3Phase Sine Wave Generator

[rob_croxford]

Hi All,

Does anyone know of an IC that can produce sine waves (all 120 degrees out of phase) at say 50Hz??

I am building an Inverter using STPWM and all works well i simply need a compact method of producing the 3phase comparator signals as what i have atm is a bit of a mess.

Any help or ideas are greatly appreciated.

Best Regards,

Rob

 

[rob_croxford]

I believe i have found the solution. Here is an IC that will generate a very accurate waveform (square, triangle, sine etc...)

http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=6866027&cm_vc=prev_uk

However some phase shift circuitry is needed to provided the 3 seperate waveforms for the comparators.

 

[Resqueline]

The XR-2206 is a well known function generator IC but I'm not aware of an easy way to shift the phase accurately.
I'm not aware of any really simple circuit solutions, but here is an article of a relatively easy & compact way of doing it.

 

[rob_croxford]

I was intending to achieve the phase delays using an RC network and then to amplify the attenuated (120 degree out of phase) signals using op amps.

These signals are then sent to a pic to produce the correct PWM signal to be sent to my IGBT drivers.

I understand using RC networks will be tedious in getting the timing precise but will this solution work??

Thanks

 

[poor mystic]

Couldn't you use a 150Hz clock and derive 3 phase-shifted 50Hz pulse trains to control phase-locked loops? Alternatively the pulse trains could be integrated to triangles and these could be bent into fairly good sines by diodes.

 

[rob_croxford]

Im not quite sure i follow...

So far this is as far as i have got...

I am, generally speaking, comparing a 5V 50Hz sine wave with a 20Khz triangle wave to create a PWM output. I have then built an IGBT driver (nothing special yet) which i have used to drive a HBridge and generate a single phase high voltage AC output.

Now that i know my driver circuits are working i would like to produce a 3 phase version where the output stage is constant regardless of the load. I would like to use this to run a large motor running a cutting machine for extruded heatsink.

So far i have managed to run this 3phase inverter using a transformer and 3 phase mains using the seperate signals as the comparator signals.

I would like to generate these signals seperate from the mains supply and intergrate this into my control board so all processes can be achieved within the singal unit.

 

[(*steve*)]

It seems to me that you may be better off generating a digital 3 phase (square wave) signal and then filtering it. The reason is that since all filters will be alike, the phase change in them will be constant and therefore you will retain the 120 degreed of difference.

My fear for using filters to achieve 120 degrees of phase shift will end up with you having to deal with a system that is not naturally stable and will need to be adjusted carefully to get the phase relationship correct and the amplitude constant.

You could look at 3 PWM outputs on a uC generating a signal that averages to a sine wave. That would probably reduce the amount of filtering required too.

 

[rob_croxford]

Those were my thorts exactly.

So probably the best way of achieving this is to use PWM. I think i have a few pic's that actually have 3 PWM outputs. So in order to average the signals to a sine wave i would use a look up table to plot the values for the sine wave. I imagine i would need to use a high frequency PWM to achieve the 50Hz Sine...

That is the bit im not sure about. How would i go about working out how to acieve exactly 50Hz sine frequency from the PWM??

Thanks for all the quidence