VFD Ramp Times with high enertia loads?
Close
Login to Your Account
Results 1 to 18 of 18
  1. #1
    Join Date
    Mar 2009
    Country
    UNITED STATES
    State/Province
    California
    Posts
    4,827
    Post Thanks / Like
    Likes (Given)
    89
    Likes (Received)
    892

    Default VFD Ramp Times with high enertia loads?

    Should the up and down times be equal?

    I'm testing a compressor with a cast iron flywheel that is dual belt and 18" diameter. 5Hp motor with 10 seconds for accel/decel times. They are default numbers for the drive. Works fine.
    I searched around a little before asking, is there a better way to chose the numbers? Do the numbers have to be equal?

    Thanks.

  2. #2
    Join Date
    Sep 2011
    Country
    UNITED STATES
    State/Province
    Virginia
    Posts
    25,645
    Post Thanks / Like
    Likes (Given)
    7561
    Likes (Received)
    8077

    Default

    Quote Originally Posted by rons View Post
    ...compressor with a cast iron flywheel that is dual belt and 18" diameter. 5Hp motor with 10 seconds for accel/decel times. They are default numbers for the drive. Works fine.
    I searched around a little before asking, is there a better way to chose the numbers?
    Trial and error, given there is probably not much breadth of data for your specific equipment. "If it ain't broke...."
    Do the numbers have to be equal?
    No. Ramp-up depends on how hard the VFD has to work to not trip-out, but ramp-down can usually be much less - perhaps even zero in this case? If it even matters. Few VFD have "regenerative" provisions, and your load doesn't seem to need such anyway, so AFAICS, all you might gain is less EMI/RFI and spike-stress than abrupt contactor opening would produce if you were running off mains 3-Phase instead of a VFD.

    Actual "braking" is a whole 'nuther story, but doesn't apply here at all.

  3. #3
    Join Date
    Apr 2011
    Location
    NE Indiana
    Posts
    108
    Post Thanks / Like
    Likes (Given)
    14
    Likes (Received)
    17

    Default

    Why ramp down on a compressor? My big 350 and 390 Quincy's stop almost immediately when the pressure switch kicks off.

    Sent from my SM-N960U using Tapatalk

  4. Likes Hopefuldave liked this post
  5. #4
    Join Date
    Jun 2001
    Location
    St Louis
    Posts
    17,768
    Post Thanks / Like
    Likes (Given)
    1755
    Likes (Received)
    2946

    Default

    if you have no problem, you must already be in the sweet spot.

    BUT, if not, if you have issues, then try BOTH slowing the accel, AND speeding it up.... Too slow and the overload on the VFD may trip out on time. By speeding it up, you may not go iver the "instant" current trip, and you may get all done before the timeout has expired.

    Most VFDs have a 150% overload for anywhere from a few seconds to over a minute. For higher currents, there may be no spec, but there will be a limit. if spec'd, it might be 150% 2 minutes, 200% 10 seconds, or similar times... so you need to get the accel done before the 10 seconds, if you go up to 200% normal current.

    Speeding the accel can often (not always) sneak you in without exceeding limits.

  6. #5
    Join Date
    Mar 2009
    Country
    UNITED STATES
    State/Province
    California
    Posts
    4,827
    Post Thanks / Like
    Likes (Given)
    89
    Likes (Received)
    892

    Default

    Quote Originally Posted by Mr.Green View Post
    Why ramp down on a compressor? My big 350 and 390 Quincy's stop almost immediately when the pressure switch kicks off.

    Sent from my SM-N960U using Tapatalk
    To avoid a bus over-voltage zone due to regeneration.

    A quote from Precision Electric:

    The most common time a VFD overvoltage fault occurs is during deceleration. Sometimes the braking torque requirement exceeds drive braking circuit capacity. Other times the deceleration is too fast for its load and inertia from the load is going faster than the designated frequency. If you hit stop during a ramp down, the load spins faster than the designated frequency, the motor regenerates power back into the drive. The motor load then turns into a generator. This power is fed back into the drive, and stored on the DC bus. Extending the deceleration time is one way to solve a VFD overvoltage fault during deceleration.

