Jump to content


Photo

Altivar 58 Trips For Overbreaking


  • Please log in to reply
5 replies to this topic

#1 Birchy

Birchy

    Junior Member

  • Full Member
  • PipPip
  • 6 posts

Posted 17 July 2008 - 07:59 PM

I'm probably going to have to call in a pro for this, but would like to see if I can stumble on something simple. I'm overbudget and work for a church, so I'm trying inexpensive DIY-diagnosis first. Unfortunately, I'm not trained in this area. I barely know what a VFD is. But if anyone has some suggestions for things to check, I'm willing to do some self study and decrease my ignorance a little.

We have this large air handler with a pretty good sized return air fan (inline in the return air duct), that is slaved to the supply air fan through code in the building controls (Delta v3). I don't know if the problem is gradually getting worse, or if it's always been an issue and I've just not been paying enough attention. Here's the problem:

During startup or recovering from a fault, about the time the system approaches setpoint (static pressure), the VFD faults for overbreaking (code = Obf). I don't notice any attempt of the drive to slow down, it seems to always still being in the ramp up phase. I can speed it attains before fault, if I sneak up on it (maybe 940 RPM, instead of 809 RPM). The only mods to config that I've done, is to max out the ACC and DEC configs to make the ramps as slow as possible. (They were 150 and 80 for ACC and DEC, respectively, now are set to 999 for both.) This allows me to set a max control signal of 47% instead of 43.8%. This air handler feeds only VAVs (54 of them, about 1/3 of them in use weekdays, about 2/3 of them in use on Sundays). I need to be able to send a control signal of about 55% on the few times when all the VAVs are open.

I would like to get the system modified at some point to hardwire slave the return air fan to the supply air fan, I don't feel like it's a good idea to just send a calling value to both and have the RAF at 45% when the SAF is faulted out or when it's ramping up from 0 after a fault. It's hard on the ductwork. I believe that our other two large AHUs are hardwired.

Here's the data that I have so far:

Trane AHU; single 50 HP SAF, single RAF
SAF is controlled by: Altivar 58 VFD
Drive settings:
460V, 60Hz, 61A, 1760 RPM (this matches the motor data plate)
mtr cos phi/power factor = .95
auto tune = no
Max out freq = 60.1
ramp type = Linear
SwitchRamp2 = 0 Hz (Freq for ramp switching, when output freq > this setting, ramp times will be AC2 & dE2 [which have no values, or at least aren't displayed in the menu] )
DecRampAdapt = no*
TorqueLimit = 200%
CurrentLimit = 107.7 (adj range is supposed to be 0 - 1.35 In) ?????!!??
Deactivation of auto DC injection at stop is allowed
SFt (SwitchFreqType) = LF (allows adj bet 0.5 and 4 kHz using the SFr parameter)
SFr (SwitchFreq) = 2; range depends on SFt; for 2, max op freq = 250 Hz
NoiseReduction = yes (randomly modulates the switching freq to reduce noise)
SpecialMotor = no (extends adj range of IR compensation
Stt (not in manual) = "NST: Freewhl stop"

Ideas? Should I set the DEC adj to a smaller value?

*" 'Activation allows the deceleration ramp time to be auto increased, avoiding an Obf fault if the ramp time was too short. This function may be incompatible with ramp positioning & with dynamic braking. If Relay2 is assigned to BrakeLogic, DecRampAdapt can only be set to "no". (Relay2 is set to "OCC".)'

#2 marke

marke

    Posting Freak

  • Moderator
  • PipPipPipPipPipPip
  • 2,603 posts
  • Gender:Male
  • Location:Christchurch, New Zealand

Posted 18 July 2008 - 10:13 PM

Hello Birchy

Welcome to the forum.

It sounds like you may have a resonance problem.
Resonances can occur at particular speeds and will result in the output current oscillating up and down and the magnitudes of the oscillations will increase over time.
If you can pass through the resonance zone quickly, you can avoid the oscillations, however where you have a high inertia load, it is not possible to accelerate quickly.

I would suggest that you do some tests to see if it always occurs at the same speed.
If it is definitely speed related, look at using a skip frequency setting to step over this speed to prevent continuous operation at this speed.
Also, check the mechanicals such as mounts, belts couplings etc as there may be a mechanical issue that is enhancing the problem.

Best regards,
Mark.

#3 Birchy

Birchy

    Junior Member

  • Full Member
  • PipPip
  • 6 posts

Posted 20 July 2008 - 02:51 PM

QUOTE (marke @ Jul 18 2008, 05:13 PM) <{POST_SNAPBACK}>
...snip...
It sounds like you may have a resonance problem.
...snip...


From what you describe, it doesn't seem to fit. If I allow a steep ramp, the overbraking fault occurs more dependably and at a lower frequency. If I slow the ramp, the fault occurs at a higher freq. It doesn't seem like I can "punch through" anything fast enough. I will run a test Monday, however, inside the closed unit, to see if I can see/feel any problems during the fault.

#4 marke

marke

    Posting Freak

  • Moderator
  • PipPipPipPipPipPip
  • 2,603 posts
  • Gender:Male
  • Location:Christchurch, New Zealand

Posted 20 July 2008 - 06:37 PM

Hello Birchy

If you decelerate too fast, you will get this problem due to the fact that you are trying to slow a high inertia load too quickly. When you are slowing the load, if the drive is slowing faster than the natural slow down time of the load, you have to pull energy our of the load and this will cause an over voltage trip.

During acceleration, there should be no such problem and if you accelerate the load too fast, you will have over current issues.

If you need to decelerate the load quickly, you can add braking resistors to the drive to help dump some of the load energy.

Best regards,
Mark.

#5 dip04051978

dip04051978

    Intermediate Member

  • Full Member
  • PipPipPip
  • 20 posts
  • Location:Anand

Posted 21 July 2008 - 09:58 AM


But with STOP Type : Freewheel stop, Dec. time will not come in picture as drive there will be no output from the VFD immediately after stop command.
Set motor control to V/F, Energy saving etc & try.


But with STOP Type : Freewheel stop, Dec. time will not come in picture as drive there will be no output from the VFD immediately after stop command.
Set motor control to V/F, Energy saving & try.

#6 Birchy

Birchy

    Junior Member

  • Full Member
  • PipPip
  • 6 posts

Posted 21 July 2008 - 08:46 PM

OK, it looks like it *was* a resonance issue, exacerbated by worn belts. As I watched the belts during ramp, there were times when the slack side vibrated, maybe six zones between 0 and 50% speed. The oscillations were worse at higher speeds. I was going to swap belt position, as the inside belt oscillated more, and was checking to see if it was belts or shieve alignment, but realized the belts were worn more than I thought. I replaced them with new grip-notch belts(previous guy didn't use them, but I've been advised by several techs to transition to grip-notch).

After replacing these belts, I was able to ramp much higher, and faster. I haven't checked to see if it will fault out at super high speeds, I'll just monitor performance. It's Vacation Bible School week, so we have the yearly max of open VAVs and should get a good test tomorrow morning.

Thanks for all the replies, I am a *little* less dumb, and you've saving me from an expensive service call that wasn't necessary at all.




0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users