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Thread: Continuous duty vs. Sever duty???

  1. #1
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    Continuous duty vs. Sever duty???

    So I lost one of my 60 HP cooling tower motors. The plate says continuous duty - my motor shop quoted a replacement rated as severe duty - there is more than a little price difference - my question is what the heck is the difference - I've been in the trade almost 25 years and I'm embarrassed to even ask this - but...

    difference between them - bearings?? windings??

    Thanks guys!

  2. #2
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    Does it run on a VFD at low speeds frequently? If so maybe a motor with an externally powered cooling fan might be called severe duty? Typical fan on same motor shaft doesn't cool so well when motor is running at a low speed and can result in hotter operating condition even though less output is being delivered.
    I live for today, I'm just a day behind.

  3. #3
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    There is no industry standard definition of severe duty motor. Some features I’ve seen from various manufacturers include both electrical and (more) mechanical features:

    All cast iron construction

    Class F insulation with class B temperature rise

    Oversized bearings

    Improved (oversized) cooling fan

    Whether or not you would benefit from a severe duty motor would depend on if any of the features in the motor proposed address the mode of failure of your motor!

  4. #4
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    Quote Originally Posted by sundowner View Post
    So I lost one of my 60 HP cooling tower motors. The plate says continuous duty - my motor shop quoted a replacement rated as severe duty - there is more than a little price difference - my question is what the heck is the difference - I've been in the trade almost 25 years and I'm embarrassed to even ask this - but...

    difference between them - bearings?? windings??

    Thanks guys!
    Do you know how/why your original motor failed? That will inform any choices about a more robust replacement motor.


    SceneryDriver

  5. #5
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    Thanks guys - my motor grounded hard out hard to case - A and B virtually zero resistance to case - C is open - so it took a pretty tragic shot when it burst. It's application in shaft down in a cooling tower. It is on a drive and we are using the drive basically as a soft start - when it gets a run command - I'm just ramping up slowly to 1800 rpm. So when this guy is running it is balls out.

    My "motor sales guy" is gently pushing us to the "severe" duty replacement - the good thing with that is it does have a 5year warranty - I like that. The continuous duty is what I had in there - right from the commissioning phase. Its almost 1200 bucks cheaper to go with the later than the "severe" - boss is already hyper-ventilating - I'm pretty sure he'll go continuous duty as a replacement.

    I was just curious what the difference in motor ratings were/are.

  6. #6
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    Quote Originally Posted by sundowner View Post
    Thanks guys - my motor grounded hard out hard to case - A and B virtually zero resistance to case - C is open - so it took a pretty tragic shot when it burst. It's application in shaft down in a cooling tower. It is on a drive and we are using the drive basically as a soft start - when it gets a run command - I'm just ramping up slowly to 1800 rpm. So when this guy is running it is balls out.

    My "motor sales guy" is gently pushing us to the "severe" duty replacement - the good thing with that is it does have a 5year warranty - I like that. The continuous duty is what I had in there - right from the commissioning phase. Its almost 1200 bucks cheaper to go with the later than the "severe" - boss is already hyper-ventilating - I'm pretty sure he'll go continuous duty as a replacement.

    I was just curious what the difference in motor ratings were/are.
    There really is no difference in motor function - severe duty motors are also continuous duty. Severe duty generally means more robust in nature with cast iron frames and may include metal shaft slingers or inpro seals, epoxy coated windings etc.

  7. #7
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    Cooling tower is a nasty environment. Not necessarily temperature wise, but water, corrosion, etc. FWIW, you will likely get what you pay for. If you get more life, what have you saved by avoiding the cost of down time, removal/replacement, etc? Very real operating costs.

    RC
    It's my name going on that drawing, not yours. If what you want ain't right, it ain't going on the drawings!

  8. #8
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    You do want a "cooling tower" motor. From what you are saying I'm assuming the fan blade is directly attached to motor shaft and motor is in the airstream. If so, I'd look at something like Baldor ECTM4314T. Should be around 6 grand. Marathon has a comparable product (364TTTS16534).

  9. #9
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    What you WANT to make sure you get is a motor designed to NEMA MG1 part 31 specifications, a motor designed for operation from an inverter. That spec will incorporate features of both the severe Duty and Continuous Duty designs, but MORE importantly the motor winding insulation will be designed to handle being run from an inverter. From the description of your failure, I'd hazard a guess as to that being the cause. It will happen again if you don't address it and there is no motor more expensive than the one you have to replace again in a couple of years.

    If the owner INSISTS on being cheap with the motor, try to then talk him into adding a DV/DT filter to the VFD output. When he sees that the filter necessary to allow the use of the cheap motor will cost more than the difference in cost of the better motor, he may change his mind.
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  10. #10
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    Quote Originally Posted by Jraef View Post
    What you WANT to make sure you get is a motor designed to NEMA MG1 part 31 specifications, a motor designed for operation from an inverter. That spec will incorporate features of both the severe Duty and Continuous Duty designs, but MORE importantly the motor winding insulation will be designed to handle being run from an inverter. From the description of your failure, I'd hazard a guess as to that being the cause. It will happen again if you don't address it and there is no motor more expensive than the one you have to replace again in a couple of years.

    If the owner INSISTS on being cheap with the motor, try to then talk him into adding a DV/DT filter to the VFD output. When he sees that the filter necessary to allow the use of the cheap motor will cost more than the difference in cost of the better motor, he may change his mind.
    From what I've read in the past is that inverter duty motors are for when you are running a motor at less than full speed because of the lack of cooling. In the OP's situation of simply using the VFD as a soft start then running at full speed is there still a necessity for inverter duty?

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