  7. #6
    Join Date
    Apr 2011
    Location
    NE Indiana
    Posts
    108
    Post Thanks / Like
    Likes (Given)
    14
    Likes (Received)
    17

    Default

    Quote Originally Posted by rons View Post
    To avoid a bus over-voltage zone due to regeneration.

    A quote from Precision Electric:

    The most common time a VFD overvoltage fault occurs is during deceleration. Sometimes the braking torque requirement exceeds drive braking circuit capacity. Other times the deceleration is too fast for its load and inertia from the load is going faster than the designated frequency. If you hit stop during a ramp down, the load spins faster than the designated frequency, the motor regenerates power back into the drive. The motor load then turns into a generator. This power is fed back into the drive, and stored on the DC bus. Extending the deceleration time is one way to solve a VFD overvoltage fault during deceleration.
    Right, I deal with finding that sweet spot all the time, I just don't see the point of even enabling that function for a compressor.

    Sent from my SM-N960U using Tapatalk

  8. #7
    Join Date
    Sep 2011
    Country
    UNITED STATES
    State/Province
    Virginia
    Posts
    25,645
    Post Thanks / Like
    Likes (Given)
    7561
    Likes (Received)
    8077

    Default

    Quote Originally Posted by rons View Post
    To avoid a bus over-voltage zone due to regeneration.

    A quote from Precision Electric:

    The most common time a VFD overvoltage fault occurs is during deceleration. Sometimes the braking torque requirement exceeds drive braking circuit capacity. Other times the deceleration is too fast for its load and inertia from the load is going faster than the designated frequency. If you hit stop during a ramp down, the load spins faster than the designated frequency, the motor regenerates power back into the drive. The motor load then turns into a generator. This power is fed back into the drive, and stored on the DC bus. Extending the deceleration time is one way to solve a VFD overvoltage fault during deceleration.
    As said, you are probably not in that sort of environment. It isn't like a motor powering a large lathe with a massive hunk of metal in rotation on good bearings.

    The mechanical side of an air compressor is still trying to ... would you believe "compress" air or "do work". It should act as a fair-decent brake.

  9. #8
    Join Date
    Aug 2008
    Location
    Sussex, England
    Posts
    3,334
    Post Thanks / Like
    Likes (Given)
    12
    Likes (Received)
    711

    Default

    Academic as you have a system that works but a VFD is an active control system and, in general, such behave best if they are always in control of the load rather than reacting to extraneous events. So I'd always run a sensible ramp down time and sensible ramp up.

    Generally with small loads like your compressor ramp down or not won't matter. But it might if the stars align just right. Never a good idea to get into the habit of cutting corners 'cos its usually OK.

    As far as ramp up is concerned I have seen a compressor stall out during acceleration of a Hydrovane compressor, not normally considered a hard starting load, and settle at half speed due to a combination of incorrect ramp up time and insufficient torque partway through acceleration. Book numbers on the Eaton drive, a decent brand, said it should have been quite happy on that compressor. Resolved that one by finding the torque boost on run-up setting which basically trades increase in maximum permitted overload current for reduction in overload time. Tweaking the run up ramp time would probably have sorted it too. Zero °C in the barn at the time which probably didn't help.

    Clive

  10. #9
    Join Date
    Mar 2009
    Country
    UNITED STATES
    State/Province
    California
    Posts
    4,827
    Post Thanks / Like
    Likes (Given)
    89
    Likes (Received)
    892

    Default

    This is the model Allen Bradley VFD I am using.

    PLC Hardware - Allen Bradley 20AB022A0AYNANNN

  11. #10
    Join Date
    Mar 2009
    Country
    UNITED STATES
    State/Province
    California
    Posts
    4,827
    Post Thanks / Like
    Likes (Given)
    89
    Likes (Received)
    892

    Default

    Quote Originally Posted by thermite View Post
    The mechanical side of an air compressor is still trying to ... would you believe "compress" air or "do work". It should act as a fair-decent brake.
    It brakes like hitting a brick wall. Too fast in my opinion.

