Learn
About Inverters
An inverter is an electronic device
which inverts DC energy (The type of energy found in
batteries) into AC energy (The type of energy that is produced
by the power company and is found in your home) Household
appliances such as refrigerators, TVs, lighting, stereos,
computer etc., all run off of AC electricity.
Modern DC to AC inverters are very
reliable, quiet, and require virtually no maintenance. There
are two different types of DC to AC inverters in common use
today. The first type of inverter is known as a modified
sine wave
inverter. This type of inverter is very high in efficiency
and produces a waveform which is an approximation of the pure
sine wave waveform that is produced by the utility
company.
Modern modified sine wave inverters
come in two varieties.
1. High frequency conversion
units which is typical of the design that you will find in
inverters that are manufactured overseas in countries like
Taiwan and China.
High frequency units take the incoming
12 Volts DC and will step up that voltage to approximately 200
volts DC through a high frequency DC to DC converter circuit
and then will take the 200 Volts and will wave shape it into a
modified sine wave using a using a device called a high
voltage H-bridge. The high voltage H-bridge is basically a
group of field effect transistors that are arranged in such a
way as to form the necessary half cycles that create the
modified sine wave at the 60 Hz frequency required for US
appliances.
By utilizing high frequency, the need
for a large iron core output transformer is eliminated and
much smaller transformers can be used. As a result of this,
high frequency inverters tend to be much lighter but do have a
lower surge capacity because they lack the fly wheel effect
found in heavy iron core output transformer based
inverters.
The technology was originally
developed for the space program where weight is a major
consideration. This type of inverter tends to be less
expensive to manufacture and are considered a lighter duty
type of product than their US made low frequency
counterparts.
2. Low frequency or 60 Hz based
conversion units that are typical of US designed inverters.
As mentioned above these inverters use
large heavy iron core transformers that tend to provide a
flywheel effect which yields a much greater surge capacity
than do high frequency inverters.
Low frequency units take the incoming
12 Volts DC (direct current) and converts it into AC
(alternating current), using a multi-vibrator or
microprocessor based circuit. This 12 volts 60 Hertz AC is fed
to an
iron core step up transformer which converts the 12 volts AC
into 120 volts AC .
Wave shaping and the increased current
that is needed to drive the transformer is performed again by
an H-bridge which is a group of field effect transistors that
are arranged in such a way as to feed high current pulses to
the primary windings of the transformer at precise moments of
each wave form half cycle.
The transformer converts the lower
voltage which was fed to its primary windings into 120 Volts
AC at its secondary windings using simple transformer step up
principles involving a 10 to 1 ratio, converting 12 Volts AC
to 120 AC. This type of inverter is considered more durable
than their high frequency counterparts and have a much higher
surge capacity.
You might be asking why anyone would
choose a high frequency inverter when low frequency units are
more durable ? The answer is cost.
Low frequency units tend to cost two
to five times more than do high frequency units, another
consideration is of course the difference in weight. For
example a typical 2000 watt high frequency based inverter may
weigh 13 Lbs, where a typical low frequency inverter can weigh
as much as 50 Lbs.
A modified sine wave looks more like a
square wave that has been time shifted in order to produce the
energy content and waveform which is found in power produced by the utility
company. It's frequency and voltage is rock solid which
prevents the brownouts and spikes typically seen with utility
company power. Most appliances such as TVs, Lighting, stereos,
computers, inkjet printers and power tools run find on
modified sine wave power.
About the only appliances that you may
see a problem with are some Laser printers and some of the
cheaper types of battery powered tool chargers. In fact 95% of
the inverters in RV's today are of the modified sine wave type.
The second type of inverter is known
as a pure sine wave inverter and you guessed it, it
produces power that is exactly like the power which is
produced by the utility company without the spikes and
brownouts of course. This type of inverter produces pure sine
waves much the same way as the two different technologies
mentioned above, but at the cost of some efficiency loss and at a
much higher price. In fact most pure sine wave inverters are
typically priced at least 75% higher than their modified sine
wave counterparts and in some cases do not have as high of
a surge capability as do modified sine wave units. See
the illustration below to visualize the difference between a
sine wave and a modified sine wave
Inverter/chargers
Sine wave and modified sine
wave inverters are available as inverter units only, or may
have additional circuits added that allows them charge
batteries when an external AC source is fed into the inverter.
