[Shophound]

Here's my situation. At my work I have a 20,000 square foot office building, 10,000 sq. feet each floor. Top floor cooled by 40 ton Carrier split system, bottom floor by 20 ton. Air handlers on both floors are variable frequency drive feeding on average six or seven VAV boxes per floor.

Air handler VFD is controlled by a VTAC 7 HVAC Drive. Variability in fan speed is determined by remote static pressure sensor in ductwork at (what I'm told) most remote location in trunk. Here's the deal:

Top floor, VFD will vary with load throughout day. More VAV boxes open up, VFD compensates by increasing motor speed to move more air. VAV boxes ramp down, so does VFD. VTAC 7 Drive readout set to indicate hertz of motor at various speeds, varies from 49 to 54 HZ recently.

Bottom floor, VFD runs dead solid one speed. Regardless of how many VAV boxes open up or ramp down. The display on VTAC 7 displays a solid 52.5 HZ.

So, being that I'm on the learning curve with this system, and being that I had to be here at my building today on my day off to supervise roof repairs, I wanted to find out why the bottom floor variable frequency drive never varies. I first checked out the top floor, hooking my meter to the sensor leads from the static sensor to see what kind of variability it feeds back to a small controller outside the VTAC 7 (all it says is UCM-420A on the small controller). It read 27.xx volts, the "xx" is what varied as I went around the building setting VAV boxes colder, then warmer. I repeated this test downstairs, and also noticed "xx" variability as I adjusted the VAV load throughout the building (building empty today...lets me play with stats without freezing anybody out).

That tells me both static sensors appear to be returning valid input to the controller, unless I'm using the wrong method to test for such. That's one thing I need to know. The other being that if everything external to the drive is giving input, but bottom floor drive seems unresponsive, is that an internal matter with the drive, such as an incorrect parameter in the setup programming?

And finally, the lack of variability doesn't seem to affect the building's cooling performance all that much, but I wonder from a static pressure point of view if there's concern. During low load conditions, the VAV's will close but the VFD doesn't slow. I recently lowered the fan speed manually as I was losing gearboxes on the Enviro Tech VAV boxes, which seems to tell me static was too high.
Also, being that my employer paid for a frequency drive system, the damn thing should vary!

I'd appreciate any info you guys may have. I haven't touched anything regarding the drive's programming, although I do have the book. I'm not willing to jump in and do anything wildass; I want to know what's involved beforehand.

Thanks in advance.

 

[R12rules]

Look up in your book what gives the sensors this 27.xx output voltage?

Since this problem is all over the building, it may be in the programing. If it was an isolated case, I would say it is with the sensor being dirty or out of calibration.
But for the whole bunch to be out of calibration ... I don't think so.

 

[rabram]

If your control signal varies as pressure changes , I would lean toward a problem in the drive. Are you sure it is not in "hand" on the control panel ? If you wanted to verify you could swap sensors with a known good one from another floor.Is this in warranty ? May want to call installer and pick his brain. Good luck.

 

[a\c don]

I agree with R12, seems like the program is not running properly.

Have Carrier reload the program for that system.

 

[Shophound]

The VTAC 7 drive consists of two cabinets. One houses the fuses, contactors, relays, terminal board, etc. and the other is the freq drive itself. On the lower cabinet, the left control switch is labeled "drive" and "bypass" and the right control is labeled "local" and "remote". The bottom floor controls are set identical to the top floor controls, meaning "drive" and "remote" are engaged.

The setup is a few years out of warranty. The company that installed the system was bought out several times (one reason we went in-house for HVAC work). Carrier sold the HVAC contractor the condensing unit and air handler; I'll have to see if any documentation I have in the closing documents specifies if there's any ties to Carrier for the freq drive. If I recall correctly I don't remember seeing anything to that effect (I'm home now at last).

