PowerFlex 755 VFD misrepresenting frequency?

phuz · Apr 10, 2019

We have a new installation with a brand new PowerFlex 755 (20hp). Motor nameplate is 1175 RPM, yet I was told it was running a little over 1200rpm at 60Hz. Two meters were used to test the output frequency and both verified that the actual output was 2.3Hz higher. What could cause this?

rupej · Apr 10, 2019

phuz said:
We have a new installation with a brand new PowerFlex 755 (20hp). Motor nameplate is 1175 RPM, yet I was told it was running a little over 1200rpm at 60Hz. Two meters were used to test the output frequency and both verified that the actual output was 2.3Hz higher. What could cause this?

Is the VFD putting out 60Hz or 62.3Hz? You're sort of saying it both ways. Also, how do you know the RPM, do you have a tach or encoder? If it's putting out 62.3Hz and you're commanding 60Hz, there may be some sort of slip compensation enabled.

If the drive is putting out 60Hz and the RPM is close to 1200RPM, the motor may just be running at very low load.

kekrahulik · Apr 10, 2019

I would assume that you have slip comp enabled. It is a 1200RPM motor that runs at 1175RPM at 60Hz due to losses. Slip Compensation tells the drive to run at a little higher frequency in order to achieve that perfect 1200RPM.

specifically: Final Freq = 1200/1175 * 60 hz = 62.277 hz

jraef · Apr 10, 2019

Did you check the output frequency or the actual motor speed? If you put a meter on the drive to check the output frequency, it had better be one heck of a sophisticated meter to be able to accurately filter out the hash that is coming from the transistors. The drive (in any kind of vector mode) is looking at the actual motor rotor speed and may be tweaking the output to attain the desired EFFECT in the motor (basically what the others were describing).

kekrahulik · Apr 10, 2019

If you would like it to run at 1175RPM @ 60Hz, enter 1200RPM as the motor nameplate speed.

phuz · Apr 10, 2019

I just found out some more information.
The output at 60Hz is actually 62.3Hz, but the RPM appears to be "hunting."
It's a very consistent load (several thousand pounds of rotating metal through a few reducing gears) so I wouldn't expect this behavior.
What parameters might affect this?

kekrahulik said:
If you would like it to run at 1175RPM @ 60Hz, enter 1200RPM as the motor nameplate speed.

Why would be enter anything other than the nameplate data?

Lynx777 · Apr 10, 2019

As others have stated, this sound like slip compensation working to keep the motor speed at the requested RPM. In order for anyone to try to give you a better answer more info is needed.

Such as,
1. are you using an encoder,
2. what control mode is the VFD programmed for,
3. have you done all of the auto tuning steps specified in the manual

There are plenty more that I haven't listed but these are a good start.

phuz · Apr 10, 2019

kekrahulik said:
If you would like it to run at 1175RPM @ 60Hz, enter 1200RPM as the motor nameplate speed.

Lynx777 said:
As others have stated, this sound like slip compensation working to keep the motor speed at the requested RPM. In order for anyone to try to give you a better answer more info is needed.

Such as,
1. are you using an encoder,
2. what control mode is the VFD programmed for,
3. have you done all of the auto tuning steps specified in the manual

There are plenty more that I haven't listed but these are a good start.

No encoder, we are just setting a speed and "hoping" the motor follows it.
Pretty sure the VFD is in sensorless vector mode, but I'll confirm when I'm back on site in a few hours.
This is where you slap my wrist because I have NOT done the Autotune. I'll add that to my list for this visit.

alan_505 · Apr 11, 2019

If it is a fairly large mass I would definately do an inertia tune with the load connected.

Alan

phuz · Apr 11, 2019

I managed to find the slip compensation in the drive was set to 249 RPM...wow!
I backed it down to 20 and motor is right on the money.

I tried to do a static tune, rotate tune, and inertia tune, but the drive never moved...it just stuck in a state of waiting.

alan_505 · Apr 11, 2019

Is it network controlled drive or hard wired?
I hard wired you may need to change the inputs to not used whilst doing the autotune.

Alan

phuz · Apr 11, 2019

Network-controlled, and while I disabled the "Stop" output (to take the drive out of StopAsserted state), I did not inhibit the drive. I bet that'll do it. I'll try Friday AM.

PowerFlex 755 VFD misrepresenting frequency?

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