The post explains the test report of a multilevel step inverter circuit which was developed by me and published in one of the earlier articles. The testing and verification of the design was successfully conducted by Mr. Sherwin Baptista, who is one of the keen followers of the website.
Notes for the Multilevel 5 Step Cascaded Sine Wave Inverter Circuit:
1. We decide the input supply to the inverter---24V @ 18Ah @ 432Wh
2. There will be an issue of NOISE generated in the whole process of building this inverter. To crack the issue of noise generated and amplified very easily;
A. We decide to filter the output signal of IC555 the moment it is produced at pin 3, by doing so a cleaner square wave can be obtained.
B. We decide to use FERRITE BEADS at the respective outputs of IC4017 to enhance filtering before the signal is sent to the amplifier transistors.
C. We decide to use TWO TRANSFORMERS and enhance filtering between both of them in the circuit.
3. The Oscillator Stage Data:
This proposed stage is the main stage of the inverter circuit. It produces the required pulses at a given frequency for the transformer to operate. It consists of IC555, IC4017 and Amplifier Power Transistors.
This is an easy to use low power timer chip and has plenty of variety of projects that can be done using it. In this inverter project we configure it in astable mode to generate square waves. Here we set the frequency at 450Hz by adjusting the 1 megaohm potentiometer and confirming the output with a frequency meter.
This is a Jhonson's 10 stage counter divider logic chip which is very famous in sequential/running LED flasher/chaser circuits. Here it is smartly configured to be used in an inverter application. We provide this 450Hz generated by IC555 to the inputs of IC4017. This IC does the job of breaking the input frequency into 9 parts with each resulting in a 50Hz output.
Now the output pins of both 4017's are having a clock signal of 50Hz continuously running forward and backward.
C. The Amplifier Power Transistors:
These are the High Power Transistors which pull the battery power into the transformer windings with accordance to the signal fed into them. Since the output currents of the 4017s are too low, we cannot directly feed them into the transformer. Therefore we need some kind of amplifier which will convert the low current signals from the 4017s into high current signals which then can be passed onto the transformer for further operation.
These transistors would get hot during operation and would necessarily need heatsinking.
One could use a separate heatsinks for each transistor, hence it should be ensured that the
heatsinks do not touch each other.
One could use a single long piece of heatsink to fit all the transistors on it. Then one should
thermally and electrically isolate each transistor's center tab from touching the heatsink in
order to avoid them from getting shorted. This can be done by using Mica Isolation Kit.
4. Next comes the First Stage Transformer:
A. Here we employ the Multi-tapped Primary to a Two Wire Secondary transformer. Next we find the volts per tap to prepare the primary voltage.
We take into consideration the Input DC Voltage that is 24V. We divide this with 1.4142 and find its AC RMS equivalent which is 16.97V~
We round the above RMS figure which results in 17V~
Next we divide RMS 17V~ by 5(since we need five tap voltages) and we get RMS 3.4V~
We take the final RMS figure by 3.5V~ and multiplying it by 5 gives us 17.5V~ as a round figure.
In final we found the Volts Per Tap which is RMS 3.5V~
B. We decide to keep the Secondary voltage to RMS 12V~ i.e., 0-12V is because we can obtain a higher amperage output at 12V~
C. So we have the transformer rating as below:
Multi-tapped Primary: 17.5---14---10.5---7---3.5---0---3.5---7---10.5---14---17.5V @ 600W/1000VA
Secondary: 0---12V @ 600W/1000VA.
We got this transformer wound by a local transformer dealer.
5. Now follows the main LC Circuit:
An LC circuit being known as a filter device has robust applications in power converter circuits.
Being used in an inverter application it is generally required for breaking down the sharp peaks
of any generated waveform and helps convert it into a smoother waveform.
Here at the secondary section of the above transformer being 0---12V, we expect a multilevel
square cascaded waveform at the output. So we employ a 5 Stage LC Circuit to get a SINEWAVE equivalent waveform.
The data for the LC Circuit is as below:
A) All Inductors should be a 500uH(microhenry) 50A rated IRON CORE EI LAMINATED.
B) All Capacitors should be a 1uF 250V NONPOLAR type.
Note that we stress on the 5 stage LC circuit and not just one or two stages such that we can get a much cleaner waveform at the output with lesser harmonic distortion.
6. Now comes the Second and Final Stage Transformer:
This transformer is responsible for converting the output from the LC network i.e., RMS 12V~ to 230V~
This transformer would be rated as below:
Primary: 0---12V @ 600W/1000VA
Secondary: 230V @ 600W/1000VA.
Here, NO additional LC network would be required at the final 230V output for more filtering since we already filtered every stage of each processed output at the beginning.
The OUTPUT will now be a SINEWAVE.
A GOOD thing is that there is absolutely NO NOISE at the final output of this inverter and
sophisticated gadgets can be operated.
But one thing to be kept in mind by the person operating the inverter is NOT TO OVERLOAD THE INVERTER and keep the power loads of sophisticated gadgets being operated in limits.
A few corrections to be made in the circuit diagram are given as under:
1. The IC7812 regulator should have bypass capacitors connected. It should be mounted on a
HEATSINK since it would get warm during operation.
2. The IC555 timer should follow a series resistance before it's signal passes forward to diodes.
The value of resistance should be 100E. IC gets hot if the resistor is not connected.
In Conclusion we have 3 proposed filter stages:
1. The signal generated by IC555 at pin 3 is filtered to ground and then passed on to resistor
and then to the diodes.
2. As the running signals exit the relevant pins of IC4017, we connected ferrite beads before
passing signal to resistor.
3. The final filter stage is employed between both transformers