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  1. #1
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    Confused Dazed & Confused

    So I had a commercial account with a bad 1/2 hp blower motor 240 volts, 825 rpm. I could not get a replacement for about a week. There was an event happening at the facility the next day with 140 people. So I had a rescue motor that was 1075 rpm and was 1/5, 1/4, 1/3, 1/2, 3/4 hp. I wired it up to 1/2 hp settings as per the motor name plate. It was supposed to pull 3.8 amps, but was pulling 9 amps and burned itself out.

    This was done to at least give some cooling knowing that I may not remove much latent heat. This was nit my brightest hour.

    What am I not understanding here?
    Can someone please explain to me -
    Why is there never enough time to do it right the first time, but plenty of time to do it twice?


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  2. #2
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    Apr 2009
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    Dazed & Confused

    Did you do a pulley/rpm/diameter calculation?

    This would have told you if the motor was going to over amp at that RPM with your given pulley sizes.

    (sheeve, pulley, yadda yadda)

    http://www.culvermotor.com/Engineeri...alculator.html

    Try it out and see what happens.


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  3. #3
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    Dazed & Confused

    Double post
    Last edited by ReeferTechy; 08-30-2017 at 07:38 PM. Reason: Double post

  4. #4
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    Sorry, I'll add to my post and my post count.

    Basically you changed a variable within a designed or engineered (somewhat, mostly, hopefully) system.

    The motor was straining because it was spinning a blower not designed for the capacity and ductwork (probably) not designed to move that much air. Resulting in a change of static pressure, etc.

    Always check motor amps after changing the motor or pulleys and try to do a calculation to make sure you stay within spec of the system.


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  5. #5
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    It was probably direct drive if it was a rescue motor.

    But I don't understand why you're confused. You didn't have the right motor so you did what you could. In doing so a motor that spins 1075 was trying to turn a blade designed for 825 and it drew way more amps than intended. End of story. Were you thinking it would be ok if you just matched the correct HP?

    We were throwing all kinds of motors on during the past few heat waves just to get people out of trouble. Most actually did ok, but I don't recall anything as dramatic as 825 to 1075.
    "...and pray it works, pray to God it works."

    ---Nick's Handyman

    https://www.youtube.com/watch?v=LSFmAznl_2o

    I proudly support DIY - I've got enough calls to run.

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  7. #6
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    Possibly if u had used the 3/4 hp tap it might have lasted until you got the proper motor. Learned this the hard way on a motor that I couldn't read the speed on. Faster speed needs higher hp.
    30 percent more work requires at least 30 percent more motor.

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  9. #7
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    Thread Starter
    Quote Originally Posted by stude283 View Post
    Possibly if u had used the 3/4 hp tap it might have lasted until you got the proper motor. Learned this the hard way on a motor that I couldn't read the speed on. Faster speed needs higher hp.
    30 percent more work requires at least 30 percent more motor.
    3/4 hp tap wast the same as 1/2 hp.
    Can someone please explain to me -
    Why is there never enough time to do it right the first time, but plenty of time to do it twice?


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  10. #8
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    In my experience, the different HP's [on the same motor] use different value caps.

    That may have been a factor in your case.


    Quote Originally Posted by joemach View Post
    3/4 hp tap wast the same as 1/2 hp.
    I don't understand happiness as an ambition . . . It's not really worthy. I understand it as a byproduct.
    Harrison Ford

  11. #9
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    Any chance you might've used a 120v motor?


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  12. #10
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    Thread Starter
    Quote Originally Posted by GoCougs View Post
    Any chance you might've used a 120v motor?


    Sent from my iPhone using Tapatalk
    No 120 volt. It was a 208/240 volt motor
    Can someone please explain to me -
    Why is there never enough time to do it right the first time, but plenty of time to do it twice?


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  13. #11
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    Thread Starter
    Quote Originally Posted by hurtinhvac View Post
    It was probably direct drive if it was a rescue motor.

    But I don't understand why you're confused. You didn't have the right motor so you did what you could. In doing so a motor that spins 1075 was trying to turn a blade designed for 825 and it drew way more amps than intended. End of story. Were you thinking it would be ok if you just matched the correct HP?

    We were throwing all kinds of motors on during the past few heat waves just to get people out of trouble. Most actually did ok, but I don't recall anything as dramatic as 825 to 1075.
    Yes, I was just try to get them by until I could get the correct motor. Thinking that the 3/4 hp connection would be get me by.

    Big lesson learned here today. Still feel like an idiot.

    1075 rpm is only about 25% more than 825, didn't figure it would more than double the amp draw. Guess the design engineers are smarter than I thought.
    Can someone please explain to me -
    Why is there never enough time to do it right the first time, but plenty of time to do it twice?


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  14. #12
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    It's really all about fan [blower] curves. They can do things you wouldn't imagine until you study them.

    In the case of what you originally posted, you would've measured the amps and kept dropping the down HP tap until the amps were within limit.
    I don't understand happiness as an ambition . . . It's not really worthy. I understand it as a byproduct.
    Harrison Ford

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  16. #13
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    Oct 2010
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    Next time use a universal 825rpm condenser fan motor. They should be readily available at any parts house, if not on the truck (I carry 2 because they're so common).

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