AB 1336 VFD Frequency Current to Freq Calculation

Hi again,

I am building a PV+ application that interfaces with a SLC5/04 and am communicating on Serial DF1.
I originally was displaying the current (Raw 0-30840)in a scale and was quite happy with the results.
Now, the supervisor is requesting that I display the Frequency of the Drive, rather than the current that I am sending to it.
Is there a calculation that I can do to take the analog signal and turn into a "somewhat accurate frequency" to display on the panel?
I am limited to completing the calculations in the PV+ app, due to the requirements and regulations of our plant. (I cannot modify the PLC code to suit)

BTW, the drives are dumb terminals, not controlnet or anything fancy...they are about 5-7 years old.

Thanks all for the responses.

Well,


Just so I follow, how are you getting the first value? Is there an analog output from the drive to the SLC? That is configured as the current (retransmitter) from the drive? This is what I am assuming to be the case. I cannot remember if there is a second analog output(I believe there is) for sure, but you could do the same thing and have the drive retransmit that value from the drive on the second analog output.

I do not know of any calculation that you could use only the current to determine the frequency. You will need at least 2 variables to determine frequency so then you are back to having to provide the second analog anyway. That being the case, it would be configured to be frequency.

Now if you are using DF1 to communicate to the panelview, there is a signal splitter available that might allow you to have 2 ports on the serial port of the SLC but you would still need to have a comm interface for the drive as well, and that is a much more expensive and complicated task to take on. It is NOT worth it for the ability to display frequency.

What industry are you in? I am in the pharma industry and understand change control.

Simple answer to this is no, you cannot. But if change control is required, the first thing to remember is this, do you have a spare analog input that is configured? If so, just hook up the signal, access the I/O register table with the PV+ and do the scaling calculation there. There would be no software change control, so that might make it paletable to your superiors.

I may be off base, but I think this is the only way you will be able to do what you are looking for.

David

The 1336 uses a frequency command (normally) which can be configured as an analog input and can be monitored as an analog output.

If you are using the 4-20ma output option for 0-60HZ then each HZ would be equal to 513.4 of the raw data, you can decide on the necessary resolution. 30804 divide by 60...if max frequency is 60.

Its actually that simple if the analog output being monitored is the frequency reference.

NOTE: I re-read the post. The same applies to using the data for the speed reference signal. It may take a little thinking because there is will be an offset...ie 4ma is equal to 0. In other words the 20ma is equal to 60HZ therefore 12ma would equal 30HZ and so on...

David, thanks for the information.

Yes, you are correct on afew of your assumptions.
I am feeding the analog output from the SLC to the VFD's. (I am displaying this raw value in the PV as the current output).

Yes, I am in the Pharm world.
Actually, we can't even add another electrical connection without approval. BS, I know. If they only knew what really went on.....

Sounds like too much work for me. I was hoping that there was a quick calculation that I could use to display the Frequency that the drive would be running at.

Cheers and thanks for the help.

Sorry, RSDoran got to the link first.

Actually, I have a drive that is maxed out in current, and is driving at 95Hz. (So I guess the scaling is off?)

So, If I interperet correctly, just wire all of the drives outputs to the AI card with associated inputs and do what in the PLC?
I cannot scale or calculate in the PLC. I can only do basic calculations in the PV+ app with the addressed online tags in order to get away with adding more logic.

Thanks RSdoran for your help.

You stated you were displaying the current in the PV, the 4-20ma current is directly proportional to the frequency. The raw data can be converted to 0-95 using the same method you did to obtain 4-20ma....which I assume is what you have done.

I will try this again

I am not as eloquent, literate or experienced in some areas as some of the others so please bear with me. Maybe with my responses it will stay high so others will provide a better answer.

I read that you were using the raw data of the analog output to the 1336 drive on the PV+ and converting it to represent the analog signal sent to the drive. You did not state specifics of the analog signal so I used 4-20ma as an example but "assumed" this because you stated current.

What I was stating is that the analog signal sent to the drive is directly proportional to the frequency if the drive is configured for frequency reference.

If the drive is configured for frequency reference then you can directly convert the raw data to 0-95HZ. I am not sure of the specifics of your raw data but if you are NOW converting that raw data to obtain 4-20ma you could just as easily change the values to 0-90 and obtain your HZ speed reference.

Thanks Rsdoran,
In the code as attached, I am referencing the RAW data.
If there is a good way to do this, please let me know.
They are scaling to a final of 0-100. Do you suppose this is Frequency?

95hz?????????????

Is anyone else bothered by a more than 50% overspeed of the motor? Or is this a special motor?

brucechase said:

Is anyone else bothered by a more than 50% overspeed of the motor? Or is this a special motor?

Click to expand...

Inverter rated motors are basically designed for at least 90HZ and in some cases up to 120HZ operation....this is assuming the motors base frequency is 60HZ.

KC, I missed one detail...how is the command being given for the speed reference. Looking at your snippet of code it "appears" that a reference of 0-100HZ is being used. IF that is correct then it seems you can just reference N7:32 to obtain your frequency (HZ) command.

Direct copy of N7:52 to Output Word O:5/1, and so on.
There are 10 drives in total, and most are running normally at 40-65Hz. There is only one Drive that is reading 95Hz.
I, too wondered about the 95Hz. Motor and components all seem OK.
Thanks for the help.

I am still not clear as to whether I can determine the Frequency based on the Integer/Analog information available.

Cheers and Thanks for the reply's

I am still not clear as to whether I can determine the Frequency based on the Integer/Analog information available.

Click to expand...

I am not clear either on some details but I have worked with 1336 drives. The older ones I am familiar with were usually configured for frequency speed reference. If the drives are configured for frequency and the PLC is sending an analog signal then that signal will directly correspond to the configured min/max settings of the 1336 drive...ie if its set for 0-60 then the raw data 0-32787 (or whatever they may be) could be directly scaled to match. I am not familiar with the PV+ so not sure about the details of scaling or math capabilities of the software.

Where I am unclear is HOW the PLC gets the "command" reference for speed/frequency. It appears that some of the N7 files already have the speed command reference available.

The other "unclear" aspect is that the 1336 drives have an output that directly "monitors" the frequency of the drive...I could not determine if this was being used or not. If so then the PLC should have the "actual" frequency of the drive available and scaled accordingly.

The PLC sends the analog data from the SCP parameter move command. This originates from a Data File, where the set speeds (integers) are stored.
I see the Frequency only on the READER on the front of the Drive.

When you create a tag in Studio, for the PV+, doesn't it have scaling parameters associated with it?
Create a TAG-DATABASE reference to the point (not a direct reference). Enter a value of (100 / 30840) (EU / RAW) into the scaling field, or 0.003242.

See if that works.

I will try once more

The 1336, by default per se, uses an analog input for frequency speed reference.

If the drive is configured for frequency speed reference then the example 0-100 you displayed is frequency being converted to an analog signal. In other words in your example N7:32 was showing 33HZ.

Look at the program and determine if that is the case, if so then you will not need to convert. If its not the case then look at the analog output at different values and match that to the frequency shown on the drives display...it can be scaled to match using something like what rdrast has given.