Time to change pressure in a VFD fan

[seanhkim]

I need to know how long it takes to change from one pressure point to another pressure in a VFD fan.
For example, from 250Pa to 500Pa.

My scenarios is once the fan gets signal to increase pressure differential from the controller, how long do I need to wait until the pressure level gets to the desired level?

And from where can I know how would be output air velocity out of the fan?

 

[RossABQ]

The ramp rate is usually a variable value that you set in the VFD software, and obviously is also affected by how much inertia the fan wheel has, where it starts from (RPM-wise), the fan curve, etc. There is no simple answer to your question.

 

[willard3]

Use a pitot tube 6' straight down the duct from the fan;do a duct traverse. See SMACNA for a duct traverse.

http://www.smacna.org/

How long the pressure change takes depends upon lots of variables and you haven't given enough information to estimate.

 

[simsd]

Like Willard3 said, you haven't given us enough information. But are you asking if should it take one minute? Two minutes? 10 minutes?

It all depends on the controller and how it's set up in the PID loop. PID stands for Proportional Integral and Derivative. Usually we only wet up the Proportional and Integral for HVAC applications. The Proportional value usually has a value of between 0-10. You usually start at 1 and continue to increase the value until the fan or pumps becomes unstable at or past the value you want. Once the value becomes unstable, take the proportional back down a bit until it slow down and is close to the set point.

Now adjust the Integral (which is in seconds) until you reach your set point and everything is stable. Then you should be good to go.

I'd say based on what I've seen that you should go from 0 to your setpoint in about 2 minutes.

 

[gepman]

You can calculate the minimum time to change from one speed to another if you know the moment of inertia of the fan and NEMA design type of the motor. As others have said the accel and decel rate are usually programmable in the VFD. And as others have said the speed and stability of the change will also be a function of the PID loop tuning parameters. One thing to be careful about is the decel rate. If you slow the fan faster than its natural decel rate (i.e. rate during a coast to stop) then the fan will turn into a generator and fault the VFD on bus overvoltage. If you need to slow a fan fast and you have a large moment of inertia then you will need to add either braking resistors or install one of the systems which allows a VFD to return the excess energy into the supply bus.