Understanding & Using Solar DC-AC Inverters – Part 3
Understanding & Using Solar DC-AC Inverters Part 3
Capacitive loading
Actually there.s a different kind of problem with many kinds of fluorescent light
assembly: not so much inductive loading, but capacitive loading.
Although a standard fluoro light assembly represents a very inductive load due
to its ballast choke, most are designed to be operated from standard AC mains
power. As a result they.re often provided with a shunt capacitor designed to
correct their power factor when they.re connected to the mains and driven with a
50Hz sinewave.
The problem is that when these lights are connected to a DC-AC inverter with its
.modified sinewave. output, rich in harmonics, the shunt capacitor doesn.t just
.correct. the power factor, but drastically over corrects . because its impedance
is much lower at the harmonic frequencies. As a result, the fluoro assembly
draws a heavily capacitive load current, and can easily overload the inverter.
In cases where fluorescent lights must be run from an inverter, and the lights are
not going to be run from the mains again, usually the best solution is to either
remove their power factor correction capacitors altogether or replace them with a
much smaller value.
Auto starting
Many inverters are provided with a power switch, and must be turned on before
they supply AC power. However some models are provided with .auto turn-on., so
they stop working when the AC load is removed, but turn on again automatically
when a load is connected. This allows the power switch of an appliance or tool to
be used to control the inverter.s operation as well, conserving battery energy
while still allowing the appliance to be operated in exactly the same way as
when it.s connected to the mains.
In most cases this auto turn-on system uses a sensing circuit connected into the
inverter.s output loop, and designed to detect when the appliance switch is
closed . to complete the high voltage circuit. This allows a small DC sensing
current to flow, and this current is used to turn on the inverter.s MOSFET drive
circuitry.
When no DC flows in the output loop, the drive circuitry is disabled and no
pulses are fed to the gates of the MOSFETs. As a result they don.t conduct, and
the inverter doesn.t operate. Only a very small .standby. current is drawn from
the battery.
Note, however, that because this kind of auto turn-on circuit uses a small direct
current to sense when the appliance has been turned on, it relies on the
appliance providing a DC path when its mains switch is closed. If the appliance
doesn.t provide such a path, the auto turn-on circuit won.t work.
So with some appliances, the inverter may still need to be turned on and off
manually when it.s needed.
Frequency stability
Although most appliances and tools designed for mains power can tolerate a
small variation in supply frequency, they can malfunction, overheat or even be
damaged if the frequency changes significantly. Examples are electromechanical
timers, clocks with small synchronous motors, turntables in older .vinyl. record
players and many reel-to-reel tape recorders.
To avoid such problems, most DC-AC inverters include circuitry to ensure that the
inverter.s output frequency stays very close to the nominal mains frequency:
50Hz in the case of Australia, New Zealand and most European countries, or
60Hz in North America.
In some inverters this is achieved by using a quartz crystal oscillator and divider
system to generate the master timing for the MOSFET drive pulses. Others simply
use a fairly stable oscillator with R-C timing, fed via a voltage regulator to
ensure that the oscillator frequency doesn.t change even if the battery voltage
varies quite widely.
Understanding & Using Solar DC-AC Inverters Part 3
By: yoni levy
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