I once pointed out to a tech while the system was still under a full maintenance agreement (beyond original warranty...long story there about how much "full" maintenance you get with a "full" maintenancea agreement) that the bottom floor freq drive was a steady state dude and never varied. His reply was "sometimes they're just like that." I never could accept that answer, but until now have not had the time to find out why.

Good replies so far. Thanks!

 

[absrbrtek]

What kind of signal is controlling the drive? Looks like 0-20ma or 0 to 10vdc are the signals you should be getting too it. Heres a quick program guide for your drive. http://www.vtacdrives.com/vtac7/d2930.pdf

 

[bobgup]

check the programming for the drive.
These drives can be programmed to come up to run at one speed and not vary. I have seen some set for two speeds.
The program should be able to tell you how it is set up.
Bob

 

[dapper]

It sounds like the UCM 420a is a Kele transducer. Go to there site and look it up. They usually require a 24v supply. My bet is that you are reading the supply voltage, not the 'control' signal. Control signal might be 4-20mA, 0-5vdc 0r 0-10vdc. Find out which you are using. It is usually configurable with jumpers or DIP switches. After you know this, get an instrument to measure the static in the duct, such as a manometer or Magnehelic and see if the control signal tracks with a change in static. The next step would be to determine if the transducer output tracks with static change .... etc.

This should be a good start. Good Luck!

 

[coolh2o]

I agree with Dapper... The UCM-420A is a 4-20 MA sensor and you have to get in-line and read current. It sounds to me like you are getting a bum reading from the static pressure sensor or the programming routine is locked up. Put a magnehelic guage in the duct and see what the static pressure is really doing. I will assure you that in a VAV application the static pressure is never stable all day long. Something is wrong with the input or output signal.

 

[Shophound]

Man, you guys rock. After I posted the original post yesterday it occurred to me to Google the UCM-420A. And I got Kele's website and downloaded both the PDF and this:

...which almost exactly depicts my system's setup.

absrbrtek, thanks for the VTAC link. That's very helpful and a lot easier than paging through the thick manual. When I was reading the book as I stood next to the drive yesterday, I was able to access and check the parameters on the first five programming steps, but stopped at six, where it asks for the password. After reading the VTAC link's info, and studying it some more, I feel confident I can enter the password and find the parameter regarding what makes the drive vary or not. I think I'll go upstairs and run through those first (the one that works properly) without changing anything (allowing for differences in motor size, etc between floors) and write everything down. Then go to the downstairs drive and do the same, noting any differences apart from motor size, etc.

Both systems do use the Kele transducer, which indeed appears to use a mA signal. I'll have to trace the line from the static sensor to see which one feeds the mA signal, which means I'll need to pop for the mA head for my Fieldpiece stick meter. Good thing to have no matter what. I do have a Magnehelic, even had it plugged into the supply yesterday as part of my education on how this system works. I have to admit that although I'd rather had spent my day off elsewhere rather than waiting on pokey-butt roofers, how I chose to spend the time learning more about this system is really paying off. And your input only boosts my confidence that I'm on the right track. Thanks again.

And...is this my 500th post??? WOO HOO!!

 

[seaboard]

You probably have a setup problem but most likely someone set it up that way on purpose. The Drive (when functioning properly) should improve comfort and save energy but neither of these are the driving force for a warranty contractor or full maintenance contractor. Depending on the capacity controls, economizer setup, and piping of the system, you may have to set a minimum speed for the system to maintain superheat and velocity for oil return. It's likely for these reasons that someone set up the drive the way they did.

 

[d_lee1]

you may want to check your static sensor to make sure it is still in the duct. if it comes off the drive would ramp up to full speed which is usally 60 hrtz but the may have the max set lower.

 

[flange]

youve got forty tons cooling the top floor at nearly 50 hertz steady. you also have the same size floor being conditioned by twenty tons below. without knowing all the other variables, it could be that the first floor system is at or near design and all those boxes are wide open and giving you a call for 52 hertz. while i would check the controls aspect carefully, dont forget to look at the big picture.