargers and Monitors
Different types of batteries have generally have different charging profiles so battery chargers need to be designed with a particular battery technology in mind. Voltage, current, temperature of the battery, and time charging are all factors that need to be taken into account.
There are many types of motors, so again, there are many types of controllers. Some examples of other factors that the controllers may get additional feedback on are RPMs, incremental position, absolute position, or force. See Electric Motor Controls for details.