Vfd logic- please review and comment

I have attached logic which i made for the operation which is described below. This is the detail of what i need. Please have a look and let me if i am on right page or not. Thanks

2 Motors, 2 VPDS

Motor (1) and VFD (1) will work as primary. Motor (2) and VFD (2) will work as a backup means if motor (1) and VFD (1) will stop working. Motor (2) and VFD (2) will wait for 10 seconds and start working after failure of motor (1) and VFD (1)

For inlet pressure we are using pressure transmitter PT1 (range is 0-0.75 psi)
For outlet pressure we are using pressure transmitter PT2 (range is 1-10 psi)

(** instrument PT1 and PT2 range is -30 HG-0-30 PSI)

Inlet pressure should always be greater than 0.25 psi and outlet pressure should always be between 3-7 psi. If outlet pressure is less than 7 psi motor will speed up but before speeding we have to see the inlet pressure also. Motor will only speed up if inlet pressure is also above 0.25 psi. If inlet pressure is less than 0.25 psi motor will not speed up and wait for inlet pressure to come above 0.25 psi.

If inlet pressure is less than 0.25 psi and outlet pressure is less than 3 psi. Alarm will come called low inlet pressure.

If outlet pressure is above 7 psi motor will slow down until pressure drops below 7 psi.

In short, if outlet pressure is less than 7 psi motor will speed up by keeping inlet pressure in mind and if outlet pressure is above 7 psi motor will slow down.

Jamalshahid,

This appears to be an RSLOigx 500 program for a Micrologix 1500. You have logic on the rung in front of the PID instruction. In RSLogix5 and Logix5000 platforms this is ok, but on RSLogix500 platforms this is a huge no-no. If the rung evaluates as false the instruction will clear the 32 bit integral sum word and that is usually not desirable and may lead to undesirable results in your control. On SLC 500 and Micrologix platforms the PID has to be on an unconditional rung.

I need feedback please.

Please share your views. Thanks

jamalshahid said:

I have attached logic which i made for the operation which is described below. This is the detail of what i need. Please have a look and let me if i am on right page or not. Thanks

2 Motors, 2 VPDS

Motor (1) and VFD (1) will work as primary. Motor (2) and VFD (2) will work as a backup means if motor (1) and VFD (1) will stop working. Motor (2) and VFD (2) will wait for 10 seconds and start working after failure of motor (1) and VFD (1)

For inlet pressure we are using pressure transmitter PT1 (range is 0-0.75 psi)
For outlet pressure we are using pressure transmitter PT2 (range is 1-10 psi)

(** instrument PT1 and PT2 range is -30 HG-0-30 PSI)

Inlet pressure should always be greater than 0.25 psi and outlet pressure should always be between 3-7 psi. If outlet pressure is less than 7 psi motor will speed up but before speeding we have to see the inlet pressure also. Motor will only speed up if inlet pressure is also above 0.25 psi. If inlet pressure is less than 0.25 psi motor will not speed up and wait for inlet pressure to come above 0.25 psi.

If inlet pressure is less than 0.25 psi and outlet pressure is less than 3 psi. Alarm will come called low inlet pressure.

If outlet pressure is above 7 psi motor will slow down until pressure drops below 7 psi.

In short, if outlet pressure is less than 7 psi motor will speed up by keeping inlet pressure in mind and if outlet pressure is above 7 psi motor will slow down.

Click to expand...

This looks like a good control design. Parameters well defined. All appropriate "outputs" well defined.

Sensing pump suction to ensure NPSH (net pos suction head) is a real good idea. IF your VFD has terminals to set motor speed at Y ie minimum speed then it may work OK.
CAUTION do not run pump too slow ie below shut off head RPM (poor term??) - it MUST pump some fluid to stay cool.

Am not sure you can do this with a VFD alone. IF that is really what you want to do it will take some careful shopping and selection. I would consider a small PLC or maybe a "smart relay"? You may need PLC just to satisfy desire for switchover on pumps.

Dan Bentler

Alaric said:

Jamalshahid,

This appears to be an RSLOigx 500 program for a Micrologix 1500. You have logic on the rung in front of the PID instruction. In RSLogix5 and Logix5000 platforms this is ok, but on RSLogix500 platforms this is a huge no-no. If the rung evaluates as false the instruction will clear the 32 bit integral sum word and that is usually not desirable and may lead to undesirable results in your control. On SLC 500 and Micrologix platforms the PID has to be on an unconditional rung.

