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Old February 26th, 2008, 11:40 AM   #1
SCADA_Dude
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VFD Pump Control PID question

Just looking for some opinions on VFD PID control design. What would you consider a better design for a VFD pump controlling pressure.

1. PID: Actual pressure (PV) compared to pressure setpoint (SP), output (CV) scaled directly to VFD 4-20ma. (Optionally, Actual pump speed and commanded speed are compared to generate an alarm if deviation is too large.)

2. PID: Actual pressure (PV) compared to pressure setpoint (SP), output (CV) scaled 0-100%. This 0-100% value is sent to another PID to control actual pump speed 0-100%(PV) to speed setpoint (SP), output (CV scaled directly to VFD 4-20ma.

I like the practice of comparing actual pump speed to desired speed, but both seem to accomplish the same thing, with the first being more simple. I'm interested in hearing opinions on this.

Thanks
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Old February 26th, 2008, 12:52 PM   #2
averytc
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I use option 1. What would be a reason to use option 2?
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Old February 26th, 2008, 01:40 PM   #3
ttichy
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What pump are you using? How close are you able to regulate the pressure? There is one (somewhat pricey) system (http://www.kadantunigy.com/index.html) that does pump modeling and torque control to give you pressure and flow control
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Old February 26th, 2008, 05:08 PM   #4
DickDV
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If you are trying to regulate pressure, why fuss with pump/motor speed?

Set your PID loop up to feed back pressure to a pressure setpoint comparator and that's it!

We do it all the time here and the system works just fine.
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Old February 26th, 2008, 06:27 PM   #5
SCADA_Dude
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Thanks for the feedback. The system has very accurate pressure control, with high capacity VFD drives (1500GPM H2O Pumps), so it works great as described in option 1 above.

I am glad everyone seems to think option 1 is better. I have seen option 2 used on some old RTU controls and it seemed like a kludge to me. I am somewhat new to VFD PID's and just wanted to make sure I wasn't overlooking something. Peer review is a great way to build confidence in a design.

Thanks again
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Old February 28th, 2008, 05:58 AM   #6
danesh
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Quote:
Originally Posted by DickDV
If you are trying to regulate pressure, why fuss with pump/motor speed?


Set your PID loop up to feed back pressure to a pressure setpoint comparator and that's it!

We do it all the time here and the system works just fine.


Dear DIKDIV,

Are you having any experience in VFD submersible pump? I use a VFD for submersible pump and cause damage in bearings pump.

Could you please help me?

Danesh
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Old February 28th, 2008, 03:26 PM   #7
DickDV
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I guess my earlier post wasn't very clear. I meant to say that I would use a VFD but only put PID control on the pump discharge pressure. I don't see any benefit in closing up the speed loop in addition to the pressure loop.

Danesh, you need to start a new thread. You probably don't mean any harm but injecting a new question like you have done is called "hijacking a thread" and is not considered acceptable.
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Old February 29th, 2008, 10:36 PM   #8
danesh
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Quote:
Originally Posted by DickDV
I guess my earlier post wasn't very clear. I meant to say that I would use a VFD but only put PID control on the pump discharge pressure. I don't see any benefit in closing up the speed loop in addition to the pressure loop.

Danesh, you need to start a new thread. You probably don't mean any harm but injecting a new question like you have done is called "hijacking a thread" and is not considered acceptable.
,

I am Very Sorry, Excuse me. I will start new thread.
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