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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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