This type of configuration is know as an inverter/charger. In
addition to the charger circuit, these units will typically
include a device known as an AC transfer switch.
These added devices allows the
inverter to charge your batteries from a
generator or utility AC power source and passes the AC power through the inverter
to your loads, allowing you to operate your appliances while the batteries
are being charged. As soon as the external AC power is turned
off or is removed from the inverter, the inverter/charger
automatically returns to functioning as an inverter, outputting
its own AC power.
HOT TIP ! Never
wire a generator or utility company's power directly to the AC
output of any inverter. Doing so will result in damage to
your inverter and or your generator. Always wire the generator
or utility company's power output to the inverters AC input though
an external AC transfer switch if your inverter includes that
option.
An example of this is
a customer who brought his inverter in to us for repair. He
explained that he did not want to add an external transfer
switch to his inverter nor did he want to wire up a separate
sub panel. What he did was wire two switches to his main
electrical panel, one which connected the utility power and
one that connected his inverter. In the event of a power
failure, he would throw the switch that would disconnect his
utility power and then would throw the switch that would
connect his inverter. He would then reverse the procedure when
the utility power came back on. Well needless to say, he got
confused and threw the swithes in the wrong sequence an bang
went his inverter ! The lesson here is unless your inverter is
specifically designed to function as a backup unit and has its
own transfer switch, never mix utility or generator power with
an inverter !
Backup Power
Another advantage that an
inverter/charger with transfer switch offers is that it can
function as a highly reliable automatic power backup unit or
UPS (uninterruptible Power Supply).
When the utility company is operating
normally the inverter/charger passes the utility company power
through its internal transfer switch to your appliances and
maintains a charge on your battery bank. As soon as the
utility power fails, the inverter automatically stops charging
the battery bank and begins producing its own AC power which
is passed on to your appliances through its internal AC
transfer switch.
When the utility power returns the
inverter goes back to charging the batteries and again passes
the utility power though the transfer switch to your
appliances. Most inverter/chargers switch from utility power
to inverter power and back again so fast that most of your
appliances will hardly miss a beat.
HOT TIP !
Unless your inverter is specifically designed to do so and
very few are, Never ever attempt to wire two
inverters in parallel to double your current or in series to
double your voltage. Doing so will instantly result in the
immediate destruction of both inverters. If you need more
power, buy a bigger inverter to begin with. If you need 240
volts, buy a step up transformer such as the Xantrex/Trace
T-240 which allows you to feed 120 in and get 240 out.
Grid Intertie Inverters
A grid intertie or grid interactive
inverter is a pure sine wave inverter that has had additional
circuits added that allows it to synchronize with and feed
power back to the grid or utility company. This type of
inverter may operate in conjunction with batteries which would
provide backup power as described above or may function
without batteries allowing power to be fed directly from solar
modules or a wind generator.
Many states are now allowing utility
companies to purchase power from such systems allowing
customers to receive credits for the excess power that they
produce. Grid intertie inverters are specially designed units
that have the capability of interacting with the grid. Never
connect the output of a non grid intertie or modified
sine wave inverter to the grid, unless you enjoy seeing a lot
of smoke and an expensive repair bill. Believe me we've
had customers that have tried it !
Inverter brands that are recommended
for multiple use: You can never go wrong with the Xantrex line of
products which include Xantrex/Trace Engineering, Xantrex/Heart Interface and
Xantrex/Statpower branded products. Xantrex/Trace Engineering,
Xantrex/Statpower, OutBack Power and Exeltech all
manufacture excellent sine wave products and Xantrex/Trace
Engineering and Xantrex/Heart Interface both manufacture the preferred
line of modified sine wave products. If you're in the market
for less expensive product, Cobra, Vector and Coleman make
excellent high frequency based inverters.
HOT TIP ! Stick
with name brand inverters. We can't tell you how many times,
we've been approached by surplus dealers who try to dump their
junk inverters which came from overseas manufacturers who have
gone out of business. When it comes to inverters, you get what
you pay for, and when it comes to off brand cheap inverters,
many times you'll be stuck with what you paid for regardless
of a manufacturers warranty because the overseas manufacturers
that built them simply go out of business, change their name
and start all over again, relieving them from any
responsibility to honor your warranty.
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