VFD Fault need ideas

[BearB]

I have a VFD (AB Power Flex 70) operating a 5 hp motor controlling a progressive cavity pump pumping starch material. The system is set up to pump the material for a time then flush for a time after material is used from tank system cycles again to fill the storage tank. This system has been in service for several years without problems and no changes have been made. Several months ago the drive started kicking out on Fault 24 (following is from manual)- Description: Decel Inhibit-The Drive is following a commanded deceleration because it is attempting to limit bus voltage. Action: 1. verify input voltage is within drive spec limits. 2.verify system ground impedance follows proper grounding techniques. 3. Disable bus voltage regulation and or add dynamic brake resistor and/or extend deceleration time.
Changed the pump and had no problems for about 6 weeks. The problem has returned intermit. Increased the Deceleration rate in the drive from 3 seconds to 4 seconds, thought this help but after 3 days the intermit problem returned. Monitored incoming 480vac for 24 hours, looked ok however during this time the unit operated ok.
Any ideas would be appreciated.

 

[DickDV]

Is the power supply 480V floating delta grounded wye? If floating, all kinds of irratic operation can result when the balance of the net to ground makes a sudden shift. I would suggest a drive isolation transformer since the hp is so low.

It your power is grounded wye, then the power is probably not the problem. Consider the other issues the manual mentions.

 

[itsmoked]

Have there been any other significant electrical changes in the facility? A new machine added? Service changed somehow?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[BearB]

I will investigate the power supply issue tomorrow if time allows.
No other machines have been installed and we generate our own power. The utility plant has added 2 new turbines to increase cap. and sell power to the grid just about the time this problem started. I am not sure how to try to tie the problem to the new turbines. I know of no other operating or power issues in the plant. Thanks for the replies.

 

[ozmosis]

It is always good to look at the company's website when you get a spurious trip, just in case it is a common one. This is from the ROckwell site:

Q? What is fault 24 and why is it not in my manual?
A! Fault 24 is the "Decel Inhibit Fault." This fault was not listed in the first manuals printed for the PowerFlex 70 drive, but are printed in the current circulation manuals.
This means that the drive is not following the commanded deceleration because it is attempting to limit bus voltage. You will want to verify that the input voltage is within the drive's limits. Verify the system ground impedance follows proper grounding techniques. Disable the bus regulation and/or add dynamic brake resistors and/or extend deceleration time. Parameter 238 bit 6 is the location to turn this fault on or off. By factory defaults this bit is on. Parameter 161, 162 and 160 are the bus regultation parameters that can be adjusted for application if the bus regulation is desired

 

[LionelHutz]

Has your supply voltage increased recently?

It sounds to me that the VFD is trying to brake the motor and pump to a stop and the energy it is removing from the motor is over-charging the capacitors in the VFD. Try increasing the decel time further. You could also set the VFD to allow the motor to coast to a stop.

 

[BearB]

Ozmsis Thanks
LionelHutz I agree, the reason I increased the Decel from 3 seconds to 4 sec (maybe I should go more). I cannot figure out why it goes for days without issue then back to back cycles it faults. The Operators run it in auto, I verified by watching it when it happen and the material (starch) is always the same.
Power is the other variable that I think is out there. I left a Fluke Meter on record on incoming A & B and was able to catch a fault the min.=475vac & max.=481vac. It has operated without problems for 24 hours and before that it faulted on 4 back to back cycles (about 1 cycle every 90min.) Measuring each phase to ground A=265vac, B=285vac, C=286vac.

 

[itsmoked]

I would change to coast. It generally causes no problems with pumps. If your process will tolerate it with no issues I suspect your problem will go away.

It sounds like perhaps the new generation added may be keeping the mains higher which means the DC link is sitting higher. So when your drive goes into decel there is less headroom on the DC link capacitor bank to store the regenerative energy.

If coasting to a stop is an issue you can go with adding a braking resistor. Since you almost don't need one, it could be pretty small, power wise.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[BearB]

Thanks for the replies. This site is always a great resource.

