Scaling issue- Powerflex 525 on SLC devicenet

lftmx3 · Sep 15, 2021

I'm having a strange issue where the operator can input a desired speed in 0-110%, and the scaling will correctly translate that in Hertz to the drive, only if the operator input stays under 60. Anything over 60, and the drive pegs to the max speed of 90Hz. I've attached a pic of the SCP function.

For example, 60% as an input, will scale to output of 2836, (0-5200 min/max) which then comes out correctly at the drive display/speed as 28.36Hz.
When I put in 75%, it scales correctly and the output integer is 3545, but the drive moves to 90.00Hz. What am I missing here?

drbitboy · Sep 15, 2021

lftmx3 said:
What am I missing here?

Maybe there is another instruction that modifies N14:3, or puts 90 instead of N14:3 somewhere else, if N24:141 is over 60.

Ken Roach · Sep 15, 2021

Thanks for those details; the PowerFlex 525 accepts digital reference commands over any of its network connections where 1 count = 0.01 Hz. There's no onboard scaling from a Reference Word -> Min/Max Hz like there is with older A-B drives.

Follow the N14:3, of course, all the way to where it's copied in the Output word for the 1747-SDN DeviceNet Scanner to see if there's some substitute logic somewhere.

P044 Maximum Frequency in the PowerFlex 525 limits the maximum frequency but doesn't scale or alter it. If your Maximum Frequency is set for 90.00 Hz, then the output value from the SLC has to be 9001 or higher to get capped by that feature. Check that value anyhow to see if it happens to be set for 90.0 Hz; the default is 60.0 Hz.

Check the Preset frequencies too, and check the Command bits for the drive to be sure that they're not switching it to a speed reference other than the network adapter.

The default Preset Frequencies are 0, 5, 10, 20, 30, 40, 50, 60 Hz, so one of them would have to be changed if that's the culprit.

lftmx3 · Sep 15, 2021

Ken Roach said:
Thanks for those details; the PowerFlex 525 accepts digital reference commands over any of its network connections where 1 count = 0.01 Hz. There's no onboard scaling from a Reference Word -> Min/Max Hz like there is with older A-B drives.

Follow the N14:3, of course, all the way to where it's copied in the Output word for the 1747-SDN DeviceNet Scanner to see if there's some substitute logic somewhere.

P044 Maximum Frequency in the PowerFlex 525 limits the maximum frequency but doesn't scale or alter it. If your Maximum Frequency is set for 90.00 Hz, then the output value from the SLC has to be 9001 or higher to get capped by that feature. Check that value anyhow to see if it happens to be set for 90.0 Hz; the default is 60.0 Hz.

Check the Preset frequencies too, and check the Command bits for the drive to be sure that they're not switching it to a speed reference other than the network adapter.

The default Preset Frequencies are 0, 5, 10, 20, 30, 40, 50, 60 Hz, so one of them would have to be changed if that's the culprit.

Just to rule it out as the problem, is it possible to set all 3 speed reference parameters to 4(network)? I’m not at work anymore but in the morning I’ll check to see if anything else is dumping into that dnet file for the speed.

Also, Ken, are you saying that if they truly want the motor to reach 90hz in a controlled man er (p44 is 90), then the scaled max needs to change to 9001?

Ken Roach · Sep 16, 2021

are you saying that if they truly want the motor to reach 90hz in a controlled manner (p44 is 90), then the scaled max needs to change to 9001?

To clarify: tthe cutoff function of P044 engages when the reference value is greater than 9000. It doesn't alter the scaling, it just limits the output frequency to 90.00 Hz even if the Reference value being given to the drive over the network exceeds 9000.

In general, if you want 110% machine/line speed to be 90 Hz, then the final output of the scaling logic that ends up being sent to the PowerFlex 525 via DeviceNet needs to have a range from 0 to 9000.

If N14:3 really is the value copied/moved into the O:slot.word address that maps to the PowerFlex Reference word over DeviceNet, then your SCP instruction should have an input range of 0 to 110, and an output range of 0 to 9000.

In addition, parameter P044 (Maximum Output Frequency) in the PowerFlex 525 has to be set to 90.00 Hz.

Ken Roach · Sep 16, 2021

Also, yes: you can set Speed Select 1, Speed Select 2, and Speed Select 3 (parameters P047, P049, P051) all equal to 4 (Network Opt) to eliminate any possible mis-selection of the Speed Reference. See page 86 of the PowerFlex 525 user manual for the meaning of each value for those parameters.

When I'm controlling a VFD exclusively over the network, I do three things:

1. Wire all the unused Digital Inputs to DC common.
2. Configure all the unused Digital Inputs to "unused" instead of their defaults.
3. Use PLC logic to set all the unused Reference Select bits to 0.

While it is very uncommon for induced voltages or fat-fingered RSLogix operators to inadvertently set a Reference Select incorrectly, those steps all come from experience.

ASF · Sep 16, 2021

One other thing to check for is skip frequencies. I've never used them on a PF525 so can't even say for 100% certain that the PF525 has that feature, but it's possible someone has configured the drive to skip all frequencies from 60Hz to 90Hz. My understanding is that this is often done to e.g. quickly bypass a resonant frequency on a fan to avoid damage, but it's possible someone has accidentally set it up to skip the whole 60-90Hz range.

But yes, I would first be tracking that signal all the way through to the drive and seeing what speed reference it is *actually* receiving. You should be able to view both the reference and the reference source in the drive status tags.

Scaling issue- Powerflex 525 on SLC devicenet

lftmx3

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drbitboy

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Ken Roach

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lftmx3

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