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arjun232
Participant

Join Date: May 2009
Posts: 4

External PID Controller in VFD for Pump Control?

05/27/2009 2:12 PM

i our company we have process water pipeline. we have to maintain 6 bar pressure on day and 3 bar on nite. so we have to chg the set pt frequently. see the loop and tell me is it need for external pid controller. currently we are changing the set point in external pid controller only.

we are using ACS550 vfd, currently vfd takes speed refference from external pid controller(that is pid controller itself takes signal from pressure transmitter and it gives control signal to vfd as 4-20ma ).

my doubt is

1. why they selected mode in vfd as external the speed reference

2. why they didnt use pid control mode in vfd

3. if pid control mode is selected, suppose if i want to chg the pressure frm 4- 6bar means,i have to enter interms of mA in vfd-- is it ?

since iam new to industry, i want to know vfd and all plz help me....
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andrew_ward
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Join Date: Jun 2008
Location: Brick, NJ
Posts: 71
Good Answers: 8
#1

Re: External PID Controller in VFD for Pump Control?

05/27/2009 11:07 PM

1. With an external PID you need to use the external reference setting on the VFD. The output basically just follows the input. Using an PID mode you would wire the PT directly to the VFD. Easily done, but see #2.

2. There are a couple of reasons that an external PID is used over the internal PID.

- The vfd is a replacement for an older vfd that did not have a built-in PID and noone wanted to change the controls

- Whoever designed the system was more comfortable with setting up the external PID

- The vfd does not have an external keypad and the operators couldn't change the setpoint (or the engineers wanted the operators to stay out of the other VFD settings)

- The external PID has remote communications (that the VFD doesn't have) and the operators control the setpoint remotely.

- The external PID controller might have an auto tune feature that the ACS550 might not have. I'm not familiar with the 550.

3. Setting the PID mode in the VFD you wire the PT to the VFD and enter the setpoint in the VFD. The setpoint in engineering units (bar). You need to scale the mA input from the PT into bar in the VFD PID setup.

The bottom line is that the designer looks at several factors when they decide where to put the control. It could be company policy, operator comfort level or "that's the way we've always done it".

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kveeresh2
Associate

Join Date: Apr 2009
Location: Bangalore, INDIA
Posts: 34
#6
In reply to #1

Re: External PID Controller in VFD for Pump Control?

05/28/2009 9:05 AM

Mr. Andrew,

Very good and detailed answer....

I appreciate the way you explained.....

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craigza
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South Africa - Member -

Join Date: Nov 2008
Location: Johannesburg
Posts: 295
Good Answers: 12
#2

Re: External PID Controller in VFD for Pump Control?

05/28/2009 1:25 AM

Hi all,

Andrew raises some valid points, but let me add a few.

The VFD is primarily the domain of the electrician. Instrumentation personell typically do not have much to do with this.

The PID controller in the VFD is used to get the VFD to follow an external setpoint as closely as possible without stalling the actual control elelment (the motor). This completely writes of using the VFD PID controller for any usefull form of process control, as your pump would not really be pumping in the lower frequency. You will notice that the PID in the VSD is very very quick, i.e get the VSD to reference as quickly as possible taking into account the current, torque etc.

The external PID controller is a different matter all together. This controller will have a completely diffrent PID settings to that of the VSD. Why - because if the loop is tuned correctly, these PID settings will match your actual process dynamics. Another point to make is that PID usually falls to the C&I personnel, where electrical do not usually have anything to do with this.

I hopw this clears things up a little bit for you.

Regards,

Craig
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Shawn33
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Join Date: Sep 2006
Location: New Jersey, USA
Posts: 178
Good Answers: 8
#5
In reply to #2

Re: External PID Controller in VFD for Pump Control?

05/28/2009 5:52 AM

I believe Craig is referring to the "Speed Loop" PID which is used by the VFD to follow the reference signal. While this PID can not be used in Process Control, many VFD's such as the 550 have other PID's that can be used. The ACS550 had a PID Macro where you can feed the actual signal in on an Analog Input and then either use a fixed setpoint or a different analog input as the setpoint. So the drive would be very capable of doing this kind of control.

There are many reasons, some valid, some not so much, why this may not have been done. But is is something that can be done and you could use a POT for your setpoint. You could even use a timer relay to choose between 2 POTs for your two time dependent set points so you wouldn't constantly have to switch between the two. Good luck!

Shawn
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Anonymous Poster
#3

Re: External PID Controller in VFD for Pump Control?

05/28/2009 2:22 AM

Ya Andrew is very write. It is not a good Idia for Operators to play with the drive because they might change some critical parameters by accident with will distabe the whole process operation. Only trained person is recommended to touch the drive.
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kveeresh2
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Join Date: Apr 2009
Location: Bangalore, INDIA
Posts: 34
#4

Re: External PID Controller in VFD for Pump Control?

05/28/2009 2:33 AM

Very Nice answer...

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Anonymous Poster
#7

Re: External PID Controller in VFD for Pump Control?

05/28/2009 6:35 PM

As previously touched upon, if the controller is sophisticated enough and can accept a contact (digital) input you may be able to configure it to change between the 2 setpoints with the flick of a switch, possibly located in a control room. From there a simple timer (clock) relay that can mimmic the switch and change the setpoint automatically could be cheaply fitted.

Rok
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