extruder vfd question

[jcraft]

We purchased a used extrusion machine a year or so ago that extrudes plastic tubing and at that time we found it had a bad variable speed drive. We redesigned the control system somewhat and added a new plc ( which the machine didn't have) and a new Allen-Bradley vfd( not sure of exact model). This machine has a 25 hp motor with encoder feedback. The speed is controlled via potentiometers on the operator panel. The drive was tuned and it has been running great for a year or so. However, now the operators are complaining that the speed fluctuates via the rpm gauge on the operator panel which makes the screw in the machine pulsate. There could be mechanical issues at work here however, I am not that familiar with vfd's so I am wondering if this could be a tuning issue for the drive or something? We actually have one other simliar extrusion line in the facility that has the same control scheme ( vfd and potentiometer) but have never really had an issue with fluctuating speed with it. That one we purchased new. Any thoughts if this could be a motor/vfd issue.

 

[jde777]

1. Could be the encoder coupling slipping.
2. If it is a digital drive you can go to the display and monitor speed error.
3. In addition I would monitor the motor current to see if the speed is fluctuating as a result of the load rapily changing.
4. Or simply a bad potentiometer/dirty spot ect..
5.. How much is the speed fluctuating? Is the meter just driven off an analog output of the drive or directly from the encoder?

 

[jcraft]

At present it it is just an analog meter driven off the analog out on the drive. Not sure at this point how much the speed is fluctuating as we haven't been there to see it under load. We are investigating that. We did change the speed pot out to a new one. This motor is coupled to a gearbox and the screw is belt driven so there could be some mechanical issues we need to check

 

[waross]

A loose drive belt may be slipping intermittently, particularly if the load is pulsating.
respectfully

 

[rawelk]

How close to full load torque is this machine running? It depends on how the drive is set up, but one possibility is the extruder is heavily loaded, hitting current limit, and dropping speed.

 

[ScottyUK]

If you suspect the pot, replace it with two fixed resistors. Solder the joint between them and hardwire them to the drive terminals. Don't overlook the possibility of an over-tightened screw in a PCB terminal on the drive cracking the soldered joint around the pin. A failure exactly like that on a fire panel intermittently tripped one of our steam turbines for a year before it was finally hunted down. Under a good light they show up as a dark halo around the pin to the naked eye. Under a low power microscope or a powerful loupe the bad joint is usually visible. It is harder to see them with the new lead-free solders which are duller in finish than good ol' 60/40 or Sn62.


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Sometimes I only open my mouth to swap feet...

 

[DickDV]

Go to the Metering section in the drive and look at the speed reference. If it is not changing and the drive output speed is changing, you probably are in vector mode and the speed loop tuning is too soft--you need more P and I gain. Or, as mentioned above, you are running into current limit and the speed is dropping to get away from the overcurrent condition.

If the speed readout is from the analog output of the drive, the problem can't very well be slipping belts or such, can it?

If the speed reference is changing, then you don't really have a drive problem. The problem is in your speed signal. The pot has already been changed so check your wiring and shielding. Remember, a noisy speed reference or a vibrating potentiometer can produce irratic speed input signals.

 

[DickDV]

One other detail I missed on first reading. Again, go to the Metering Section of the drive software and find the speed feedback signal from the tach/encoder. With the extruder screw empty, use the drive keypad in local control mode to set a speed near the usual operating speed. Then watch the speed feedback signal. It should be rock steady. If not, you've got a tach/encoder problem.

 

[sunrays]

You could check if the pot, encoder & analog output signal of the drive all are through screened cables and earthed at both ends.

 

[DickDV]

Another thought: With the extruder working at normal load, go to the Metering Section of the drive and watch the motor torque. Extruder torque should be pretty constant. If it is jumping all around, you have a mechanical issue, not a drive issue.