Can some one give me an idea as to why a 4 to 20 milliamp signal from the controlling computer will not control a variable speed drive, but if a 4 to
20 milliamp isolator is installed, it works and controlls. There doesn't seem to be difference between the two signals that can be measured, but one works and one doesn't.Thanks
What are you using in the controlling computer to produce the 4-20 mA signal? Where is the loop power coming from? Were both measurements taken with the drive connected?
Ben Miller
The immediate thought that comes to mind is the interrelation between the signal commons on the computer and the drive.
I have been involved in similar problems where the output from the computer can be either a "Sink" or a "Source". Source being a variable positive supply for the current, and a sink is a variable negative or return. It all sounds confusing and I have no idea who dreamed it up or why these things exist, but if you check into the schematics of the various cards involved in the computer and the Drive, you might discover that with the direct connection, The Signal terminal of one device might be connected to the Common or Ground on the other.
Adding in the isolator electrically isolates the two device commons.
I am assuming, Of course, you have access to the Schematics. Good Luck Tom Grayson
Maybe you've got a sneak path through ground that's shunting the drive circuit. Measure the *voltage* across the input terminals at the drive and I'll bet that you'll see a difference.
Best regards, Spehro Pefhany
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