PowerFlex755 output frequency is different to the reference command

[Matthis]

Hello guys,

I am new to this forum and have some problems with my PowerFlex755 EENET frequency inverter.

I want to do a point to point positioning task but my problem before I can have a look at the point to point positioning is, that the inverter is always running with 5Hz as soon as I give the start command. Even if I write 0.0 to the reference the output frequency is about 5 Hz. When I change my reference speed to 10.0Hz the output frequency is 12.X (see attached screenshots). . I thought that maybe a min frequency of 5Hz was set somehow, but I couldn't find a parameter and that wouldn't explain why I have 12.X Hz as the output frequency with a setpoint of 10Hz. --> see attached screenshots

And an other question, have someone experience with positining tasks with a PowerFlex755 and Point to Point positioning? I think in the first step it is not working because of the problem I described above, but maybe someone have a programming example or could tell me where I can find some information about how to use the Point to Point positioning?



Thank you for advice!

 

[mathieu1881]

If you want to do positioning, you might want to look into programming it as an axis. There is an encoder in the system? The feedback is at 10, based on motor parameters, the drive will compensate the output frequency to get you to 10hz.

 

[Matthis]

But I think I don´t need programming it as an axis if I do it with PtP positioning? Yes, I have a SSI Encoder in the system, this encoder is already installed an the value I get seems good and is changing if I move a magnet on my encoder bar.

I am wondering, why the motor is running with 5Hz even if I stoped the drive...

 

[Lynx777]

It looks to me like you're drive is holding the motor at zero speed. Telling the drive to "stop" the motor is not necessarily the same thing as "stopping" the drive from controlling the motor. Since you have an encoder, the VFD is looking for the encoder input to tell it that it is at zero speed. The output you see is the small amount of torque needed to hold the motor at that zero speed state.

Also, from your tag names it appears that you might be doing some sort of lifting application, so the torque output from the drive is what is required to hold the load against the pull of gravity.

 

[Matthis]

It looks to me like you're drive is holding the motor at zero speed. Telling the drive to "stop" the motor is not necessarily the same thing as "stopping" the drive from controlling the motor. Since you have an encoder, the VFD is looking for the encoder input to tell it that it is at zero speed. The output you see is the small amount of torque needed to hold the motor at that zero speed state.

Also, from your tag names it appears that you might be doing some sort of lifting application, so the torque output from the drive is what is required to hold the load against the pull of gravity.

That makes sense, but in my case I just have a small 0,3kW motor for testing and I do not have a load here in the office.
But because of my parameters regarding the PtP mode this is maybe normal?

I tried to do a PtP task and I thought the problem why it doesn't work is that the motor runs even if I don't give a reference command.... But the problem is probably that the PAR784 PTP command does not change and is always equal to my actual position of the SSI encoder...

 

[Lynx777]

I've never used PTP mode with a PF755, but I have used them in servo mode (positioning mode, axis mode) and they do behave this way, even with no load. So my guess is that in PTP mode it would be very similar. All of that being said, IMO, what you are seeing is normal. The drive is simply keeping control of the motor to hold a given position.

 

[Matthis]

Alright, thank you for the input!

Is it also possible to use an asynchronous motor in axis mode?

 

[Lynx777]

As long as the motor has an encoder it is possible. The control is not as tight as a true servo setup, but it works well for certain situations.

For anyone that's curious. I've got several large "needle style" flow control valves that the flow of material through the valve used to only be controllable within ~ +/- 50lbs from target per minute. Now we can control it within ~ +/- 10 lbs from target per minute. The old control used a PF40 VFD with no encoder feedback and the logic would simply look at the flow rate and then bump the needle in the direction needed based on a timer. The new control uses a PF755 VFD with an encoder added to the same 5hp motor from the old system and a PID for logic control.

 

[Matthis]

I have only an UFB Encoder Card with a SSI Encoder bar connected, so no direct encoder on the motor.
I think this will be a problem?

Sounds nice! So the accuracy is mutch better with the encoder feedback.

 

[Lynx777]

I have no experience with a servo setup where there is no encoder directly connected to the motor, so I'm can not say if it would be a problem or not.