VFD Problem

[RSTEng1]

Hi,
I am currently working at one pumping station with 3 VFD pumps (asynchronous motor, 208V, 42kw.
The strange thing is when panel is powered with no pump running VFD draws about 50A per phase.
Rest of the load is disconnected. How this is possible?
Could someone explain this situation.
RSTENG

 

[ozmosis]

I encountered, a number of years ago, a pump system running off VFDs with a similar issue.
It was found that the control logic operating the drives was such that the "run/stop" command (digital input) was never actually used and the method to "run/stop" was to use the analog input. This meant to 'stop', the reference signal was set to 0v( or 0mA) but the run command was always active (because the digital input was always active), even though the drive was at 0hz.
Although not actual running, it was still drawing current due to the fact the drive was being told to run at 0hz.
We recommended they change the configuration to utilise the Run/stop via digital inputs. Drives were happy, motors were very happy (and cooler) and so was the customer.
Just a tip worth looking at.
A VFD shouldn't be drawing current if it is in a real stop situation.

 

[iop995]

Why was designed such RUN/STOP in analogue mode when such commands have 2 very precise states only?

 

[iceworm]

Check to see if the VFD has line side caps - I'd guess about 20 kVAR. The motor is not running so the drive is not drawing any real power. The 50A must be reactive - probably line side caps.

ice

Harmless flakes working together can unleash an avalanche of destruction

 

[ozmosis]

iop995
the idea is that we try and help RSTEng1, not to try and understand why I found a company who didn't/couldn't read a manual.

 

[itsmoked]

Just to further iceworm's likely scenario, keep in mind that a clamp-on ammeter has no clue as to whether the current is due to a cap bank or an actual work load.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

That was my first inclination as well. How is this "50A per phase" being measured? Cheap meters cannot always accurately interpret high harmonic loads. Even when not running, the drive is still rectifying into the DC bus and bleeding off some of that power to run the electronics themselves. If the meter is only seeing peaks, however short, it might be falsely showing something that is not really happening.

I like ozmosis' story too though. Using the analog signal as the run command was a common "trick of the trade" for automated pumping systems that had no automation intelligence, primarily agricultural pump systems where the users are notoriously cheap. In other words, the designer (typically a pump supplier) was too cheap to put IN a "Start" command device if there was some way he could get away without it! When people discovered they COULD do it with just the analog signal only, they jumped on it. But on occasion, it did create problems like what ozmosis witnessed; 0 speed is not really "Off". Now days, many drive mfrs have fixed the problems by adding what is referred to as "Sleep Mode" which monitors that exact situation and takes it to the next step of actually turning the VFD off if the input analog signal is below a threshold for a programmed amount of time. That was not always the case, it was developed by a pump supplier here in California for Saftronics (Yaskawa) drives about 10 years ago and Saftronics had an exclusive on it for years. That didn't stop the major players from developing their own versions however once they knew the need was there. But even now not all VFDs have that, I think the marginal players can't afford the R&D or have never chased that niche market so they are unaware of it. So if the system designer was PLANNING on using that feature, but the job went to the lowest bidder who used a drive that doesn't have it, that scenario is very plausible.

"Will work for (the memory of) salami"

 

[RSTEng1]

Hi everyone,
First of all, I want to thank to you guys for the very good insight.
In this particular case, the problem is related to the VFD Tuning Capacitors (Eaton VFD SVX9000 Series).
Although, not related to this situation, I consider the explanation related to the Start/Stop very important and good to keep in mind.
Again, thank you so much.
RSTENG

 

[mikekilroy]

What are "VFD Tuning Capacitors?" And where are they located?

 

[Marke]

Something else to consider, if the drive has been installed with a passive harmonic filter, then the capacitors in the filter will draw a high current if they are not turned off when the drive is not running.
Commonly, tuned trap filters will draw around 50% current through the capacitors. Consequently, many installations include a contactor to turn the capacitors OFF when less than 30% load.

Mark Empson
Advanced Motor Control Ltd