Green Collar Careers AC - DC
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.
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:
- Line monitors Commercial AC voltage frequency and compares that
with a built in clock.
- Converts AC power to DC - only for AC windmills.
- 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.
- Pulsed power is fed into a transformer to convert the voltage to
the proper level.
- Steady DC voltage is maintained at the output of the transformer
using a large capacitor.
- 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
- 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.