AC ground Isolation Issue
AC ground Isolation Issue
(OP)
Gday,
I'm having trouble with the analog input of my PLC (S7-314IFM).
I am currently trying to run a DC VSD using voltage following, connected to the PLC's analog output with +-10V, and then a tachogenerator is used to measure the speed. The voltage from the tacho is stepped down from 105V to 10V (voltage divider) and connected to the analog input of the PLC.
The PLC outputs the correct voltage and the motor follows it exactly, the problem is the input value fluctuates dramatically. Example if the speed is set to 1750RPM it would chnage from 300 to 550 to 287 to 1300 to whatever.
The problem was eventually found to be caused by the connection of the analog output of the PLC as the VSD terminals were not isolated from the AC source and in the VSD user manual it states that a Isolated voltage source is needed and that it should not be connected to ground (which it is in my case, when connected to the PLC).
So by disconnecting the voltage following terminals of the VSD and connecting them to a DC power supply, the readings of the analog input of the PLC was steady and accurate.
So now I have the background situation explained, my question is... How do I go about isolating either the analog input or output of the PLC, so that I can use the analog output to control and the analog input to monitor the DC motor?
I hope I have given enough detail, any help would be greatly appreciated. I dont know if this is the right place to ask this question.
I'm having trouble with the analog input of my PLC (S7-314IFM).
I am currently trying to run a DC VSD using voltage following, connected to the PLC's analog output with +-10V, and then a tachogenerator is used to measure the speed. The voltage from the tacho is stepped down from 105V to 10V (voltage divider) and connected to the analog input of the PLC.
The PLC outputs the correct voltage and the motor follows it exactly, the problem is the input value fluctuates dramatically. Example if the speed is set to 1750RPM it would chnage from 300 to 550 to 287 to 1300 to whatever.
The problem was eventually found to be caused by the connection of the analog output of the PLC as the VSD terminals were not isolated from the AC source and in the VSD user manual it states that a Isolated voltage source is needed and that it should not be connected to ground (which it is in my case, when connected to the PLC).
So by disconnecting the voltage following terminals of the VSD and connecting them to a DC power supply, the readings of the analog input of the PLC was steady and accurate.
So now I have the background situation explained, my question is... How do I go about isolating either the analog input or output of the PLC, so that I can use the analog output to control and the analog input to monitor the DC motor?
I hope I have given enough detail, any help would be greatly appreciated. I dont know if this is the right place to ask this question.





RE: AC ground Isolation Issue
RE: AC ground Isolation Issue
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If we learn from our mistakes I'm getting a great education!
RE: AC ground Isolation Issue
I thought of using a signal isolator but couldn't really find one that would suit my needs within my budget <$100AUD (as it is for uni).
I'm not entirely sure what a HF filter is, is that a High-pass filter?
RE: AC ground Isolation Issue
You really shouldn't have mentioned 'uni' on this site - the memberships rules states that the site is for practising engineers only.
Check ebay for isolators - they do turn up from time to time. I'm surprised you can't find one for <$100 even through the mainsteam stockists.
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If we learn from our mistakes I'm getting a great education!