Click to expand...

Alaric,

You are correct. Undesirable circumstances definitely happen, especially with over a thousand threads running on a machine.

Lesson learned.

Sir,

I followed your suggestion.
· PID in now on unconditional rung.

My next question is :

How to control Ramp for accerlation and deaccerlation in VFD.
Can i have write some logic or VFDs are set via parameters in the VFD themselves means i have to do that manually. Please help!!

If i have to write logic what i have to write as i already attached my program.

I need feedback please. Please share your views. Thanks

Click to expand...

Jamal,
It looks fairly good, except like Alaric said, On Rung 007, remove the two instructions to the left of the PID block. It appears that you are trying to only use the PID block when a change is speed is needed. That will not work very well, as the PID instruction is designed as a continous function. I think that only enabling it intermittently cause eratic results.

For most VFD drives with which I am familar, usually there are preset speeds that can be controlled with a PLC output. You could possibly use your SpeedDown and SpeedUp relays to control preset speeds, but I don't think that would work very well.

Instead, you need to somehow put restrictions on the output of the PID that only let the speed change under the acceptable pressure conditions.

One method is to add two new rungs after Rung 008 that creates a new value called "Last Speed" N12:2.

New Rung 009: Do Comparison for N12:1 and N12:2. If N12:2 < N12:1 AND SpeedUP B3:35/0 = ON, then MOVE N12:1 to N12:2.

New Rung 010: Do comparison for N12:1 and N12:2. If N12:2 > N12:1 AND SpeedDown B3:35/2 = ON, then MOVE N12:1 to N12:2.

If neither SpeedUp or SpeedDown is ON, then speed should stay the same as old last speed N12:2. Also change the old Rungs 009 and 010 (now rungs 011 and 012) to MOVE the new N12:2 to Output O:10.0 and O:10.1.

Thank you sir!!

How to control Ramp for accerlation and deaccerlation in VFD.
Can i have write some logic or VFDs are set via parameters in the VFD themselves means i have to do that manually. Please help!!

Click to expand...

There are two methods, (1) using only the functions internal to the VFD, OR (2) creating a ramp inside the PLC program, and switching the PID block to Manual Mode when this ramp is applied.

Method 1: You VFD will at least have several parameter that can be manually set to control the Acceleration Time. If you set the VFD Acceleration Time to 10 seconds, then the drive must not accelrate any faster than 0 to Maximum Speed in 10 seconds. There will also be a setting for Deceleration Time

In addition, many VFDs now have other built in acceleration ramps that can be pre-defined.

First, decide which method is most useful for your particular VFD drive brand and model. If you need a very specific and complicated ramp, then you may need to use Method 2 and do it in your PLC program.

I am using BLADOR VFD DRIVE.
Which method you recommend. programming?

Brand BLADOR
VS1MD microdrive

I am sure that the Baldor VFD has the Acceleration and Deceleration Time settings. I would try setting those first and see if that does the job.

For the Baldor VS1 Microdrive, Acceleration Time is set with Parameter P41. The range is 0 to 6000 seconds, default factory setting = 5 seconds.

Parameter P42 sets the Deacceleration time, range 0 to 6000 seconds, default = 10 seconds.

There are also 8 user-defined Ramps that can be used for Accel and Decel. These ramps are controlled with external inputs to Terminals t14 to t27. See the Baldor Installation & Operating Manual, page 7-14. You will only need these if your accel and decel ramps are not straight lines (have steps).

If you need more specific ramps, then we can look at how to add Accel and Decel Ramps to your PLC program.

I noticed something else that may be useful. On Page 7-10 of the Operating Manual, this Baldor drive has built-in Frequency Increase (UP on Terminal 15) and Frequency Decrease (DOWN on Terminal 16) commands. You could use those to control your drive speed with your PLC program UP and DOWN relays. Maybe that is what you had in mind all the time!

I have updated the project. Please have a look and let me know how it looks now. Please see attached file with thread. Thanks

Jamal,

I can not find where you set (control) B3:35/3 VFD_PRIMARY, which you use in Rung 004. I think you only need the new B3:45/3 contact on Rung 004, so you can delete all uses of B3:35/3 on Rungs 004 and 005.

Quick question:

when i save this project in PDF it shows different address and when i open this electronically it shows different address

for ex in pdf it shows B3:35 but when i opened electronically it shows B3/563 why this is??