Two analogs to one VFD
Two analogs to one VFD
(OP)
I need to supply a speed reference (4-20MA) to a vfd from two separate plc systems (4-20 ma outputs) for redundancy. Would prefer to have no user input required to make the selection. Considered using a high-select analog module but this would require a separate loop supply which is problematic. Anyone have a alternative suggestion?





RE: Two analogs to one VFD
It will obviously depend on the drive you are using.
RE: Two analogs to one VFD
RE: Two analogs to one VFD
Phoenix Contact radios requires a DC power supply. The OP reveals that a loop power supply is problematic. If power is problematic for one, it's probably problematic for the other.
RE: Two analogs to one VFD
A few of the mid-range VSDs have a small set of logic elements which allow rudimentary ladder logic or a simple PID loop to be configured. Some offer expansion modules which give increased PLC-style processing with a bigger instruction set. One we've used successfully is the Control techniques Unidrive with the PLC module. There are almost certainly others with similar capability although I don't know the specific models.
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RE: Two analogs to one VFD
I assumed that loop supply is not only power but the wire run itself. Radios commenly run on 9v-30v dc. The OPS is not running a remote loop powered device,its a VFD. I can't think of any senareo where there is a PLC and a VFD where 24v cant be made. I just had a salesmen pushing accutech equipment. These things run on battery and transmit things like temp and pressure along with analog. Havent tried anything yet but I'm thinking about it.
RE: Two analogs to one VFD
I cannot imagine that PDL was the only company to make drives with this feature, particularly as some of the newer drives I've seen are far more fancy.
RE: Two analogs to one VFD
Like the ACS-550.
RE: Two analogs to one VFD
JRaef.com
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For the best use of Eng-Tips, please click here -> FAQ731-376
RE: Two analogs to one VFD
RE: Two analogs to one VFD
while idle: any sensor indicating speed is bad
While spinning: the acceleration must be less than
a physical limit.
While both are identical: both work
so the logic can decide which sensor to believe.
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