Just to make sure, when the alternate temp shuts off
the low speed reverse output at 140 degrees and After the
T4:4.TT timer times out to allow the low speed to stop. Will the High speed out turn on and run then?
Thanks again. Just wanted to make sure it would
Yes, I believe the high speed out will become 1 when T4:4/TT transitions from 1 to 0, as long as, at that transition, the ambient temperature is above the [high fan setpoint N7:13].
And you're welcome.
TL;DR
To be pedantic
, if the [low speed reverse output B3/30] is running when the [alternate temperature N7:21] reaches (is greater than or equal to) 140deg, then the [Low Speed Coast model T4:4] timer starts timing, and it is that state of T4:4/TT being 1 that actually shuts off the [low speed reverse output B3/30], because of the [XIO T4:4/TT] that is the last contact in rung 0003.
Also, I call the first four rungs Start/Stops, although rungs 0000 and 0001 with timers look like Sealed-in Coils (cf.
here).
However, I still call them Start/Stops, because using only [XIC /TT] in the seal-in branch is effectively the same as using [XIC /EN] in the seal-in branch and [XIO /DN] in the main rung, which would then look like a Start/Stop.
The point is that
- each coast model (shutoff) timer only starts when its corresponding fan speed is 1
- once a timer starts timing,
- it will always assign 0 its correspondng fan speed because /TT becomes 1 and the [XIO /TT] in the fan run logic becomes False
- it will always run until the timer expires, i.e. until /TT is 0.
Neither fan speed can run while any timer is running. This does mean that when the system is first started (B3/31 transistions from 0 to 1), and none of the timers are running, then one of the fans may start immediately, depending on the temperature setpoints.
Finally, the fans turning on is dependent on the temperature behavior, and the assumptions are that
- the [alternate temperature N7:21] will increase when [low reverse fan speed B3/30] is 1, eventually exceeding 140
- as well as
- the [ambient temperature N7:20] will decrease when [high fan speed B3/29] is 1, eventually going below 29.
Those are the assumptions, and I know very little about the process, and the fact that there two separate temperatures controlling this logic makes unsure what the edge cases are, so it is up to you to think about what may happen e.g.
- if one or both temperature sensors break and return invalid data,
- or if the operators invert the sense of the Low and High fan temperature setpoints.
So I suggest experimenting with the simulation to see if there are any unexpected behaviors. E.g. what happens if the low fan setpoint temperature that triggers the low rev fan speed is higher than the high fan setpoint temperature that triggers the high fan speed?
note that there are other approaches that may be more clear e.g. five states: 0; 1; 2; 3; 4. I.e.
0. B3/31 is 0, system is off
- low reverse fan speed is 1
- low reverse fan speed is 0, fan is coasting in reverse to a stop
- high fan speed is 1
- high fan speed is 1, fan is coasting to a stop
and then write the logic that transitions based on the current state, the temperature sensor readings and the timers.