Why are Power Inverters Rated in KVA

Why are Power Inverters Rated in KVA

Power inverter or inverter is a power electronic device or circuitry that changes direct current (DC) to alternating current (AC).

The input voltage , output voltage and frequency, and overall power handling depend on the design of the specific device or circuitry. The inverter does not produce any power; the power is provided by the DC source.

power inverter can be entirely electronic or may be a combination of mechanical effects (such as a rotary apparatus) and electronic circuitry. Static inverters do not use moving parts in the conversion process.

Power inverters are primarily used in electrical power applications where high currents and

voltages are present. Circuits that perform the same function for electronic signals, which usually

have very low currents and voltages, are called oscillators . Circuits that perform the opposite

functions, converting AC to DC are called rectifiers . With an increasing capacity of solar,

wind etc being generated and exported to the grid, power companies and utilities now require

inverters to be able to support the power quality of the grid and control the overall voltage rise

from the point of supply to the inverter to within set thresholds specified by national standards or

the local utility.

Newer inverters thus allow settings and modes including fixed power factor and reactive

power to be changed and enhance the systems performance and support the power quality of the

grid. When the power factor or reactive power control setting is changed the actual power

(kW) "power converted into an output " no longer correctly defines the inverter#39;s capacity.

The inverters capacity in this case is made up of actual power (kW) and reactive power (kVAR)

which through the cos/sine rule specify the apparent power (kVA) of the inverter.

The apparent power (kVA) describes the total amount of power being used by a system

Types of Power Inverters

Based on the output waveforms, there are three types of Inverters. These are Sine wave,

Modified Sine wave and Square wave inverters.

Sine wave power inverter

Alternating current has continuously varying voltage, which swings from positive to negative.

This has an advantage in power transmission over long distance. Power from the grid is carefully

regulated to get a pure sine wave and also the sine wave radiate the least amount of radio power

during long distance transmission. But it is expensive to generate sine waves in an inverter. Its

quality is excellent and almost all electrical and electronic appliances work well in sine wave

inverter.

The sine wave is the AC waveform we get from the domestic lines and from the generator. The

major advantage of sine wave inverter is that all of the house hold appliances are designed to

operate in sine wave AC. Another advantage is that the sine wave is a form of soft temporal rise

voltage and it lacks harmonic oscillations which can cause unwanted counter forces on engines,

interference on radio equipment and surge current on condensers.

Modified Sine wave or Quasi Sine wave

Modified sine wave is designed to simulate a sine wave since the generation of sine wave is

expensive. This waveform consists of a Flat Plateau of positive voltage, dropping abruptly to

zero for a short period, then dropping again to a flat plateau of negative voltage. It then goes

back to zero again and return to positive. This short pause at zero volts gives more power to 50

Hz fundamental frequency of AC than the simple square wave.

Inverters providing modified sine waves can adequately power most household appliances. It is

more economical but may present certain problems with appliances like microwave ovens, laser

printers, digital clocks and some music systems. 99% of appliances run happily in modified sine

wave. Instruments using SCR (Silicon Controlled Rectifier) in the power supply section behave

badly with modified sine waves. The SCR will consider the sharp corners of the sine wave as

trash and shut off the instrument. Many of the laser printers behave like this and fail to work in

inverters and UPS providing modified sine wave power. Most variable speed fans buzz when

used in modified sine wave inverters.

Square Wave Power Inverter

This is the simplest form of output wave available in the cheapest form of inverters. They can

run simple appliances without problems but not much. Square wave voltage can easily be

generated using a simple oscillator.

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25th Jun 2020 Ogochukwu Agina

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