700h vfd writing freq command to the drive

remullis

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Oct 2012
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Georgia
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Typically our 700h drives get the freq output number from the Plc by multiplying the percent speed, ex. 50% x 280.86 which writes to a predefined tag for the 700h. Our integrator is now writing ex. 50% x .6 to a predefined tag called commanded.frequency. at 100% we output 60hz. So with the example at 50% it gives it 30hz from the plc but at the drive I'm not getting more than 1.5 hz. I changed this math from .6 to 280.86 and got the driver's to output and actually run. But it's not right. Should it be writing to a different tag other than considered. Freq for the 700h. What number does the drive actually look for. Integrator is on vacation.
 
When I calculate a command for a VFD, I try to include the whole derivation in the rung comments. That helps future troubleshooters (frequently including myself !) figure out what ratios and limits I was considering.

I presume you're sending the speed reference value to the PowerFlex 700H drive over Ethernet or ControlNet.

For those who haven't used them, the PowerFlex 700H was a large-frame version of the PowerFlex 700. "H" for "Heavy" or "Huge" or "Honking"; they ranged from 200 to 1000 horsepower.

The size of the drives is why I'm a little hesitant to give Internet advice. A value of 280.86 for 60 Hz isn't an intuitively obvious value.

Have a look at the PowerFlex 700H Programming Manual.

The PowerFlex 700H could be configured for speed-related parameters to all be in units of Hz or units of RPM. There's a parameter (Parameter 79, Speed Units) that automatically goes through and converts them all from Hz to RPM if you change that parameter. PLEASE don't mess with it, but it's worth looking at to see the value.

Then go to Page 24 and look at the diagram for Parameter 83, the Overspeed Limit. This shows you the relationship between Min Speed, Max Speed, Max Frequency, and Overspeed Limit.

PowerFlex 700H drives don't use engineering units on the network reference. Instead, the full range of the value sent from the PLC is a 16-bit signed integer.

So for most PowerFlex 700H drives, the value of 32767 in the network reference should give you the Max Frequency (Parameter 55).

You can look at that value directly in whatever tag is sent out to the drive, or indirectly as Parameter 272 (Drive Ref Result).

This is different from the PowerFlex 755 family drives, which use actual engineering units, where "30.0" would be 30.0 Hz. It's just speculation, but maybe your integrator copied some code or got confused between 750-family drives and the older 700-family.

So I guess my only really usable advice is... "be careful".
 
Ken,

This was great information. We always setup the our drives for 70hz (par 55) with max speed at 60hz. Parameter 83 is set to 10.

We give the operators control of the motors 0-100% of the max speed. So the math in the plc is normally the speed cmd (ex. 20%) x 280.86 which writes to parameter 275 port value. The problem was parameter 274 "port select" was set to "0" not used. I set this to DPI port 5 which seemed to fix the problem. This gave me 5617 in the value. At parameter 272 the value is 368115712. I appreciate your help in getting down the right path.
 
What PLC type and network are you using to control the drives ?

For exposition:


Parameter 82: Maximum Speed is the high limit for speed reference after scaling is applied. This can be in Hz or RPM.
Parameter 83: Overspeed Limit is the amount of overspeed that slip compensation and other features can assert.
Parameter 55: Maximum Frequency is the maximum frequency that the drive will output.

Maximum Speed + Overspeed Limit <= Maximum Frequency

60 Hz + 10 Hz <= 70 Hz

So this makes sense that a maximum reference (10 volts, 20 mA, or a value of 32767 from the network) will give you 60 Hz output.


DPI Port Select (274) and DPI Port Value (275) are just for monitoring of DPI reference values. When you set DPI Port Select to 5, you told the drive to put the Reference Value from DPI Port 5 (a 20-COMM-? Adapter) into Parameter 275.

The PLC writes to something else, probably a tag in your ControlLogix that is linked to the 20-COMM-? adapter. Its value is shown by Parameter 275.

5617 / 32767 = 0.171, so that's roughly 20% but not exactly.

Parameter 272 is Drive Reference Result, which should be the same as Parameter 275 (DPI Port Value) when the drive is getting its reference from DPI Port 5.

I can't explain why the value of Parameter 272 appears to be 368115712. That's outside of its stated range (it should be only -32767 to +32768), so either I'm reading an incorrect user manual or something else is wrong.

I still think your integrator was probably mistaking a Powerflex 700H for a 755 because of the difference in units, but I can't be sure.
 

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