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.

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