Cooling Tower Motor
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  1. #1
    Just installed new BAC Cooling Towers on job. The motor is rated at 230v - not 208-230v just 230v. Its a Leeson two speed motor. The incoming power to my starter is 204/202/206v (below the allowable 10%)and drops to 198v when motor is operating at high speed. I am having a problem with the new starter pitting the contacts badly and quickly. The operating amperage is within range - rated 33/13 actual 26/8. It seems the problem is at startup or when the contactor pulls in.Sometimes its ok but the contacts are wearing out quickly.This being a cooling tower the starting torque is high. The decreased voltage I believe is creating very high starting current ( measured it in the 150- 160 range)causing the contacts to pit and soon after fail.

    Question - will changing the motors to 208-230v motors help. The incoming power is what it is. Everything else on the job Chillers, AHUs, ACs, CUs,Exhaust Fans are 208 -230v and have had no problems. I realize that the decreased voltage increases the amperage but should be ok as long as it within Nameplate rating, but wondering if this help my starter problem. Also the control tranformer for the cooling tower starter is only producing 107v because of the low incoming power - coils are 115-120. This maybe adding to the problem and not giving the starer the full control voltage needed to pull in properly. Do the coils have the same voltage tolerance as the motors?

    Any ideas?

    Thanks in advance.

  2. #2
    Join Date
    Jul 2000
    Location
    Dallas,Texas
    Posts
    4,948
    Based on the amps you are in the 7.5 -10 HP range. Not a cheap 2 speed motor.

    How about 3 buck/boost tranformers on this circiut in the mechanical rroom. A small buck/boost will handle the load. A 32 volt will bring your voltage up to at least 230 volts. It will fix your control voltage problem too.

  3. #3
    Join Date
    Mar 2003
    Posts
    7,321
    Call the genuis who bought the equipment and ask for his opinion. if he is happy, document it and walk away. you will be back to fix the problem soon enough. at that time you will have an open checkbook.

  4. #4
    Join Date
    Oct 2001
    Posts
    2,096
    Smokies,
    I am not familiar with two speed three phase motors.
    How do they work? What makes them change speed?

  5. #5
    Join Date
    May 2006
    Location
    N.E. IA
    Posts
    512
    How about comparing to variable speed drives. The price is coming down. They will do anything you want with the voltage, hertz, and amps. Wear and tear on belts and pulleys is much less as well as electrical useage.
    "Wheel" mechanics work on "Wheel" chillers

  6. #6
    Join Date
    Feb 2006
    Location
    Lexington, Ky
    Posts
    169
    You may be on to something with the low control voltage. It is very possible that the coil is not drawing in as quickly/firmly as it should.

    Another thing I would check is your cycle time between starts and stops. It may be that the fan is simply cycling too much. Can you spread out the range?

  7. #7
    Join Date
    Jun 2001
    Location
    San Diego, CA
    Posts
    1,149
    Yep yep, all good points. I second getting the tower on a variable speed drive. It will pay for itself quickly in reduced wear and tear while eliminating the 200amp starting surges that mostly determine your commercial billing rate.

    Make sure your control voltage is within spec and the LRA rating of your contactor/starter is in excess of the motor's LRA. Maybe you got a control voltage issue that chatters the contactor? If your Trans has a 200/208 input tap it should be used. If not, change it out so you can get the 120v up.

  8. #8
    Join Date
    Aug 2006
    Posts
    2,579
    You need time delays between your speeds, (minimum application high to low) set them so you're almost to a stop, before it switches to the next speed. Has been a long time since I set one of these up. BAC or Evapco dealer should be able to tell you how to retro fit the two speed starter. PS At least you don't have two motors!

  9. #9
    Join Date
    Aug 2000
    Location
    Austell, Ga.
    Posts
    1,294
    Smokies..
    You undoubtelt have a "T" Frame motor...
    "All" T-Frame motors are "Voltage-Specific", by that it simply means it "Cannot" be used in a 208-230 volt condition..
    Your options are many...
    If your relationship w/BAC is great...then perhaps an exchange motor might be considered...
    As mentioned earlier, a Buck/Boost transformer could be installed at a fairly considerable cost..
    I do believe that the most economical & by far the most reliable option would be obtaining a single speed motor along with a VSD..The payback in energy & failed belts alone makes this the most favored option.
    The 2-speed switching is not that bad when going from Low to High.....But when going from Hi to Low it is imperative that a time delay of several seconds be used so as to keep the belts on...When swithing from a 4-pole to an 8-pole configiration, the 8-pole torque is awwwwwwwwesome' and will snatch the belts off in a hearbeat...
    Myself...I would seek the single-speed with a VSD without reservation...
    PS...
    Locate & castrate the one who ordered this configaration...We do not want him multiplying..
    Ain't "None" of us as smart as "All" of us..

  10. #10
    Join Date
    Apr 2005
    Location
    where the beer flows like wine
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    2,871
    Originally posted by sigma
    Smokies,
    I am not familiar with two speed three phase motors.
    How do they work? What makes them change speed?
    I've worked on many BAC and Marley and I've never seen one either!

  11. #11
    Join Date
    May 2006
    Location
    N.E. IA
    Posts
    512
    RichardL and Madhat touched on the switching. Essentially there are two separate sets of contactors to configure the motor as 4 pole or 8 pole. It is imperative that the contactors be interlocked so both speeds do not energize at the same time. THAT WOULD BE DIASTROUS. Also there needs to be some time for the high speed to bleed off before the low speed engages.
    "Wheel" mechanics work on "Wheel" chillers

  12. #12
    Join Date
    Apr 2005
    Location
    where the beer flows like wine
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    2,871
    refer could you post a link?

  13. #13
    Join Date
    May 2006
    Location
    N.E. IA
    Posts
    512
    I'm going from memory and my "OLD" electiric motor control text book. Been "googling" motor controls but don't have any "links" for you, yet. Anybody else out there???
    Essentially it's the same as trying to put your vehicle in two gears at once or down shifting to fast.
    "Wheel" mechanics work on "Wheel" chillers

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