Two varieties of modified sine wave
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.
