Powerflex 755 not outputting over motor nameplate current

remlapw

Member
Join Date
Oct 2013
Location
BC
Posts
34
Hi All,

I have a lifting/hoist application that I am using a powerflex 755 1hp drive. I am using a 1/3hp SEW gear motor that is rated at 0.57A 480V delta.

I was unable to complete a static autotune with the powerflex 755 drive (the drive faults with code 77 IR Volts Range). But i am still able to control the hoist. Although the problem is that i don't have a great amount of lifting power. I contacted rockwell and they basically said that the drive doesnt like something from the nameplate when doing the autotune. I contacted SEW and confirmed all values were correct. Now aside from not having the best control over the motor which i was expecting to be improved with at least a static tune I have started thinking about other things that seem odd to me.

First of all, I have 2 of these same applications on the same machine, both behaving in the same manner (failed autotune, limited lifting power).

What i find very difficult to understand is that during the cycle of the equipment the hoist is commanded to lift until it hits a cylinder at which point it will lift the cylinder which in turn changes the state of a prox that is barrel mounted, at which time the hoist is directed to lower.

I have seen multiple times that when there is a heavy load (which is should lift under normal circumstances) it will come in contact with the cylinder and not have enough power to oppose the compressed air, effectively stalling the motor. What i would expect to happen would be the drive would trip with a motor OL. However this is not the case. I simply see the drive sitting there putting out no more than the rated nameplate current. I have the overload factor set at 1.0 so i would expect to see it trip quite quickly.

Can anyone think of a reason as to why this is? the duty cycle is very low on this equipment and could probably be run at 150% rated current for it's lifting time (approx. 3-5 seconds every 60 seconds).

I have checked the drive parameters to ensure that the maximum current parameter is set above the nameplate value (it is set to the default 3.78A that the drive is rated to output).

I'm at a loss here because I have definetly seen applications where i am running over current for a period of time on conveyors due to jams which inherently clear themselves (in which case i typically set the overload factor to around 1.15)

Any help would be appreciated.
 
If it isn't tuned, it can't operate the motor correctly in any vector mode. Most likely the problem is that the motor is too small compared to the drive. I've seen this when motor FLAs are less than 50% of the drive current rating, I think because the accuracy range on the current sensors begins working with values that are too small.

What I would do first is to reset EVERYTHING back to the factory defaults (parameter 7) just to make sure there were no conflicting values anywhere, then start over with the autotune. But if possible, do a rotating autotune, that's more accurate than the static and I have used that to overcome the issue of the motor being too small. If you have to uncouple the motor, it's a pain I know, but sometimes that's the only thing that will work. If you CAN'T uncouple the motor, see if you can do it coupled but safely, then try an "Inertia Tune". If you can't do that either, I don't know...
 
It is common for a lot of drives when oversized to fault on the resistance of the circuit. This is a very important part of the calculation which determines how much voltage is lost before the current gets to the motor, so s to produce work.

If all else fails, contact SEW for the resistance value of the stator, and enter it in the drive. Worth a try!
 
It is common for a lot of drives when oversized to fault on the resistance of the circuit. This is a very important part of the calculation which determines how much voltage is lost before the current gets to the motor, so s to produce work.

If all else fails, contact SEW for the resistance value of the stator, and enter it in the drive. Worth a try!
True. The "autotune" feature is there as a convenience for avoiding needing to wrestle the necessary data points out of the motor mfr. You can just enter the data manually if you know it. The parameters for that immediately follow the Autotune parameter. You may need to push hard on your supplier to get it for you, but they have it, somewhere.
 

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