Inverters and Battery Based AC

An inverter is a circuit for converting direct current to alternating current. An inverter can have one or two switched-mode power supplies (SMPS).

Early inverters consisted of an oscillator driving a transistor, that is used to interrupt the incoming direct current to create a square wave. This is then fed through a transformer to smooth the square wave into a sine wave and to produce the required output voltage.

More efficient inverters use various tricks to try to get a reasonable sine wave at the transformer input rather than relying on the transformer to smooth it. Capacitors can be used to smooth the flow of current into and out of the transistor. Also, it is possible to produce a more sinusoidal wave by having split-rail direct current inputs at two voltages, or positive and negative inputs with a central ground. By connecting the transformer input terminals in sequence between the positive rail and ground, the positive rail and the negative rail, the ground rail and the negative rail, then both to the ground rail, a stepped sinusoid is generated at the transformer input and the current drain on the direct current supply is less choppy.

Modified Sine Wave inverters convert the (usually 12 V DC) battery voltage to high frequency (20 kHz) AC, so that a small transformer can be used. This is then stepped up to a higher voltage (say 160 V) AC. This output is converted to DC at the same voltage, and then inverted again to a quasi sine wave output (about 120 V RMS). Another disadvantage of the modified sine wave inverters is that the output voltage depends on the battery voltage.

It is quite expensive to obtain a good sine wave from an inverter. The quoted accuracy (harmonic distortion) for most is less than 60%, and this will have an effect on the appliances connected to the output of the inverter. This leads to noise in lot of appliances and damages electric motors, as they run significantly hotter.

High end inverters (> $2,000) produce waveforms which are closer to a mathematical sine wave than those produced by the utility.

Date: 2014-12-29; view: 994