  12. #11
    Join Date
    Mar 2009
    Country
    UNITED STATES
    State/Province
    California
    Posts
    4,827
    Post Thanks / Like
    Likes (Given)
    89
    Likes (Received)
    892

    Default

    Quote Originally Posted by Clive603 View Post
    Academic as you have a system that works but a VFD is an active control system and, in general, such behave best if they are always in control of the load rather than reacting to extraneous events. So I'd always run a sensible ramp down time and sensible ramp up.
    How about a sensible equation for up and down times basewd on:
    motor hp
    input/output pulley dimensions, weight.
    stroke of pump
    etc.

  13. #12
    Join Date
    Mar 2006
    Location
    Auburn CA
    Posts
    356
    Post Thanks / Like
    Likes (Given)
    0
    Likes (Received)
    10

    Default

    Quote Originally Posted by rons View Post
    It brakes like hitting a brick wall. Too fast in my opinion.
    The compressor was designed for a zero ramp de-accel time. Non VFD operation simply removes the power and the compressor stops in a very short amount of time all on its own. Simply set the down time to 0 with no braking to emulate what it was designed to do.

    The accel time is more of a crap shoot. It has to overcome what the VFD is unable to do, provide extreme start up current. You could tune it by trying longer and shorter times and setting midpoint between any issues found, if any.

  14. #13
    Join Date
    Aug 2008
    Location
    Sussex, England
    Posts
    3,334
    Post Thanks / Like
    Likes (Given)
    12
    Likes (Received)
    711

    Default

    Quote Originally Posted by rons View Post
    How about a sensible equation for up and down times basewd on:
    motor hp
    input/output pulley dimensions, weight.
    stroke of pump
    etc.
    RTFM!

    Its all in there somewhere for the harder loads that do stretch the VFD capability. Probably somewhere the far side of page 100.

    For any ordinary load that is easily inside the VFD performance envelope any few seconds will do just fine to keep the VFD in control of whats happening rather than reacting.

    As Bill says the compressor and motor assembly was designed for simple switch power on switch power off operation relying on the "natural" behaviour of the motor to set how long it takes to spin up and spin down.

    VFD systems are designed to ramp the motor up to speed and ramp it down so thats the way you should operate them. Like everything they can work OK if used in a different manner to what they were designed to do. But there will be edge effects where you can't get away with that. Down to you to ensure none of the edges effect your installation. Ever! Unless you like troubleshooting "only does it sometimes" issues. Far better to stick with the book 'cos the makers will have sorted all the safe operating area, edge performance effects, yadda, yadda things. If there is a real unexpected oddity chances are the makers have already heard of it and can suggest a fix for when the box doesn't do what it orta.

    Clive

  15. #14
    Join Date
    Oct 2004
    Country
    UNITED STATES
    State/Province
    Iowa
    Posts
    3,089
    Post Thanks / Like
    Likes (Given)
    56
    Likes (Received)
    346

    Default

    Set the VFD to RAMP UP, and do it as QUICKLY as you can, without putting too much a hurt on your mains.
    Set the VFD to NO RAMP on shutdown... just drop out... do NOT attempt to control the stop whatsoever.

    The compressor was designed around an across-the-line start/stop by pressure switch. The unloader for that compressor, in conjunction with the compressor head output volume (downstream of the exhaust valve) serves several functions, but during starting, the unloader trip oint, and that output volume provide a stroke or two of 'no compression', followed by several strokes of 'partial' compression load. After about 4 or 5 strokes, you've got full head pressure load.

    You DO NOT want to ramp it SLOW, because this will run you through many strokes, and introduce head pressure load BEFORE getting the motor up to speed.

    Next: lubrication. The compressor either has splash, pump, or differential pressure lubrication. Splash is obvious, pump is obvious. Differential pressure lubrication uses difference between pump outlet pressure and inlet pressure to siphon lubricating oil from the pump's sump, to it's necessary parts. IF you run a compressor TOO SLOW, it will not lubricate properly under splash or pump operation. If you run a compressor with insufficient differential pressure, it will not lubricate properly... so get it up to proper speed.

