Page 1 of 3 123 LastLast
Results 1 to 13 of 29
  1. #1
    Join Date
    May 2011
    Location
    CA
    Posts
    123

    VFDs on a compressor

    I've searched all over the 'net for a good explanation of what, exactly, VFDs do for a compressor. I know they vary the frequency and allow motors to start slowly and build up to speed, but am not exactly clear why this is beneficial. Is it to lower the LRA (inrush current when starting) and save energy? Does it affect the way the compressor runs (isn't that determined by load?) or is it only beneficial when the motor is starting?
    Thanks....

  2. #2
    Join Date
    Nov 2009
    Posts
    385
    Are you talking about recips?

    You will hear the VFD manufacturers tell you that it will save energy and baffle you with graphs and pamphlets. The reality is that it doesn't really pencil out.

    The benefit is for capacity control, smoothing out the suction pressure, reducing compressor cycles, longer equipment life, etc

    I believe that digital discus is a better solution. A recip on a VFD can only be slowed down to 50% of full speed due to lubrication limitations. Digital discus can go down much lower as the compressor stays at full RPM.

    Not real sure that either one saves much energy,
    "Don't wish it were easier, wish you were better"

  3. #3
    Join Date
    Dec 2010
    Location
    Down by the river
    Posts
    1,664
    I just did a start up on a 50HP motor with a ABB freq. drive this morning. Motor FLA was 56 amps nameplate, at 60Hz I was drawing 48amps on the load side of the drive and 22 amps on the line side of the drive. That 22 amp vs 56Fla is a big differance. Now I have never done this on a recip compressor but I wouldn't imagine the end results would vary. Thats a big drop in power consumption.
    It's hard to stop a Trane. but I have made one helluva living keeping them going.

  4. #4
    Join Date
    May 2011
    Location
    CA
    Posts
    123
    I was talking mostly about vfds on reciprocating compressors. This manufacturer's website really got me interested in them, but there's not much info out there on them. http://www.advancedrs.com/

  5. #5
    Join Date
    Nov 2009
    Posts
    385
    Quote Originally Posted by Southern Mech View Post
    I just did a start up on a 50HP motor with a ABB freq. drive this morning. Motor FLA was 56 amps nameplate, at 60Hz I was drawing 48amps on the load side of the drive and 22 amps on the line side of the drive. That 22 amp vs 56Fla is a big differance. Now I have never done this on a recip compressor but I wouldn't imagine the end results would vary. Thats a big drop in power consumption.
    There is a big difference. With fans and blowers the fan curve theory applies. There is a large energy savings when reducing the fan speed. Compressors are linear loads. Not a lot of energy savings.
    "Don't wish it were easier, wish you were better"

  6. #6
    Join Date
    Feb 2008
    Location
    California
    Posts
    2,066
    out here where I work some stores run 1 compressor per suction group off of a VFD.

    the benefit is capacity control you will be able to nearly match the exact load requirement. your energy savings is not going to come from the compressor on the VFD, but rather from the other compressors not on the VFD that will cycle less.

  7. #7
    Join Date
    Sep 2008
    Location
    Western PA
    Posts
    25,670
    Quote Originally Posted by Phase Loss View Post
    out here where I work some stores run 1 compressor per suction group off of a VFD.

    the benefit is capacity control you will be able to nearly match the exact load requirement. your energy savings is not going to come from the compressor on the VFD, but rather from the other compressors not on the VFD that will cycle less.
    This is the correct answer.

    Compressors are deemed a "constant torque" load. VFDs do not save energy on these. It still takes 'X' amount of force to turn the motor at 50% speed as it does at 100% speed.

    Fans (both centrifugal and prop) are deemed "variable torque" loads. This is where you can see some very real energy savings by use of a VFD.

  8. #8
    Join Date
    Jul 2009
    Location
    Michigan
    Posts
    103

    Not all compressors are the same.

    Ultimately the saving is derived from the use of the VFD's to more closely match the load. Isn't that the savings. The first biggest VFD compressor savings is on the centrifugal, second is found on the screw, and third is found on a reciprocating compressor with cylinder unloaders. Anytime at which 100% capacity isn't required you are saving energy.

