×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

AC ground Isolation Issue

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.  

RE: AC ground Isolation Issue

Look into a signal isolator.  Action, Red Lion, and others make them.

RE: AC ground Isolation Issue

An HF filter between the tacho and the PLC input might help. Do you have any option to use a 4-20mA signal for the speed reference?
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: AC ground Isolation Issue

(OP)
Unfortunately the speed control of the VSD is only voltage following and the output of the tacho would not work with 4-20mA range.

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

Low pass filter - keeps the switching noise out of the sensitive inputs.

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.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources