3phase VFD drives project

[bassbob]

Hi

Ive been asked to build a 3 phase variable frequency drive controlled via a Siemens PLC as a project. As i have never done a project like this i was wondering if any one has any starting points for me to follow??
As i haven't got a clue where to kick off

Any help would be much appriciated.
Cheers

Bassbob

 

[ScottyUK]

Why would you want to build one? The smaller ones are cheaper to buy than the PLC!


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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[bassbob]

Its going to be training aid for the apprentices at my place of work.

 

[Barry1961]

You might want to check out

http://www.national.com

for some applications notes.

You could also search for some schematics of old VFD's.

Sounds like fun.

Barry1961

 

[jraef]

I don't think we are clear here. Are you looking to build a VFD test stand to demonstrate how a VFD can be controlled by a PLC, or are you wanting to construct a VFD from raw components using a PLC as the control for the transistors?

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[itsmoked]

If you want to build a VFD with a PLC it ain't gonna happen.

That's like trying to build a space launch vehicle out of a coffee table...

 

[jraef]

Hmmm... I probably shouldn't have said transistors. With a high speed PLC you might possibly be able to make a 6 step inverter work. Sort of. Maybe not. I would never even think to try.

But the point I was really trying to make was just what you said itsmoked. I think he meant to say he wanted to build a test stand WITH a PLC and a VFD. Hopefully bassbob will respond and clear that up, because if so, then we can help.

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[Marke]

Hello bassbob

I do not believe that you will be able to build a VSD using a PLC as the core. It is just too slow for that type of operation. You may be able to get building blocks that comprise the "inverter"section of the VSD and use the PLC to provide the user interface. At that point, I would not say that the PLC is part of the VSD. - I would not even attemp this as the costs would far outway any advantage.

I am assuming that what you are really trying to achieve is an example of a standard VSD controlled by a PLC.
Many of the drives on th market today, have a MODBUS, Profibus or similar serial input. This can be easily coupled to a PLC with a similar serial port.
Start with the PLC, determine the serial protocol and then find a drive with a compatible protocol. NOTE Many drives use MODBUS RTU as standard and other protocols require additional modules.
If you already have a site standard coms protocol, it would make sense to use that.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[electmech]

I work at Siemens, and the PLC may be fast enough...I am not sure there. I know that there are some pretty high powered processors out there offered in standard products.

This seems like a very expensive, and very difficult way to do something that has been done already (millions of times). If you are going to be working on an R&D project, you may want to consider designing a PLC program to run a standard VFD instead of literally reinventing the wheel. There is going to be a whole lot more involved than just firing transistors to accomplishing your goal. Even if you are successful, it is going to be extremely complicated to design, and I doubt that it to be able to do much but fire a series of gating signals based on an algorithm at best, and be an expensive experiment in futility at worst.

 

[itsmoked]

bassbob; If you really are going to scratch build a VFD(a year project) go to Microchip's site and search their app notes. They have a nice reference design.

 

[bassbob]

Thanks for the info fellas.

What i was thinking of doing was build a vfd from raw components and control the firing from the plc. But from the info i gathered from this forum it seems that it might be to difficult to complete??

 

[itsmoked]

Like I said look at those app notes. Takes a few minutes and you will see just how big the job is. You'll learn too.

A lot of VFDs use high speed digital signal processors to get through the math in time to close the loop. That's not plc territory.

 

[Euphaz]

Here is a 3-phase inverter program for PIC16C55 microcontroller and similar processors.

http://www.piclist.org/techref/member/HB-operamail-885/index.htm