# Help with designing a 3-phase inverter!

#### SyuGalSpinal

Joined Oct 5, 2011
4
so im doing a 3-phase inverter to run a 3-phase induction motor for my project n yes ive read every similar thread but im just like,, lost,,
so i'd glad if any of you cn give me start up for this one as this is not my specialty,,

so im instructed to design a 3-phase inverter,, using pwm ( still trying to understand this ) and im still a new learner! so ill be glad to take any advice from u guys thx =)

#### crutschow

Joined Mar 14, 2008
24,711
PWM outputs a varying duty-cycle pulse with pulse frequency much higher than the waveform your are generating. The average value of the pulses is the desired waveform. So you vary the duty-cycle of the PWM signal to give the desired sine-wave signal (you will need three separate generators to generate the three phases.

The output PWM driver will operate from equal plus and minus supply voltages that will generate peak PWM pulses equal to or greater than the plus and minus desired peak AC voltage.

What is the voltage and current requirements of the motor?

#### SyuGalSpinal

Joined Oct 5, 2011
4
ahh i see, so pwm basically enhances the signal that we will get?

ohh n im still at the early phase so im yet to think about the voltage and current since im asked to design it i guess ill need to figured it out myself or ill ask my superior later about the requirement,

mind giving me some more heads up? and i thank you a lot sir =)

#### GetDeviceInfo

Joined Jun 7, 2009
1,702
build a block diagram of the overall circuit, then expand each block with a more specific block diagram. Work from the final product backwards. If you list the various technologies that fullfill the requirements of each block, then you can wiegh the pros and cons of each.

#### praondevou

Joined Jul 9, 2011
2,942
When I look at the circuit diagrams for three-phase motor inverters from my company (which took a long time to complete) I do not understand how this could be a project for a beginner.

You will need three independent PWM signals, modulated with a 120 degree shifted sinewave signal.
Since you want to drive a motor, the sinewave modulation should start ramp-like.
The easiest, if you know how to program would be to use a microcontroller for the PWM generation, passing the signals to a three-phase IGBT fullbridge.

Start by determining the power part, how big is the motor you want to drive, what load is the motor driving, can the motor run faster than the the frequency given by the inverter etc. (in this case you'll need a means to dissipate the energy you get back from the motor).
From these parameters you can decide what rectifier components you'll need.
Then it comes to the PWM part and IGBT drivers... You see it's not a project you can finish in a few weeks. Not if you design this from scratch.

#### crutschow

Joined Mar 14, 2008
24,711
ahh i see, so pwm basically enhances the signal that we will get?
It doesn't enhance the signal, it's just a form of modulation. It's an efficient way, using switching techniques, to convert a low level signals (sine waves in this case) into the high power sine waves needed for the motor. For more info look here.

#### dataman19

Joined Dec 26, 2009
135
I believe the purpose of the PWM use in a 3-Phase Inverter is to give you a way to fcontrol "when" each Phase voltage starts.
..
You can control when a PWM starts - it is harder to correctly phase a sine wave oscillator (which would be your start for the Inverter).
...
Most "cheap" inverters are basically Multivibrator Circuits. Even though you can in effect control when a multivibrator starts - it isn't all that easy to get three working together with enough accuracy to actually trigger a 3-phase signal source. Add to this the problem of matching transformers and you have a literal nightmare.
...
Good luck with this project. I'm definitely gonna watch this one progress...
Dave

#### crutschow

Joined Mar 14, 2008
24,711
I believe the purpose of the PWM use in a 3-Phase Inverter is to give you a way to fcontrol "when" each Phase voltage starts.................
Not really. The reason for using PWM is because it's a very efficient method of generating a high power sine wave. Using a linear amp for this, for example, would waste a lot of power.

#### SyuGalSpinal

Joined Oct 5, 2011
4
im back!! and thanks for all of u guys help, passed the theoretical part for my project, now i gotta do the real thing,, and yeah im mostly clear about the pwm parts, and still not quite sure about inverter ( so, IGBT is chosen, cuz its easier? or something to do with the cost, efficiency?) and as of now im still scrolling down for anything regarding the inverter, need to refresh back after a few months holiday, ^^

okay, so, my block diagram will be basically like this; pwm -> driver circuit -> inverter

i will be using a dc power supply, and the parameters of my motor is this,

400V/0.77A (Y)
0.25kW

i was told not to worry much about the pwm components, and needs to start simulating the inverter/driver circuits component

so, base on my undrstanding, the driver circuit is, err to raise the volts? cant find much info about this, but im still scrolling lol,,

and the inverter, i take it ill be using IGBT due to its popularity,, so any suggestion/opinion/advice will be aprreciated, thankss!

#### praondevou

Joined Jul 9, 2011
2,942
What power supply will you be using?

The driver is there to drive the IGBT/MOSFET switches.

You will find much more easy IGBTs with a high collector emitter breakdown voltage than MOSFETs with a high drain source breakdown voltage.

#### SyuGalSpinal

Joined Oct 5, 2011
4
hi praondevou! nice to see u again, and
oh yeah, the power supply, from the battery, have been forgotten bout this but i think its 12v , sorry ill cnfirm it back asap, but its around that,, the most important part, yet i 4got bout it x.x

okay, so igbt then, i think ill cntinuosly post wip here, im sure there're lot more that needs help bout this and u guys have been a great help!