Two analogs to one VFD

[baxtersdad]

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?

 

[ozmosis]

A number of VFD's will have 2 x AIN and these will accept 4-20mA. I assume the AIN is used as the setpoint and therefore you can typically select between AIN1 or AIN2. The selection of either can sometimes be determined within the drive itself, maybe with AND/OR logic functions that reside in the drive and activated by the same command that determines the redundancy switchover.
It will obviously depend on the drive you are using.

 

[pooslinger]

Wireless analog. I use phoenix contact or Dataradio. You can use your existing wire for analog in 1 and use a radio for analog in 2.

 

[danw2]

pooslinger,
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.

 

[ScottyUK]

Most VFDs have a low power on-board 24V supply for contact wetting and loop power. Often they're only capable of 100mA or so but that's more than adequate for loop power on a couple of loops.

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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[pooslinger]

Danw2,
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.

 

[FreddyNurk]

Unfortunately this specific case is no longer relevant, as PDL no longer make drives, but I believe that some PDL drives, at least the smaller ones I dealt with, had multiple 4-20mA inputs, with a configurable interface that allowed for one loop to fail.

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.

 

[dV8r]

Some of the ABB drive have 2 analog inputs too.
Like the ACS-550.

 

[jraef]

Many advanced drives have 2 analog inputs and enough math functionality to do this all internally. The trick to having both of them control in a relatively bumpless fashion is to integrate them into high values, average them, then reduce the high average value again to use as a command signal. By integrating them into a high values first, then reducing the averaged value, error differences between the two will mean less to the averaged value. Then tie the averaging function to a minimum value comparator in each signal so that if one signal is lost, you don't perform the average function any more, which means the remaining value is the only one being used for control. The only change in the commanded speed is the loss of whatever error there had been between the two.

JRaef.com
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[bgood1]

I always just use a couple of dry contact discrete output to switch in the one I want via logic...make them mutually exclusive though.

 

[nbucska]

A speed sensor can fail either while spinning or while idle.

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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