bake time verification

janner_10 said:

You are over thinking it, whatever gearbox changes etc have occurred in the past doesn't matter, you just have to deal with what you have got right now.

Just time the runs at 2 different frequencies and write a new formula to achieve them. Forget all the fog about slip, it's 20 minutes work at tops.

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I concur, but I would take one more step if the VFD reference is an analog signal:

Validate (and calibrate if necessary) the speed command signal going from the PLC to the VFD. You don't want to get this all working with new numbers and then find out that a 4 to 20mA signal is out of whack.

I'd check the command signal first (before measuring bake times), and while you are at it, make sure that the VFD speed scaling of that command is programmed to match. Some drives let you scale the analog input in one place and apply that result to the speed command reference through yet another set of scaling parameters. 4 to 20mA might be 0 to 100% in one monitor which might correlate to 30Hz to 60Hz by the time it comes out the power terminals.

You don't have to do all that, but I would recommend it so that at least you can recreate the settings in the drive next time it fails and get the same performance. If the speed command is digital via Ethernet or serial, skip all this and KISS.

drbitboy said:

So the original CPT formula (60/7.08)*(40/target_time)*(32767/100), which is the same as approximately (111075/target_time)* sent 12342 to the VFD, which is 38% of 32767 (signed 16-bit full scale), suggesting full scale (32767 sent to VFD) would be 49Hz/.38 = 130Hz.

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From here (and many other places (e.g. manual):

danny.michael said:

For the PowerFlex 700 drive, the speed reference is 0-32767 representing either 0-Max Frequency or 0-Max Speed. The default is 0-Max Frequency and the default for Max Frequency is 130 Hz. ...

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So even though OP did not say this is PowerFlex 700, it seems likely that VFD is that or similar, and is consistent with an eventual solution of changing the 7.08 (after calibrating the I/O as @OkiePC suggests), or make a simple [DIV constant target_time frequency_reference] rung with comments summarizing your timing experiments.


[Posting this for future reference; being a noob, I was clueless about the 32767 and 130Hz max; presumably that is to allow motors to run at least twice 60Hz speed]

Often the min/max HZ of the VFD is adjusted in the parameters to stop people from under/over driving them, so it is likely that a new VFD has been fitted but not set up as per the original.
Choices:
1. Adjust the calculation
2. Adjust the analogue output
3. Adjust the VFD MIN/MAX speeds