An inverter uses two switching transistors arranged with feedback circuits so when one transistor turns on it turns the other off and vice versa. These transistors (arranged in what is sometimes calle a "push-pull" configuration) drive a transformer.
The input to the inverter is the 28 volts DC. When DC power is supplied, minor differences in the transistors ensure that one turns on before the other, thus initiating inverter action. Exactly identical transistors wouldn't work because both would pass equal currents simultaneously and inverter action wouldn't be initiated!
In the output circuit of the switching-transistors is the output transformer. As each transistor turns on it sends a current pulse into the primary windings of this transformer and the ratio of number of turns on the secondary winding in relation to the number of turns on the primary winding determine the output voltage from the output side of the transformer.
As soon as current flows in one transistor, a feedback circuit arranges to turn this transitsor back off again and turn the other one on, hence they alternately switch current pulses into either end of the primary winding, thus feeding it a form of alternating current.
The shape of this AC is usually a square-wave, or a modified sine-wave, as opposed to the output of a genuine (rotary) alternator which is a pure sine-wave.
The transformer faithfully reproduces the waveform appearing in the primary in the secondary winding, both in frequency and waveform shape. However, the voltage is stepped up by the ratio of secondary turns to primary turns.
The current passing through the transformer primary may be quite high, so the windings will be thick, heavy wire. At the secondary end, the voltage may be very high and the current corespondingly lower for the same power output as that in the input circuit (transformers are quite efficient devices), so the windings will be much thinner wire.
The rating of an inverter is usually expressed as so many Volt-Amps, rather than Watts (the electrical unit of power) because power in Watts only relates to loads which are purely resistive. In reality, the load placed across an inverter output might not exhibit a pure resistive characteristic...it may look like a capacitive reactance or an inductive reactance, hence the use of the unit Volt-Amps rather than Watts. A term sometimes seen is the VAR, short for Volt-Amps Reactive.
The frequency of output side of the transformer is set by that of the input frequency which is in turn set by the timing components (resistors and capacitors usually) associated with the switching transistors.
The voltage is established by the ratio of output turns to input turns in the transformer.
The waveform shape is set by that of the inverter switching circuit. Most produce square waves and a lot of equipment is not happy with square-wave AC input. Square-wave inverters are cheaper to make than pure sine-wave inverters. The usual compromise is to produce what is called a "modified sine-wave" output which is usually sufficient for most normally ac-operated equipment.
Hope this helps!
