OT:Exhaust Fan VFD Accel & Decel

[curlyandshemp]

I am on a jobsite we inherited from previous integrator. One issue the customer has is the PID control of a 30hp exhaust fan moving 1000 cfm of air from a dryer. The previous intergrator had the accel @ 40 seconds and the decel @ 60 seconds to compensate for programmming issues that was messing up the PID control which I have since resolved. I wish to speed up the response of the VFD, but I know there a limit as to how low the accel and decel should go. The previous intergrator decide on a variable speed fan rather than adjustable dampners. I am thing dropping the accel and decel down to about 20 seconds each. Before actually trying this, based on the size of the motor and volume of airflow, do these numbers seem reasonable?

 

[bikerdude]

HI,
20 sec. sounds right for 30HP, to me anyway. I've seen it less, hell I've seen it straight across the line start, belts squaalling, smoking, frames a rattlin. That probably wouldn't impress anyone though. Most of the drives we use have provisions for a digital input that can be assigned as a second accell time. Can you use 2 different accell times? One for start & another while running?
Just some rambling
BD

 

[Mechtrician]

Hi there,
As far as accel time goes, 20sec is probably fine, but take care that if you decel it in too short of a time you may fault the drive or depending on the drive (not sure what drive your using) you can possibly cause damage if it tries to stop too much mass too fast. Like I said it depends on the type/size of drive, some are more efficient than others and some regenerate better and or different than others. Also consider the size of the braking resistor(s.)

 

[leitmotif]

Did it run OK at the longer accel and decel times? Was performance of dryer OK?
If so I would consider leaving well enough alone.
I agree with comment re braking resistors - a longer braking time will more than likely not create problems with braking resistors and will reduce likelihood of high bus voltage trips / faults.

Dan Bentler

 

[zova]

Did it run OK at the longer accel and decel times? Was performance of dryer OK?
If so I would consider leaving well enough alone.

Exactly.

Instead of the decision of the problem is not necessary to create a new one. At least, energy consumption of exhauster will be more under decreased dec&acc time.

It is typical windup problem and it already was discussed here and may be somewhere else just a search is needed.

 

[curlyandshemp]

Did it run OK at the longer accel and decel times? Was performance of dryer OK?
If so I would consider leaving well enough alone.
I agree with comment re braking resistors - a longer braking time will more than likely not create problems with braking resistors and will reduce likelihood of high bus voltage trips / faults.

Dan Bentler

No, before the airflow control was horrible. If the customer put a setpoint of 800 cfm, the PID would have an oscillation of +/- 250 cfm. This would being caused by some logic the original intergrator put in the PLC code that altered the CV from the PID during the shaking cycle of the filters on this dryer.

I now have the PID at +/- 10 cfm of setpoint.

 

[milldrone]

curlyandshemp,

Many newer drives have "stall prevention" on accell and decell. You can adjust the accell and decell to low numbers and if the drive reaches a trip point during accell or decell, the stall prevention will overide the accell or decell rate.

 

[zova]

No, before the airflow control was horrible. If the customer put a setpoint of 800 cfm, the PID would have an oscillation of +/- 250 cfm. This would being caused by some logic the original intergrator put in the PLC code that altered the CV from the PID during the shaking cycle of the filters on this dryer.

I now have the PID at +/- 10 cfm of setpoint.

All the more so it is speed windup - if the actuator is slow, that I-part shouldn't save up an error when speed of change of PID's output exceeds really possible speed of change of the actuator.

 

[Clay B.]

curlyandshemp,

Many newer drives have "stall prevention" on accell and decell. You can adjust the accell and decell to low numbers and if the drive reaches a trip point during accell or decell, the stall prevention will overide the accell or decell rate.

This is an effective feature but can cause issue with the PID loop.

Several Oxidizers I have worked on over the years have presented me with the same issue. Start up tends to be the main issue. My solution was to create a break out routine ... What this did was allow for slower accell on startup. Once I got above a certain speed I would then allow the PID to take over. Shutdown was handled the same way. When I needed to stop I used a slower decell (actually in most cases I just allowed it to coast to a stop).

I did have one system that kicked my rear for a period. What would happen was the blower was used on 2 seperate systems and when one drying would shut down My PID would go nuts. I got help on this one but basically we made use of a filter to smooth things out.

FYI... IF your not familar with Oxidizers, they are used to control the consentration of Explosivie gasses in a printing process. Basically your trying to maintain a certain level to make the Oxidizer self sustaining to limit fuel consumption. You do however have to keep the consentration low enough not to pose a hazard. The gasses are created in the drying process and evaporation from the ink tanks so it can be highly variable.

If this is what you are working on Curly you have my deepest sympathy....