 

[roydm]

A progressive cavity pump won't coast like a centrifugal pump as there is quite a lot of friction and not much rotating mass (I don't understand the de-acceleration issue).
Is it possible you have an intermittent fault (loose connection) in the drives speed setpoint so it tries to go from normal to zero in no time flat which causes the code 24?
Roy

 

[BearB]

I will go over that part of the circuit.
Thanks.

 

[ozmosis]

"coasting stop" for the VFD means simply that the output from the VFD stops immediatly and does not 'control' the deceleration. In this way, any regenerated energy would not come back to the VFD and cause it to trip on overvoltage.
As far as the motor/load is concerned, if it stops in a few seconds then ok-it has no effect on the VFD, or it could take minutes to stop if the inertia is high and there is no friction (e.g. fans). Either way, the VFD would not 'see' it.

 

[BearB]

This is a follow up from this problem. Thanks for the feedback, I extended the decel time to 6 seconds (double the time it was set at and operated for several years without problems, problem returned. This morning I set from ramp stop to coast stop, problem returned 1 out of 4 cycles. Again always at end of cycle and intermit. The Bus regulation is halting decel due to excess bus voltage is reason for decel inhibit fault. Since Drive is set to coast stop now regen is not the problem. Any thoughts on an isolation transformer on the 480vac input to drive. However I measured the incoming with a fluke when drive faulted and voltage was ok, see above post. I am concerned about disabling the Bus Regulator in the drive because I am not sure how that would effect the process when pumping.

 

[controlsdude]

this might be off the wall, but if the drive has been operating years without any problems.

Do you think someone fat fingered a default parameter to other than default? I have had so many problems at customer sites before where this was the issue.

Do you have a list of parameters? I know this drive has hundreds. Do you have a list of parameters to compare it with todays list?

Maybe default it then change the parameters that need to change from default.

 

[LionelHutz]

Something wrong in the mechanical/fluid system allowing the pump to "motor"?

You might need to connect some high speed recording equipment to see what's happening. There could be sub-cycle transients that are causing the trip. A VFD fed with 3-phase power has 6 charging peaks per cycle. You only need a transient which boosts 1 or 2 of these peaks to cause the trip you see. A Fluke meter would never record this type of problem. I believe the Fluke use is 100mS averaging which is 5 or 6 cycles. Way too slow.

Do you have any capacitors around this VFD? Do you have any contactors switching the line power near this VFD? Is there a line reactor on the VFD?

 

[DickDV]

This almost has to be fast input power transients jacking the DC bus while running so the Decel Inhibit function becomes activated.

I would immediately install a 5% line reactor ahead of the drive. It's not an expensive device and would be good protection for other power-related problems too. If the problem persists after that, a more detailed study might be in order.

As to a progressive cavity pump "motoring", I don't think so. That's about inconceivable due to their construction.

 

[Compositepro]

Actually, progressive cavity pumps are used as motors downhole to drill oil wells. They are quite capable of motoring.

 

[ozmosis]

I don't think there would be a problem mechanically 'coasting stop' as this would only be the same as going DOL with the motor and then pulling a contactor out to stop.
The VFD controls deceleration to a set time, the time programmed. If this is too quick then you will possibly get a level of regeneration. If you need a controlled stop, try a coast stop first and see how long it takes to come to a stop from 60Hz (or 50Hz depending on where you are in the world) and this will give a rough guide as to the decel time.
Alternatively, just increase the decel time to something like 10 secs (if your process can cope with this) or more and see if you get an alarm.

Maybe I'm a little too long in this industry but when companies make a point of highlighting a specific alarm or fault, it is often a 'problem' fault. FAQ's are often a subtle way of saying this is a bug but we don't want to call it a bug so we'll call it an FAQ.

 

[DickDV]

In the typical starch pumping application using a Moyno type pump, there is no other pump in the system, very little standing head pressure, and very high friction losses in the pump. Motoring of the pump under the conditions the OP has described is nearly impossible.

He has already stated that he has extended the decel time out with no relief and went to Coast to Stop with no relief so there is near zero chance of the pump motoring on decel.

I'd say that motoring of the pump is not the cause of his faults. The next most likely cause would be input power transients, it seems to me.

 

[itsmoked]

Yep I agree DickDV. That's ship that's sailed. Time for input scrutiny.

Keith Cress
kcress -

http://www.flaminsystems.com