AB VFD Help on a Saturday!?

[Beryl]

I am in need of some help this weekend. I sadly have to work on a AB 1336 VFD. I have it set up in a PI control mode. It works fine but, I can't seem to get the min Frequency to work. I have the min Feq set up in both the set up and Advanced set up areas. I would imagine that I just need to enable this to work or set up a different param somewhere else. Anyone know off hand.


Thanks,
Beryl

 

[Thomas Sullens]

4to20

Hi Beryl
Here`s an example out of the book. Hope this helps.
Input Mode 2 - Application calls for HIM
speed command or 4-20ma from PLC. The drive is programed as follows:
[Freq Select 1] = Adpter1
[Freq Select 2] = 4-20mA
With Speed Select inputs 2 & 3 open and the selector switch set to Remote (Speed Select 1 closed) the drive will follow [Freq Select 2] or the 4-20mA. With the switch set to Local (Speed Select 1 open)
all speed select inputs open and the drive will follow the local HMI (Adapter 1) as selected with [Freq Select 1].
terminal 26 = Speed Select 3
terminal 27 = Speed Select 2
terminal 28 = Speed select 1
When Select 1 is closed terminal 28 is Hi
when select 1 is open terminal 28 is low
Good Luck :site:

 

[Beryl]

So let me get this strait. For me to be able to have the minimum feq I have to cause the VFD to ignore the PI with an input to 28? This makes no sense. I would think that you could enable the min freq withing the HMI in the PI options params. I don't have a PLC or anything connected to the VFD. It is a closed loop PI. So would I use a DO to input back into the DI? What settings would I used to say when to switch to standard control instead of the PI?

I'm getting lost...

banghead

 

[PhilipW]

I'm reasonably familiar with 1336 ..which version are you using? 1336 Classic, 1336 Plus, 1336 Plus II? and which firmware version?

Can you please enlarge on what you mean by the Minimum Frequency not working?

In normal operation the drive always starts at zero and when commanded to start ramps to Min Freq. The Speed Reference signal is then scaled between Min Freq and Max Freq.

If however you are using the PI loop in a 1336 Plus II, parameters 223 and 224 also define the Negative and Positive Limit for the PI Output.

 

[DickDV]

When you set the Minimum Frequency and Maximum Frequency Parameters, you define the limits in which the drive output is able to operate. It doesn't matter whether the drive is operating in PID mode or any other mode.

Do not confuse Min Freq and Max Freq with the Frequency limit settings for the analog inputs. You can set the Analog Input Minimum Frequency for whatever minimun you need to get the scaling and offset your input signal requires. The same is true for the Analog Input Maximum Frequency. These can be inside or outside the Min and Max Frequency limits however, the limits prevail if the Analog Max and Mins are outside.

When operating in PID mode, you have to be careful in setting the Minimum Frequency too high. If it is, the PID controller cannot respond properly when the process signal runs high and the drive must slow down to reduce the error.

 

[Beryl]

It is a 1336 Plus II.

The problem I'm having is that in certain conditions the PI control will allow the drive freq output to drop to say 12Hz. I don't want the drive output drop below 25Hz. It appears to me that when the drive is in PI control mode, the min freq output does not apply.
I am not refering to the Pos and Neg output freq that are set in the PI control section of the drive. Those are set to an aceptable range for allowing the drive to ramp up and down based on the A/I input freq.
Thanks for the help so far. I'm still looking. AB tech support didn't have any ideas other than using an external contoller to enable and disable the PI control. That is not an option...

Thanks,
Beryl

 

[Beryl]

Well I guess I'm just screwed. I can't find any simple options to accomplish what I need. Thanks for the help/ideas.

Beryl

 

[Tom Jenkins]

I don't believe A-B can't accomplish what you want. Keep bugging tech support, and contact the distributor that you bought the drive from and raise holy heck with them. They often have some guru back at the factory, not accessible to the great unwashed masses, that they can go to for answers on tough problems.

One thing you can check out is using the frequency skip function. Most VFDs have the ability to skip certain frequency ranges to avoid harmonic vibrations near critical speeds. You could set the skip range from 1 Hz to 25 Hz and stay out of that operating range no matter what the PID does.

 

[Beryl]

Thanks Tom, I hadn't thought of the Skip. If I can't get any answers from AB I'll give that a try. I don't think starting the pump/motor at 25Hz should be a problem.


Thanks,

 

[Thomas Sullens]

Hard to understand

Berl if no speed select function is being used Set Par. 5 Freq Sel = 0
if speed is being controlled by 4 to 20 Set Par 6 = 2 (4 to 20mA)
in Par 84 Set how you want to follow the signal 1 or 0.If Par 84 is set to 0 when the drive sees 4 mA it should run at what ever Hz you have in Par 16 Set Par. 19 to what ever your 20mA will be. To understand these manual what ever you do DON`T TRY TO THINK LOGICAL!!!!!! And don`t forget some Par. ONLY take affect after the MAIN power is cycled.
Tom I`m like you I believe it can be done . It`s just trying to understand the manuals. banghead

 

[Beryl]

Thomas,
Thanks for the ideas. You may have lost me but I think the 4-20 control you are talking about is different than my pressure transducer input. Havn't looked at the manual though regarding your ideas. I'll let you know if it worked.


Thanks,
Beryl

 

[eastkodakguy]

AB

I am not sure about the 1336, but can select the minimum and maximum value it sees on the analog input, if so find out the what the value of the input is at when the drive is running 25hz the set that as the minimum value. I have done this before us AB's powerflex drives and it seemed to work ok

 

[tom_stalcup]

Just thought I'd toss something out there.... What are your current levels when the freq is down 25 Hz? I don't know about this specific drive, but I've had problems before on the 160's where the application would bog down at the lower values of frequency.

 

[Beryl]

What are your current levels when the freq is down 25 Hz? I don't know about this specific drive, but I've had problems before on the 160's where the application would bog down at the lower values of frequency.

I haven't looked and the current at this freq, and I can't look right now but, the system doesn't bog down. However it does have a mechanical braking that starts to kick in around 12Hz.

Still trying.
Beryl

 

[Beryl]

Here is yet another option from the factory. Just doesn't make sense to me...

If we used a 700 with a vector cassette, we could do it. That not being an option, the Plus II needs external help to deal with this. We don’t need to disable the PI. Assuming he is maintaining a pressure in a pipe, simplest answer is a pressure switch. A pressure switch has two settings. The high setting opens the contacts, the low setting closes them. So, by running the start input to the drive through the pressure switch, we can control when the drive starts and stops. As the demand falls off the drive tries to go to a minimum speed. However, because the demand is so low, the pressure continues to build because the drive is at a minimum speed. At a certain point, the pressure switch will open up (some pressure just above the setpoint). The drive stops. Then, when usage picks up, the pressure falls off. When it drops to the low level setting on the pressure switch the switch closes again providing a start input to the drive. The drive goes back to maintaining the setpoint.

BTW I'm not going to use this method. I think I can tune the system to keep above the 12Hz I'm worried about. Besides it really wouldn't work for what I want because the pressure doesn't drop off. Oh well thanks for everyone's help.

Beryl