VFD - Frequent Frequency changes

[Highland Controls]

D has its place, particularly in positioning applications, but is completely unnecessary here. This is such a slow process that a little P and some I should be all that is needed.

 

[FactoryTalktotheHand]

Sounds like the Proportional gain is too high. Lowering it will change the frequency less. The integral gain might also be too fast (it depends on the type of loop, sometimes the higher number means slower, sometimes it means faster) where it sees the big jump and thinks "whoa, I've gone way lower in a very short amount of time, I need to go full speed!" I would try decreasing the proportional gain first, and then increasing the integral time (see the controller's manual to know whether increasing or decreasing the integral value increases the integral time). Hopefully, a longer integral time will allow the signal to settle out a bit more, or at least the loop won't overreact so much. The Derivative could be coming into play here, too, but don't mess with it until you've messed with the P and I terms first, as it may be dependent on those. The Derivative may be the problem, too. Most loops can run "pretty good" without a Derivative, so I would actually try setting that to 0 first. If nothing changes or it gets worse, than the Derivative wasn't the problem.

In basic terms, a PID works this way:

P = How much the loop reacts each step, based on WHAT THE ERROR IS (difference between Setpoint and Value) right now.
I = Analyzes what the error is now compared to WHAT THE ERROR WAS the last time it checked (you change the interval that it checks), and adjusts the output to either increase or decrease according to what it found.
D = Analyzes the slope of the integral line and calculates WHAT THE ERROR WILL BE when the integral checks again. You change how much the Derivative corrects the output.

 

[Tharon]

I tried to go in and do the fastest possible solution, adjusting the Accel/Decel time on the drive.

It is a Siemens Simovert VC, 6SE7031-53F60. But unfortunately the parameters seem to be locked out (Parameter 359 and 358 I think, for inputting a password).

And when I try to connect to it using Drive Monitor v5.5 SP2, it knows something is there, but cannot identify it. Comes up as "Unit Type ?????? Version ??????"

Is there another, newer, piece of software to connect to the Simovert VC drives? Not sure if they have it locked out over RS232 as well because of the Password lockout on the P358/359 parameters, or if I'm just not using the latest software, and it can't identify the drive.

 

[redbarron55]

How do you know the demand?
Even with steady pressure (regulated by the VFD) the demand could be changing rapidly and the vdf is making up the difference to maintain the pressure.
What is the measured power doing during this time of fluctuations?
If the power is changing the the demand is also changing and you are chasing a normal situation.
This is why you have the VFD in the first place.
At low speed there is very low demand and it is easily met.
At near 100% you have very high demand and the motor speed reflects that.
In between the system is doing it's job and varying the speed to match demand and keep the pressure constant.
The rapid change in speed will do no damage to the compressor. It was made to run at base speed and anything less is less wear and tear.
I wouldn't worry about it unless the pressure was surging as well. Then it is a matter of tuning th loop as normal.

 

[Highland Controls]

Everyone has an opinion, and here is mine. Don't listen to anyone who says this is normal. This is not a rapidly changing process. It can't be unless you have a 12" valve that is opening and closing on that tank. Oscillating speed, if not bad for the mechanical parts(I think it is), is wasting energy.

 

[DickDV]

I'm with Highland Controls. There are always a thousand excuses for systems that are not properly commissioned and tuned. We've heard a large share of them in this thread.

I'd suggest you get someone that has solid experience in this work and settle that system down. I know, it not your money that's being wasted and it does work, but, just on principle, why not get it right and enjoy the benefits (like a quieter office!)