In the previous part of this article we learned the main section of the 4kva synchronized stackable inverter circuit which explained the synchronization details of the design. In this article we study how to make the design a sinewave equivalent and also ensure correct synchronization of the PWMs across the involved inverters.
Synchronizing Sine Wave PWM across the Inverters
A simple RMS matched PWM equivalent sinewave waveform generator can be made by using an IC 555 and IC 4060, as shown in the following figure.
This design can be then used for enabling the inverters to produce a sinewave equivalent waveform at their outputs, and across the connected mains line.
Each of these PWM processors would be required for each of the stackable inverter modules individually.
The different stages involved in the above PWM geneartor circuit can be understood with the help of the following point:
Using IC 555 as the PWM Generator
The IC 555 is configured as the basic PWM generator circuit. To be able to generate an adjustable PWM equivalent pulses at the desired RMS the IC requires fast triangle waves at its pin7 and a reference potential at its pin5 which determines the PWM level at its output pin#3
Using IC 4060 as the Triangle Wave Generator
For generating the triangle waves, the IC 555 requires square waves at its pin#2, which is acquired from the IC 4060 oscillator chip.
The IC 4060 determines the frequency of the PWM, or simply the number of "pillars" in each of the AC half cycles.
The IC 4060 is mainly employed for multiplying the sample low frequency content from the inverter output into a relatively high frequency from its pin#7. The sample frequency basically makes sure that the PWM chopping is equal and synchronized for all the invetrer modules. This is main reason why the IC 4060 is included otherwise another IC 555 could have easy done the job, instead.
The reference potential at pin#5 of IC 555 is acquired from an opamp voltage follower shown at extreme left of the circuit.
As the name suggests this opamp delivers exactly the same magnitude of voltage at its pin#6 which appears at its pin#3....however the pin#6 replication of its pin#3 is nicely buffered, and therefore is richer than its pin3 quality, and that's the exact reason of including this stage in the design.
The 10 k preset associated at pin3 of this IC is used for adjusting the RMS level which ultimately fine tunes the the IC 555 output PWMs to the desired RMS level.
This RMS is then applied to the bases of the power devices in order to force them to work at the specified PWM RMS levels, which in turn causes the output AC to acquire a pure sinewave like attribute through a correct RMS level. This may be further enhanced by employing an LC filter across the output winding of the all the transformers.
The next and the final part of this 4kva stackable synchronized inverter circuit details the automatic load correction feature for enabling the inverters to deliver and maintain the correct amount wattage across the output power mains line in accordance with the varying loads switching.