# What goes bad with VFDs?



## erics37 (May 7, 2009)

Okay here's the scenario. Storm water pump station, built mid-90s or so, with two 100 hp 480V pump motors, controlled by identical VFDs in a typical lead/lag alternating setup.

To the extent of the Public Works guys' memory, every time there is a rainstorm and heavy flow with both pumps running full tilt, the drives will ocassionally kick out on an over temp fault. This only happens when both pumps are running and there is not much downtime between cycles (if any). According to the city guys, it's been getting progressively worse each storm season.

We've already done a few things to try and mitigate the problem. More ventilation fans on the drive enclosure doors, more ventilation in the electrical room as a whole. Even blatantly violating safety considerations and leaving the cabinet doors open only somewhat works. I went today and looked at it again. At this point, I'm wondering what to recommend.

First off I think the system and building were poorly designed. NO ventilation whatsoever, and clearly the adjacent drive enclosures in a small room simply can't dissipate heat fast enough when both pumps are running. I'd like to suggest getting a mini-split A/C unit installed, they're not too expensive and they work great in other cramped, hot electrical rooms (like data rooms). But the fact that the problem is getting worse makes me wonder what on these 15+ year old drives is slowly failing. And if that's the case, how would a pair of new, modern VFDs fare in this setup?


----------



## joethemechanic (Sep 21, 2011)

erics37 said:


> Okay here's the scenario. Storm water pump station, built mid-90s or so, with two 100 hp 480V pump motors, controlled by identical VFDs in a typical lead/lag alternating setup.
> 
> To the extent of the Public Works guys' memory, every time there is a rainstorm and heavy flow with both pumps running full tilt, the drives will ocassionally kick out on an over temp fault. This only happens when both pumps are running and there is not much downtime between cycles (if any). According to the city guys, it's been getting progressively worse each storm season.
> 
> ...


I was going to say the AC thing too.

Are all the heat sinks and mounted components tight and clean at the heat conducting connections/physical mountings. And if you do take it apart and clean it, I use some heat conductant grease like the stuff you use for computer chips.


----------



## joethemechanic (Sep 21, 2011)

I'm also wondering if the run cycles are growing progressively longer do to land development and increase in impervious surface within this pump station's drainage basin


----------



## erics37 (May 7, 2009)

joethemechanic said:


> I was going to say the AC thing too.
> 
> Are all the heat sinks and mounted components tight and clean at the heat conducting connections/physical mountings. And if you do take it apart and clean it, I use some heat conductant grease like the stuff you use for computer chips.


Yeah we got in there a little while back and torqued all the connections to spec. It regularly gets vacuumed out as part of the City's maintenance regimen. The equipment has been well taken care of, to say the least.


----------



## erics37 (May 7, 2009)

joethemechanic said:


> I'm also wondering if the run cycles are growing progressively longer do to land development and increase in impervious surface within this pump station's drainage basin


Nope not much growth in that area. It's in the industrial part of town, big pulp mill right next door. No significant development or roads or anything since this station was built. Its basin is probably about as developed as it will ever be.


----------



## joethemechanic (Sep 21, 2011)

I kinda used to do what you do, lots of pump stations, and I sold, installed, serviced, and was the "Field Engineer" for the largest distributor of these clarifier drives.











http://www.dbsmfg.com/index.htm


----------



## joethemechanic (Sep 21, 2011)

erics37 said:


> Nope not much growth in that area. It's in the industrial part of town, big pulp mill right next door. No significant development or roads or anything since this station was built. Its basin is probably about as developed as it will ever be.


Do you do much for the pulp mill? We sold lots of drives to the pulp mills. We put one in Georgetown SC. that weighed 35,000 pounds. We had to get a 500 ton crane in there just to sit it. The biggest PIA was getting enough sludge out of the tank so the H2S levels were safe


----------



## erics37 (May 7, 2009)

joethemechanic said:


> Do you do much for the pulp mill? We sold lots of drives to the pulp mills. We put one in Georgetown SC. that weighed 35,000 pounds. We had to get a 500 ton crane in there just to sit it. The biggest PIA was getting enough sludge out of the tank so the H2S levels were safe


I started my apprenticeship there actually. Although I was involved mostly in the construction of a brand new steam turbine generator building. Didn't get much into the paper/pulp process stuff (which is good, them's some nasty chemicals going on in there).


----------



## joethemechanic (Sep 21, 2011)

That H2S coming off the paper sludge is really nasty. You only smell it for a split second. The first effect on your body is it kills your sense of smell. In any kind of confined space its deadly in a big hurry


----------



## John Valdes (May 17, 2007)

The drives have a thermistor or another type temp sensor located on the drive heat sink. The heat sink must be kept clean. The IGBT's (transistors) that are mounted to the heat sink should be set in "heat sink compound". This substance ensures the IGBT's are in good contact with the heat sink. It is not the electrical connection as much as the mounting contact between the heat sink and the IGBT's. Bad contact causes more heat.
Another thing to do is connect a meter capable of reading temperature to the drive heat sink. Compare this temperature reading to the drive readout. You could have a bad sensor or both could be suspect and are not calibrated. Since the drives are old, you may need to calibrate or replace the sensors. Check the manual and see if this is discussed.
You may just want to find the sensors in the drive manual and get the part numbers. If you can still buy the sensors, you may want to replace them. (I would if cleaning does not correct the problem).
The drive manual will also give you the high temp setting. This is not adjustable, but will let you know what temperature will cause the drive to fault.
Compressed air used in a safe manner can be used to blow out the heat sinks. Since they are on the back plane of the drive, accumulated dust and debris can exist without you being able to see it.
Blow them out real good making sure to keep as much dust as possible from entering the drive housing. Also, compressed air can compromise or destroy fragile drive components. 
High heat sink temperature faults are usually quite easy to remedy. Clean it, cool it and replace the temperature sensors if needed.
The description that they have gradually gotten worse over time is a good indicator that a good cleaning is in order along with a check of the fault remedies.


----------

