Inverter in UPS

Depending on the application and VA load design and switching frequency for the pulse transformer, thyristor components such as SCRs (or Triacs) may be used to turn "ON" the thyristor using short duration pulse at the gate and allowing the shunt effect of the load current to almost instantaneously energize the transformer. Thyristor components are designed to turn "ON" when voltage and current follow each other in one phase but turn "OFF" the component when the supply voltage and current are reversed. Thyristors are therefore designed for AC (sine or square wave) type switching in most cases. Many SMPS designs use power switching transistors, Power MOSFETs or IGBTs. These components may typically be more expensive but are easier to control when switching at higher frequencies or for heavy current loads. PWM is an acronym for Pulse Width Modulation. Mathematically, it is possible to "average" the output voltage based on a fixed input voltage by varying the duration of the "ON" state of the switching device (there are other components used to produce the output as well such as diodes, inductor, capacitors etc. As an example, If the input supply voltage equals 20V @ 10A, and the PWM duration pulse was 50% of the frequency, the typical output expected would be 10V @ 5A. There are many variations of inverter and SMPS (Switch Mode Power Supply) designs and depend upon switching frequency input and output load requirements as well as cost.

Correction; the average output voltage should have read as the average output "power".

PWM is Pulse Width Modulation as stated in post by covector. A DC supply is the source for electrical power, and by switching at ANY frequency with a bipolar switch (explained later), a 50% duty cycle results in equal average positive and negative currents in a load. When varying the duty cycle of square-wave pulses from 0 - 100%, the power delivered to a purely resistive load will be constant while power delivered to loads that are not purely resistive will be a function of switching frequency, duty cycle, and of course, the electrical impedance (inductive or capacitive reactance and resistance) of the load.

A bi-polar switch is one which alternately connects one end of a load to the positive power supply terminal and the other end to the negative side, and then switches the end previously connected to the positive terminal to the negative terminal; similarly for the previously connected end of the load that was on the negative terminal.

The voltage developed across an inductive load, such as a transformer winding, is a function of the inductance of the winding, the resistance of the wire forming it, and the rate at which current changes: V = L di/dt. The DC resistance of the wire will limit the amount of current that can flow, whereas the switching frequency and the inductance of the winding will limit the voltage developed.

Since components (inductors and capacitors) to make a 60 Hz tuned circuit (one which has a resonant frequency of 60 Hz) are very large, such things are just not done. Instead, high currents at low voltages (such as 12V battery) in the primary of a transformer are controlled by such means as PWM, and the lower currents at higher voltages (like 120V) are induced in the secondary.

There is lots and lots more to this than my short explanation, so seek knowledge from the web and library before jumping into a design effort.
This forum is not practical for learning everything you need to know.

Thyristor (SCR) based inverters are really only suitable in applications where there's a sine-wave voltage already impressed at the output, e.g., an induction motor drive or an electric utility application. This is because the thyristor needs to the voltage across its terminals to reverse in order to switch off. This is why most UPS inverters use on/off switches such as MOSFET or IGBT devices for the inverter stage.

Pulse width modulation is used to create high fidelity sine waves at the inverter output. By varying the width of the voltage pulses provided at the output, and thus the average voltage over time, the output can be made to closely approximate a sine wave. The higher the switching frequency of the transistors, the better the fidelity of the resulting sine wave. This is important because lower fidelity sine waves result in higher harmonic currents. The input transformers to the equipment being run from the UPS could overheat and be damaged by poor quality sine waves. This is why many "recreational" inverters carefully specify what type of devices can be connected.