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Green Collar Careers AC - DC Electronics

When people think of solar or wind power the image that comes to mind is a solar panel or a windmill.  Electricity generated by those systems isn't quite ready to connect to your electrical panel needing first to be converted to "commercial grade AC" power.  Conversion is done by an inverter, usually a drab gray little box that few ever see or pay attention to.

Solar cells and most windmills produce a Direct Current (DC) output, and usually at a voltage different then needed.  Some windmills may produce an Alternating Current (AC) output but usually the wrong voltage and the frequency of alternation is determined by the speed of the turbine blade.

To understand a little more about the inverter and how it works, some basics about electricity are all you need to know.  In Module 201 we reviewed Volts, Amperes and Watts the units of electrical power measurement.  DC power is fairly straight forward, one wire is a steady negative voltage, where the other wire is a positive voltage.   Connect something like a battery to a speaker and all you'll hear is one click when its connected, and another click when its disconnected.

Alternating Current is different.  Commercial AC oscillates at 60 Hertz (Hz) or 60 times a second.  Connect a 60 Hz signal to a speaker and it makes a low frequency sound equal to that of the "A" string of a bass guitar.  Shown on an oscilloscope commercial AC oscillates almost identical in shape to the A-string in a waveform known as a sine wave.

Now imagine you have two guitars with perfectly tuned strings - pluck them both at exactly the same time and the sine waves "resonate".  Pluck them just off a little bit from each other and they have a slight muting effect.

Commercial AC is like those guitar strings in the shape of the electricity - add a second source of electricity and the waves have to be synchronized.  Connecting your home to the power grid requires use of an inverter system that the electrical company will first want to qualify, otherwise its possible that a loss of synchronization could affect the operation of your appliances - or even your neighbor's appliances.

The inverter will take "dirty power" and tidy it up into a proper voltage and waveform that will synchronize to the commercial AC power.  It does this in the following order:

1. Line monitors Commercial AC voltage frequency and compares that with a built in clock.
2. Converts AC power to DC - only for AC windmills.
3. Creates a pulsed DC output with semiconductor switching transistors.  The frequency is synchronized to the commercial AC when present, otherwise the clock sets the 60Hz timer.
4. Pulsed power is fed into a transformer to convert the voltage to the proper level.
5. Steady DC voltage is maintained at the output of the transformer using a large capacitor.
6. A series of "stepped switching" transistors build a sine wave;  the shape is more like a series of 8 steps up and 8 steps down, but that's good enough.

Inverter specifications requires only the basic electrical understanding:

• Input Voltage
• Input Amperage
• AC or DC Input
• Output Voltage (240VAC in America)
• Output Watt
• Cycles per Second (Hz)
• Grid compliant - usually requires a UL1741 rating and IEEE 1547 compliance to be acceptable to most utilities

And you thought this would be a hard lesson.