  9. #9
    Join Date
    Jun 2006
    Posts
    203
    Years back i saw a VFD on a cope (RECIP) supplying it with 220v 3 phase from a 220v single phase power source. I know this is not what you questioning but i thought that was very interesting and never saw any thing like that b4.

  10. #10
    Join Date
    Jul 2000
    Location
    Guayaquil EC
    Posts
    10,364
    Quote Originally Posted by jpsmith1cm View Post
    This is the correct answer.

    Compressors are deemed a "constant torque" load. VFDs do not save energy on these. It still takes 'X' amount of force to turn the motor at 50% speed as it does at 100% speed.

    Fans (both centrifugal and prop) are deemed "variable torque" loads. This is where you can see some very real energy savings by use of a VFD.
    Compressors do in fact require significantly less power at lower speeds. The BHP/Ton vs RPM relationship is fairly close to a straight-line curve. If you look at Fig. 20 on p.31 of this Carlyle 5F/5H Application Manual (5F-5H Compressor Application Manual.pdf), you'll see what I mean. The power requirement per ton actually gets lower as speed is reduced. This is due primarily to lower mechanical losses at lower speeds.

    What this chart doesn't show is how current variable speed compressor technology takes advantage of DC motors and VFD to achieve additional capacity with overspeed.

    The Copeland Variable Speed Scroll can run at a 60% speed reduction up to a 40% overspeed. This gives a pretty wide range of operation to match the load. Watch this video from Copeland.

  11. #11
    Join Date
    May 2011
    Location
    CA
    Posts
    123
    I may be way off base here, but...

    Don't the utilities somehow "peg" commercial usage to the largest amount of current used at a given time? Not sure how to explain this because I don't fully understand it, but I've heard that if you have a huge current spike--say, when a very large compressor starts and draws its locked rotor amperage--that commercial customers can be put into a higher billing "tier" or something like that. Or maybe it's just evil PG and E.

    Anybody ever heard of this before? If it's true at all, then I can see how slowly starting a compressor and limiting the "inrush" current could help lower utility bills...

  12. #12
    Join Date
    Apr 2010
    Location
    southern california
    Posts
    535
    Quote Originally Posted by Iceneck View Post
    I may be way off base here, but...

    Don't the utilities somehow "peg" commercial usage to the largest amount of current used at a given time? Not sure how to explain this because I don't fully understand it, but I've heard that if you have a huge current spike--say, when a very large compressor starts and draws its locked rotor amperage--that commercial customers can be put into a higher billing "tier" or something like that. Or maybe it's just evil PG and E.

    Anybody ever heard of this before? If it's true at all, then I can see how slowly starting a compressor and limiting the "inrush" current could help lower utility bills...
    Most commercial customers are billed for electric usage by kw-hrs.and demand. Demand charges are usually based on peak demand and is based on the greatest registered demand in each billing period.This would be called Facilities-Related Demand, there is also Time-Related Demand which is the peak demand registered during a summer season billing period. Demand charges are the means in which the utilities recover the cost for installed transmission and distribution facilities required to serve the customers needs at the time of highest demand. With that said, If a customer can reduce their monthly demand or reduce the peak then there is a possibility of tremendous savings. Load shedding equipment is the best means, energy analysis of each building is the only way to determine if savings are possible. Vfds and power factor correction are means to improve efficiencies and lower actual kws.

  13. #13
    Join Date
    Sep 2010
    Posts
    21
    Remember that demand charges are usually based on a sliding window, in which the greatest demand average over a period of 10 or 15 mins. is what the customer is billed for. Which means that motor starting currents don't account for much on the demand load as it only lasts for a second or two out of the 900 seconds of the 15 minute window that are averaged (around 0.1% of 15 minutes).

    One of the biggest way to save on the demand charges is to sequence the equipment and control it such that no more than a certain number of compressors/motors/etc. are running at any given time.

Page 1 of 3 123 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event