    Finally... ramp down... there is no reason to utilize dynamic or dynamic regenerative braking- there is no useful energy recovered, and no advantage to control the stop of compressor. Slowing the compressor will cause the unloader to flutter, rather than disengage... AND... if your lubrication system uses differential pressure AND the unloader dumps, but you keep spinning the compressor, you'll be running it with crippled lubrication.

    Just set it to ramp up with as much expedience as you can (I would shoot for no more than 5 seconds) and have it simply cut-off at shutdown... no ramp, no DB, just drop out. The parameter on my drives usually call this "coast-to-stop".

    The advantage to the VFD in this application is INRUSH. The Across-The-Line start inrush will be really high, but using the VFD to ramp it, even a FAST ramp, will quell that inrush substantially.

    If you REALLY want to quell inrush, you can alter the drive ratio of your motor (smaller motor sheave) and recalculate the motor speed required to re-establish proper pump speed, and then program the VFD for higher run frequency. This will reduce motor torque during the ramp.

  16. Likes JCByrd24 liked this post
  17. #15
    Join Date
    Mar 2009
    Country
    UNITED STATES
    State/Province
    California
    Posts
    4,827
    Post Thanks / Like
    Likes (Given)
    89
    Likes (Received)
    892

    Default

    Quote Originally Posted by DaveKamp View Post
    Set the VFD to RAMP UP, and do it as QUICKLY as you can, without putting too much a hurt on your mains.
    Set the VFD to NO RAMP on shutdown... just drop out... do NOT attempt to control the stop whatsoever.

    The compressor was designed around an across-the-line start/stop by pressure switch. The unloader for that compressor, in conjunction with the compressor head output volume (downstream of the exhaust valve) serves several functions, but during starting, the unloader trip oint, and that output volume provide a stroke or two of 'no compression', followed by several strokes of 'partial' compression load. After about 4 or 5 strokes, you've got full head pressure load.
    The extended copper and filter bowl (volume) is much greater than the original 1/2" tube of just a few feet. The increase in volume is 49.5 times. Then there is the volume of the black filter bowl. The time it is taking to fill up the new volume will allow a faster ramp up. The unloading time before was a couple of seconds. Now it is 30 seconds. So I can probably try 3-5 seconds for the ramp up and 0 seconds for the ramp down.

  18. #16
    Join Date
    Aug 2010
    Location
    Tucson, AZ
    Posts
    162
    Post Thanks / Like
    Likes (Given)
    4
    Likes (Received)
    34

    Default

    With air compressors running off of a VFD, stop Mode selection I typically recommend is a free run stopping w/o electronic braking, this disconnects the output section of the VFD and the compressor will stop on it's own compression which is typically 2-3 seconds. Ramp up as you have outlined 3-5 seconds and you can adjust the VFD parameters/time so one does not incur a fault mode or lubrication issue. Most piston compressors have an operating range (RPM) and may use with different motor sizes and pulley ratios, so there is a range of acceptable RPM for a specific compressor pump.

  19. #17
    Join Date
    Jan 2009
    Location
    Maine, USA
    Posts
    127
    Post Thanks / Like
    Likes (Given)
    64
    Likes (Received)
    56

    Default

    Quote Originally Posted by rons View Post
    So I can probably try 3-5 seconds for the ramp up and 0 seconds for the ramp down.
    Make sure you understand your VFD settings/terminology with respect to ramp down vs coast to stop. You don't really want to ramp down in 0 seconds, with the VFD trying to control the stop. That implies braking and doing so in zero seconds will almost surely trip the VFD on DC bus overvoltage. You want to VFD to disconnect power and the compressor to come to a stop on it's own, as has been said. Often referred to as coast.

  20. #18
    Join Date
    Mar 2009
    Country
    UNITED STATES
    State/Province
    California
    Posts
    4,827
    Post Thanks / Like
    Likes (Given)
    89
    Likes (Received)
    892

    Default

    Thanks guys.

    Right now I have 5 seconds ramp up and coast to stop.
    The main tank is not accumulating any significant moisture, the black filter is collecting water. The system works better than I thought it would.


Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •