subject: Understanding & Using Solar DC-AC Inverters – Part 4 [print this page] Understanding & Using Solar DC-AC Inverters Part 4
Sinewave inverters
As explained earlier, most DC-AC inverters deliver a .modified sinewave. output
voltage, because they convert the incoming DC into AC by using MOSFET
transistors as electronic switches.
This gives very high conversion efficiency, but the .alternating pulses. output waveform is also relatively rich in harmonics.
Some appliances are less than happy with such a supply waveform, however .
examples include light dimmers, variable speed drills, sewing machine speed
controls and some laser printers. Because of this, inverter manufacturers do
make a small number of models which are designed to deliver a pure sinewave
output.
Generally speaking these inverters use rather more complex circuitry than
the .modified sinewave. type, because it.s hard to produce a pure sinewave
output while still converting the energy into AC efficiently. As a result pure
sinewave inverters tend to be significantly more expensive, for the same output
power rating.
The most common type of pure sinewave inverter operates by first converting the
low voltage DC into high voltage DC, using a high frequency DC-DC converter. It
then uses a high frequency PWM system to convert the high voltage DC into
.chopped.
AC, which is passed through an L-C lowpass filter to produce the final
clean 50Hz sinewave output. This is like a high-voltage version of the single-bit
digital to analog conversion process used in many CD players.
Don.t reverse polarity!
A lot of electronic equipment designed to be operated from a battery is fitted
with an internal diode in series with one battery lead, to protect the equipment
from damage if the connections to the battery are accidentally reversed.
But this type of reverse polarity is generally not fitted to DC-AC inverters, because of the heavy current drain involved. The additional voltage drop introduced by a diode
would degrade the inverter.s regulation too much, quite apart from wasting power and hence reducing the overall efficiency.
If reverse polarity protection is provided, this is usually in the form of a
protective fuse or circuit breaker in series with the battery leads, plus a reversedpolarity power diode connected across the inverter.s DC input (after the fuse: This means that when the battery leads are connected with the correct
polarity, the diode is reverse biased and remains dormant. But if the battery
connections are accidentally reversed, the diode is forward biased and suddenly
conducts . blowing the fuse and hopefully protecting the inverter itself.
This system generally does provide protection against reversed-polarity damage
to most of the inverter circuitry, without degrading its efficiency or output
regulation. However the price you pay is that an accidental reversal of the
battery connections still blows a fairly expensive high-current fuse, and
sometimes also destroys the protective shunt diode as well.
Probably for this reason, some lower priced inverters don't incorporate any
specific protection against accidental reversal of the battery leads. So with these
inverters, you have to be especially careful to connect the battery leads correctly.
Even with inverters which are provided with protection, it's still a very good idea
to double-check the connections before clipping on the second battery lead.
Remember that a mistake will almost certainly mean a blown fuse at least, and
possibly a long delay until you can replace it!
Understanding & Using Solar DC-AC Inverters Part 4
