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Old June 25th, 2014, 05:51 PM   #1
Cyanogen281
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Siemens S7-300 Analog Output

So I am trying to control the speed of a motor with a Siemens S7-300 PLC. I have an analog input card as well as an analog output card.

I am wanting to be able to view the speed of the motor as well as use the analog output card to write speed control changes to it.

Does anyone know what kind of block I would use for this?

I am scaling the speed input with an FC108 scale block, which is working fine... I am just a little confused on how to take that value that I am getting from the analog input card, make it viewable on the HMI (Already done), and enable setpoints for changing the speed of the motor through the analog output card.

Any advice would be greatly appreciated.

Thanks
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Old June 25th, 2014, 08:37 PM   #2
dchartier
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Hello cyanogen;
What are these inputs and outputs connected to, a drive? If so, what make and model?
By the way, to scale an input (using standard S7 functions) you should use FC 105 ("SCALE"); to unscale an analog output you should look at FC 106 ("UNSCALE").
Hope this helps,
Daniel Chartier
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Old June 26th, 2014, 09:24 AM   #3
Cyanogen281
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Yes, they are connected to a Variable Frequency Drive(VFD) I'm not sure of the make and model but I don't believe it would matter just as long as my PIW addressing is correct

I saw the "Unscale" block but I couldn't figure out what I am supposed to be inputting in order to send an output to the VFD.

Could you please give me an example of a scaled analog input value put into the unscale block in order to control the speed of the VFD?

Thanks for the advice.

Last edited by Cyanogen281; June 26th, 2014 at 09:27 AM.
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Old June 26th, 2014, 01:31 PM   #4
dchartier
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Hello Cyanogen;
You understand what FC 105 does: it reads in a numerical value from the analog input address (PIWxxx) in a standard (Siemens) numerical value, between 0 and 27468 ( for a unipolar input, that is o-20mA) or from -27687 to +27648 (for a bipolar input, -10 to +10 V); then it output the value in engineering units that you configure, so you read 45.0%, or 123.0 gpm, or 42,4 Hz. Values are read from the periphery in WORD format, and expressed in engineering units as RESAL floating-point values.

FC 106 does the reverse: your program calculates in REAL values the output you want to send to the analog output address, say 55.0% or 28,0 Hz, calculates the unipolar or bipolar numeric value that the analog output module needs to output in WORD format and sends it to the required peripheral address.

The analog output module then generates the current or voltage value it is designed or configured to produce, so the drive can respond to the signal.
Hope this helps,
Daniel Chartier
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