powerflex 700 commanded frequency drops

I've come across an unusual drive problem on a powerflex 700
the scenario goes as follows the drive is hooked up on analog input (looking into the integrity of this signal)

the drive ramps up to 60 hertz without issue but every once in a while it will suddenly drop to 50 hertz this causes a current limit drive fault and alarms the refrigeration system.

Information on its hook up.

runs 4 fans on S curve ramp 5 sec ramp time
running in volts vs hertz mode
uses XLPE cabling each motor leads are in seperate conduit
amperage checks are within range and phase to phase are balanced (unfortunately no line reactors)

Other than the commanded frequeny I am open to any suggestions as to what could be causing theis problem.

mordred, you may very well have told us the reason in your post. The speed reduction doesn't happen first, the current limit does.

The drive will reduce the speed reference automatically when the current limit is reached. If the current limit persists, the drive will fault. Some drives, when a rather slow current limit occurs, will reduce their speed reference all the way to zero without faulting but, if it occurs fast, the fault will be involked.

Thanks DickDv I was hoping you would read this. That was one of the possibilities I was thinking of but was not sure on that scenario thanks for the info. Now I can approach the problem at the motor, cabling end as I could not find any problem on the analog inputs. So I'm going to get an inspection done on each motor and check for any high resistance through bad cabling or poor connections. Then if nothing is found I'll probably disable one motor run for a while, change motors and see if I can isolate the problem to a specific motor.

I've seen extraneous induced voltages on Discrete Inputs trigger reference changes to Preset Speed or to Speed Reference 2. Change the unused discrete inputs to "Not Used" and/or tie them to common to prevent any possibility of an unintended input. Preset Speed 6 is 50 Hz, so with the default parameter set you would have to get Digital Inputs 5 and 6 true at the same time to select it. Not impossible but unlikely.

I think DickDV's explanation is the most likely one, especially because the Max Frequency minus the default Overspeed Limit = 50 Hz.

your advise on floating inputs is a good point. Decided to set that anyways as its sounds like a good policy in this case the parameters should suffice rather than hardwire pull down connections

Mordred
This sounds like a 4 fan evaporation unit (chill coil) in a refer or freezer. Air flow (sitting here at home and visualizing from memory) is thru coil then thru fan. Could it be that as air temp drops thus denser the denser air is loading down motors??

Think I would also check bearings on motors if they have been installed for say 5 years.

Maybe heat trace on conductors to prevent chilled thickended "lethargic electrons?? Had to do for humor.
Dan Bentler

You've visualized the application closely in this case its used to support a cooling air circulation without nearby chiler units. Essentially to maintain pressure zones to assist in over all air circulation over a large volume. Also assists in a dehumidification process. This is one of 5 similar setups so that easily narrowed out any software based problem.
Currently the sparkies are checking connections and doing megger checks. This was a problem a fellow tech was working on but he went for holidays so I'm going off the details he provided lol.

Tracking the cause of the current limit symptom,

Golly, leitmotif! I was just reading about a new scientific discovery involving cooling some materials until they turn into "superinsulators" with infinite resistance (seriously). They had done a huge amount of research and spent tons of money.

They sure could have saved a lot of time and money by just asking you about it!!!! Now, ain't we smart!!! (Please, this is a joke, Dan!)

DickDV said:

Golly, leitmotif! I was just reading about a new scientific discovery involving cooling some materials until they turn into "superinsulators" with infinite resistance (seriously). They had done a huge amount of research and spent tons of money.

They sure could have saved a lot of time and money by just asking you about it!!!! Now, ain't we smart!!! (Please, this is a joke, Dan!)

Click to expand...

I am going to throw a study on it.

Both were good for a grin

Dan

rofl evidently missed that part lol must have been tired

anyways the sparkies think they found the problem will have to wait till they run production top know for sure