High/Low Speed Caps in Logic

So i came across this set of logix in a control logix plc, i understand what the logic is doing, but however, i don't understand why they are using the specified constant values.

The logic is used to calculate HI and low Caps to compare with the feedback speed coming from the VFD. (unit in Hz)

Ex. using a CPT instruction, the commanded frequency speed set point is multiplied by 1.1 then the result is stored as a High cap, for the Low cap we multiply the commanded frequency speed with 0.9 and store result in Low Cap.

Then if the speed Feedback for the VFD is GRT than the High cap we trigger an alarm after a set delay, if Speed feedback is LES than the Low cap we trigger an alarm after a set delay.

So my question arises, why use 1.1 as the constant multiplier for the high cap and 0.9 as the constant multiplier for the low cap I know that they is a way we can trigger an alarm if the commanded speed and the feedback we are getting aren't in a certain threshold, but why 1.1 and 0.9??????

it's just a way to do it, but 1.1 and 0.9 constants mean +/- 10% of something.

Simple Safe engineered limits? Without knowing the impacts any changes in those limits might have on the final driven elements of your machine is it impossible to say.

ok, thanks and obviously those constants can change depending on how your systems feedback react

dbh6 said:

ok, thanks and obviously those constants can change depending on how your systems feedback react

Click to expand...

The constants change depending on what your set-point is.
Is the set-point a variable ( can/does it get changed)?

@ mickey the commanded speed is the set point which is of REAl data type and yes that can change as it can be manually set or automatically adjusted by the specific application, but the constants 0.1 & 0.9 don't change, but the High and Low Caps will change, like you mentioned depending on the Commanded speed Set point.

Typo i meant to say 1.1 instead of 0.1

From Random House College Dictonary, 1st Edition:
"cap,...definition 15. to serve as a cap, covering, or top to; overlie."

High Cap would be the top allowed speed, and Low Cap would be the bottom allowed speed or the range allowed above and below the set speed, before an alarm event. As Widelto said, the allowed range before alarm has been chosen to be from 90% of set speed up to a cap of +110% of set speed. Therefore the limits (Low and High "Cap") continually change as the speed setpoint changes.

Thanks guys