PowerFlex 525 Accelerates Out of Control

[sym_man]

I have a 15HP PowerFlex 525 running in sensorless vector mode. It is running a positive displacement pump that is moving material from a tank (45' high) to open air at ground level.

It will run at its set frequency for a period and it seems at some point that the VFD loses control. My theory is that the head pressure from the tank is causing the pump to spin faster than the VFD is driving it.

I don't have any braking resistors or other hardware on the drive. I have 10+ facilities with the same setup in place and I've never run into this issue.

Any suggestions?

Here's what it looks like:
http://webmshare.com/play/wwePG

 

[Gene Bond]

It sounds like the motor is being over-driven by the head pressure, and the drive doesn't have anywhere to put the energy (like a DB/Shunt resistor), so it is accelerating the motor to try to dump the DC bus back down to a reasonable level.

You must have something set to avoid Over-Voltage trips? Typically a drive would trip under this condition, rather than go into bus-regulation (runaway) mode... I have had this happen numerous times (with other drives) when I set braking/regen torque limit too low, and have an overhauling load on the motor.

You could try a DB/Shunt resistor to dump the excess energy, or go with an active front-end (regen) drive...

 

[sym_man]

It sounds like the motor is being over-driven by the head pressure, and the drive doesn't have anywhere to put the energy (like a DB/Shunt resistor), so it is accelerating the motor to try to dump the DC bus back down to a reasonable level.

You must have something set to avoid Over-Voltage trips? Typically a drive would trip under this condition, rather than go into bus-regulation (runaway) mode... I have had this happen numerous times (with other drives) when I set braking/regen torque limit too low, and have an overhauling load on the motor.

You could try a DB/Shunt resistor to dump the excess energy, or go with an active front-end (regen) drive...

That's the weird thing. I would expect the drive to fault but it doesn't. I haven't modified any parameters besides the control source (Ethernet/IP speed and start reference), IP address, acceleration time (2.0 seconds start/stop), and overload current.

They're progressive cavity pumps, I'm going to have the operator tighten up the stator on the pump to reduce the slip between the stator and rotor, which should reduce the amount of material forcing its way through the pipe and hopefully reduce the driving effect on the motor.

The weird part is, as mentioned before, we have several other facilities with the exact same setup that have never encountered this issue.

 

[sym_man]

Another weird part, once I give the drive the stop command it stops almost instantly (~2 seconds). It's set to "Coast/CF". I would expect it to trip on a DC overvoltage once the stop command comes through.

 

[GaryS]

a few questions
is the speed command local (VFD Keypad ) or remote networked 0-10VDC 2-20MA or what
with an open tank the only head pressure would be the level of the tank
a positive displacement pump normally will not be driven backswords from the static pressure.
the rapid stop in a cost to stop would be the head pressure to pump the liquid. nothing to be concerned about there
I would look at the speed command
is the higher speed above the max speed set in the VFD there normally 60 or 120HZ
a command coming in higher then the max speed the VFD will cap the speed to the max in the setup.
a low speed at startup and then going up may be due to current limiting at slow speeds
(lower speed = Lower HP available )the 15HP rating would be at 60HZ below that the HP drops off and above the HP remains flat at 15HP with the torque dropping off

 

[Gene Bond]

Another weird part, once I give the drive the stop command it stops almost instantly (~2 seconds). It's set to "Coast/CF". I would expect it to trip on a DC overvoltage once the stop command comes through.

It doesn't trip on stop, since it 'lets go' of the motor...

It is funny, though, once you remove the magnetizing current from the motor, friction brings these loads down. Like I said, I've seen it. Mostly on fans rather than pumps.

 

[jraef]

That's the weird thing. I would expect the drive to fault but it doesn't. I haven't modified any parameters besides the control source (Ethernet/IP speed and start reference), IP address, acceleration time (2.0 seconds start/stop), and overload current.

They're progressive cavity pumps, I'm going to have the operator tighten up the stator on the pump to reduce the slip between the stator and rotor, which should reduce the amount of material forcing its way through the pipe and hopefully reduce the driving effect on the motor.

The weird part is, as mentioned before, we have several other facilities with the exact same setup that have never encountered this issue.

Do you have the Bus Regulation enabled? Parameter A550, the default is 'Enabled' (1). If enabled, the drive attempts to regulate the DC bus by changing the frequency reference.

When you set the Stop to "Coast", it will not regenerate because the drive ceases to provide excitation to the motor. There might be some residual magnetism that gets a little bit of voltage back off of it, but the transistors would be turned off, so it would not get into the drive. However if the drive is told to decel instead of coast, then it is going to keep energy on the motor, exciting the windings and allowing regeneration back into the DC bus.