The post discuses the how to make a 3 phase inverter circuit which can be used in conjunction with any ordinary single phase square wave inverter circuit. The circuit was requested by one of the interested readers of this blog.
The Circuit Concept
A 3 phase load can be operated from a single phase inverter by employing the following explained circuit stages.
Basically the involved stages can be divided into three groups:
- The PWM generator circuit
- The 3 phase signal generator circuit
- The mosfet driver circuit
The first diagram below shows the PWM generator stage, it may be understood with the following points:
The Oscillator and the PWM Stage
The IC 4047 is wired as a standard flip flop output generator at the rate of the desired mains frequency set by VR1 and C1. The 555 IC is configured as a PWM generator which sets the RMS of the 4047 output to the required degree by chopping the output transistor base signals.
The dimensioned push-pull PWM now becomes available at the E/C junction of the two BC547 transistors.
This PWM is applied to the input of the 3 phase generator explained in the next section.
The following circuit shows a simple three phase generator circuit which converters the above input push-pull signal into 3 discrete outputs, phase shifted by 120 degrees.
These outputs are further bifurcated by individual push-pull stages made from a NPN/PNP transistor stages. These 3 discrete 120 degree phase shifted, push pull PWMs now become the feeding input signals (HIN, LIN) for the final 3 phase driver stage explained below.
This signal generator uses a single 12V supply and not a dual supply.
Complete explanation can be found in this 3 phase signal generator article
The circuit below shows a 3 phase inverter inverter circuit stage using H-bridge mosfets configuration which receives the phase shifted PWMs from the above stage and converts them into corresponding high voltage AC outputs for operating the connected 3 phase load, normally this would be a 3 phase motor.
The 330 high voltage across the indivdual mosfet drivers sections is obtained from any standard single phase inverter integrated across the shown mosfets drains for powering the desired 3 phase load.
Full-Bridge Circuit using 3 Half-Bridge Drivers
ISSUES FOUND IN THE ABOVE 3 phase inverter circuit design
The above circuits are not entirely correct and needed a few serious corrections.
After reassessing the circuits I have come up with the following revised diagrams of a 3-phase inverter circuit.
Presently it's a square wave type, I'll soon explain how it could be converted into a modified pure sine wave simply by inserting a dimesioned PWM input to the shut down pins of the half bridge driver ICs.
The ground rail of all the circuits should be terminated into a common line.
Improved Oscillator Stage
3-Phase Generator Stage
The 3-Phase Full-Bridge Driver Stage
In the above 3 phase generator circuit (second last diagram) using a sine wave doesn't make sense because the 4049 would ultimately convert it into square waves, and moreover the driver ICs in the last design employ digital ICs which will not respond to sine waves.
Therefore a better idea is to use a 3 phase square wave signal generator for feeding the last driver stage.
You may refer the article which explains how to make a 3 phase solar inverter circuit for understanding the 3 phase signal generator stage functioning and implementation details.
Simplifying the Above Designs
In the above explained 3-phase inverter circuit, the 3-phase generator stage looks unnecessarily complex, and therefore I decided to look for an alternative easier option for replacing this specific section.
After some searching I found the following interesting 3 phase generator circuit which looks pretty easy and and straightforward with its settings.
Therefore now you can simply replace the earlier explained IC 4047 and the opamp section entirely and integrate this design with HIN, LIN inputs f the 3 phase driver circuit.
But remember you will have to still use the N1----N6 gates between this new circuit and the full bridge driver circuit