AB 1336 VFD Frequency Current to Freq Calculation

[davidg68124]

I guess the way I am understanding things is this, he has one of the analog outputs from the 1336 drive set up to retransmit the drive current back to the PLC. I was also understanding that he wanted some type of formula to use in the PV+ to convert that current to frequency. Unfortunately, that is not possible without more information coming from the drive.


On the other hand, if I understand RSDorin, if things are the other way around, and you are reading the commanded output to the drive from the SLC, ie, the speed reference to the drive, you can use the scaling of the tag in the PV+ to allow you to display the COMMANDED frequency. But that still is not the actual running frequency of the drive.

David

 

[SNK]

I guess the way I am understanding things is this, he has one of the analog outputs from the 1336 drive set up to retransmit the drive current back to the PLC. I was also understanding that he wanted some type of formula to use in the PV+ to convert that current to frequency. Unfortunately, that is not possible without more information coming from the drive.


On the other hand, if I understand RSDorin, if things are the other way around, and you are reading the commanded output to the drive from the SLC, ie, the speed reference to the drive, you can use the scaling of the tag in the PV+ to allow you to display the COMMANDED frequency. But that still is not the actual running frequency of the drive.

David

Yes, this is the situation.
So, is the 0-100 integer used in the SCP supposed to be directly corelated to the Frequency? If so this is easy,however, I don't beleive it for a second.

I will have to go online and check visually.

 

[SNK]

Should have quoted the second paragraph...

 

[rsdoran]

This is getting more complicated than it should be. The thing to do is obtain the appropriate manual for the drive and determine what "reference" is being used for speed...by default it is set to "0" which is frequency.

If the 1336 is set to frequency then its basically using 12 bit resolution ...ie 0 to 32767 (I am not sure where the 30804 comes from) to send to the drive.

In the example you have shown using 0-100 and the fact that the drive mentioned is configured for 95HZ then it seems that its a direct correlation to frequency....this is easily verified by looking at the N7 register then looking at the drives display. This may also be where the 30804 comes in, it would limit the max output to the drive at approximately 95HZ...ie 32767-30804 = 1963 32767 divided by 100 (hertz) would = 327.6 1963 divided by 327.6 units per hz = approximately 6 HZ 100-6 = 94 which is approximately what the drive's max frequency is set too.

Personally I think it is that simple, in the old days many simple people like me did it just that way.

As noted this will display "command" speed, not actual speed.

 

[SNK]

I like it Rsdoran. I will test this tomorrow.
Thanks for the help,

 

[panic mode]

actually 12 bit is 0-4095 while 0-32767 is 15-bit...

 

[rsdoran]

panic you are correct...did not think before writing. Most of the 1336 drives I dealt with primarily used 4096 ...ie 2^12. The newer (still possibly old) models could use 15 or 16 bit resolution.

 

[davidg68124]

rsdoran is quite correct. IT is probably just that simple.


David