PLC, PowerFlex 70, and DeviceNet

I'm relatively new to VFD's in general so if I'm missing something that seems obvious, it's probably not obvious to me.

I have several new PowerFlex 70's that were installed recently on a DeviceNet network. When I send a speed value from the PLC to the VFD, it appears to be scaled from 0 - 32767 (where 32767 is the max speed). My customer would prefer a scaling from 0 - 9000, where 9000 is 90 Hz (our max speed). Is there an easy way to change the scaling, or do I have to live with using numbers from 0 - 32767?

Thanks in advance

+32767 is the largest 16-bit signed integer number that a PLC-5, SLC-500 or Micrologix controller can handle. In the interest of maximum resolution, that's what the interface designers decided to make the PLC equivalent of the output frequency span of Min Hz to Max Hz in the drive.

That way, it wouldn't matter as drives evolved and got more precise in their frequency resoution... the PLC was already at it's limit of resolution.

That standard was decided back in the Remote I/O interface to 1336 Plus drives, and carried over into the DeviceNet interface to 1336 family and now PowerFlex drives.

I usually use an SCP (Scale with Parameters) or CPT (compute) instruction in my ladder logic to scale my user units to drive units. With any drive you're often going to have to run through a whole set of ratios anyhow: PLC units to output Hz is just the first one. Then it's Output Hz -> Output RPM and through your mechanical drivetrain to feet/min, gallons/sec, or furlongs/fortnight.

This is just the way that 1336 and PowerFlex drives work with DeviceNet interfaces because they were designed to work with A-B PLC's. Some other drives work the same way on DeviceNet, and others (like the A-B Bulletin 160 and the Yaskawa GPD515) you send Hz*100, or RPM, or something else they define. Often it's different depending on what I/O Assembly you specify.

I only ask because I've used the same drive with a Remote I/O connection instead of DeviceNet, and I'm almost positive that my 90Hz speed reference was 9000 and not 32767

So are you saying that the actual value going from the PLC-5 to the DeveiceNet scanner is always scaled from 0 - 32767, and that it can't be changed to Hz*100 ?

I have commissioned dozens of 1336 Plus drives using 1203-GD1 adapters for RIO, but no PowerFlex 70, so I did a little research.

1336 and Powerflex drives all use a network speed reference value from 0 to 32767 to command zero through Max Frequency.

There is no way to change this feature to Engineering Units. There are Min/Max scaling parameters for analog inputs, but not for the Speed Reference when it comes from a network interface.

Aha ! There's a wrinkle with the PowerFlex 70 that was not present in the 1336 Plus.

Max Frequency (parameter 55) is not the only thing that governs how fast the drive will go. There are more parameters, including Parameter 82, Maximum Speed.

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With PowerFlex 70 settings at default, Maximum Speed (Parameter 88) is 60 Hz, and Maximum Frequency (Parameter 55) is 130 Hz.

The 0-32767 span is actually scaled to 0 - 130 Hz (Max Frequency), not 0 - 60 Hz (Max Speed). This effectively cuts the resolution in the normal 0-60 Hz speed range from 32767 to (60/130 * 32767=) 15123.

So, I'm sure that your command range on your 20-COMM-R was not 0-9000, but it's also likely it wasn't 0-32767.

Matt Wuteska from the A-B drives division wrote an excellent technote on this matter. It's available from the Rockwell Automation Knowledgebase as document A6338615.

This is a good question, bbb. I happen to be setting up a PF70 on DeviceNet today and tomorrow and I'll report back what I get out of it.

(it's a very strange DeviceNet... it goes via a Profibus DP gateway !)