PowerFlex700VC "Decel Inhibit" Flt 24

Good Day Everyone,


I’ve been reading this forum whenever I get time and very pleased with the various information given that in the end help me a lot in my technical know how. I need a little help with the problem I am facing for two weeks now and this is regarding our PowerFlex700VC VFD. To give a little information regarding the system we used, please find below.

• We used PowerFlex700 VC Series A 90kW, 400V to drive our crane long travel motors (2 nos.), 30kW each.
• The PowerFlex700 VC is connected in the DeviceNet Network together with our RSLogix5000.
• I used 1024PPR encoder for the motor installed to one of them, the other one doesn’t have.
• I used external brake chopper and external resistor.
• The crane is approximately 300T in weight, and being used for our Anode Baking Furnace.
• Precise positioning and stopping of the Long Travel is very much required.

Here are the problems I’m facing;

• When the long travel moving forward at slow speed 10Hz or below in order to position to section or pits, the time I give a stop a command the drive starts decelerating till it reaches to 0Hz (normal) but instead of stopping completely the drive start moving in opposite direction and then few seconds later “Decel Inhibit Fault” Flt 24 is generated.
• Our Anode Baking Furnace is around 400 meters long and some of the sections it is working well and only few of them is doing the same.
• The problem on other sections is rare but on one section every time you position the crane it is always happening.

Drive settings/parameterization are:

• Par 053 Torque Perf Mode is 0 – Sensrls Vect
• Par 080 Speed Mode is 3 – Encoder
• Par 090 Speed Ref A Sel is 22 – DPI Port 5
• Par 140 Accel Time 1 is 10 Secs
• Par 142 Decel Time 1 is 6 Sces
• Par 146 S Curve is 20%
• Par 161 Bus Reg Mode A is 2 - Dynamic Brake
• Par 163 DB Resisotor Type is 1 – External Resistor
• Par 153 Regen Power Limit is -150%
• Par 155 Stop Mode A is 1 – Ramp

I have tried to switch other “Torque Perf Mode” and ‘Speed Mode” but still. Now I’m lost and don’t know where is the problem. I am thinking of the rail misalignment or some kind of wind influence as the region where the problem is really seen is often experiencing big blows of wind. Can anyone help me out there?

Thank you,
Don

It seems to me what you are saying is:
1. You know your positioning controlis working well.
2. In several spots you get it into position but then crane drifts from that position.

I wonder if there is a slope or low spot in the rail or if there is enough cross sectional area on bridge rails (beams ?) for wind to generate enough force to move crane.

In either case a mechanical brake may be the cure?

Dan Bentler

I dealt with a very similar problem last week using Telemechanique drives. One was for adjusting the width of a conveyor and when I stopped the motor, after operating at 20Hz, it would rotate backwards. I did not have this problem at higher frequencies. It was recommneded that I change from a Flux Vector Control to a Constant Torque control which worked well.

On your PF700 make sure you go through the autotune process. If you still have no luck then start looking at the parameters related to "torque perf mode" in the manual.

Hi Dan,
Thank you for the reply. Yes I know that my positioning control is working and my drive in general. In one spot I get into position but after it is ramping down to 0Hz it moves to opposite direction for about 100mm to 200mm and tripped on Decel Inhibit Fault” Flt 24. Other spots are rarely happening but when it does it is exactly the same thing.
The rail is badly design in fact we experience breaking on the joints and a couple of re-alignment. The thing is that I find difficulties to prove this in mechanical and production department as they don’t understand VFD’s. I will try to suggest installing mechanical brake but this will go through a lot of debate.
Here are some observation I’ve noticed in the process;
Drive HIM displays “STOP ASSERTED”, this is the time when it moves itself in opposite direction
· Output Freq = 0
· Commanded Freq = 0
· Output Power = 0
· Output, Torque, Flux current & Output voltage still reads some values other than 0.
Normally when the VFD is “Running or At Speed” you will get the reading of Output freq, Commanded freq and the Output power.
Regards,
Don

Hi Giz99,
I may have to do auto tuning again. I tried other function of “torque perf mode” except the FVC Vector, I may try this tomorrow.
Thank you,
Don

Thanks for so much good data. I'm not totally familiar with the AB Pf700 drives but i suspect that there is a "DC Hold" option in the software somewhere, probably in the braking or stopping section. Typically this can be set up to send DC thru the motor while standing still.

This tends to make the motor hold its position so you don't get the rollback you are experiencing. The problem is that you can't send DC thru a stationary motor for very long before it overheats. So, if you have to hold position for only say 10 seconds or so, you can get it done with DC. If longer, I would still use DC Hold but use the hold time to engage a mechanical brake. Then you can hold position as long as you like.

Perhaps a motor with a built-in brake might be a solution. One that you can electronically release when you need to move and engage to hold the motor in place once the VFD reports zero speed.

sorry, didn't realise I was basically restating the previous post

Thank you

I would like to thank everybody for the helping hand and valuable contribution. To Dan Bentler who gave me courage to stand and explain them the real cause of the problem.

Today with the presence of all concerned department and at around 0630am we tested the crane on that particular section with a normal wind blows, in general a very calm weather condition and the result is the crane is stopping exactly to its target position, no movement, no drifts and no backward movement.

At 1130am with the same team we did the test again. This time big wind blows started picking up. And very true it happens again but this time I outsmart them because they now believe that it is some kind of wind“sail” effect. We are now planning to close the open area of our maintenance bay. The problem is now solved, thank you all.

Just for little info, the length of crane (girder) is 40 meters long and the girder width is 5 meters and most of the time the wind blows from N-S direction directly hitting the girder.

Regards,
Dondon

In stop mode A, you could change it from "ramp" to "ramp to hold". You would then have to go into the DC current settings and set it to probably about 30% of motor FLC. This should hold the motor in position quite well unless it is a heavy load, then you may have to go a little more. With today's motors 30% should not be a problem to hold pretty much indefinitely.