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Thread: Continuous duty vs. Sever duty???

  1. #11
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    Quote Originally Posted by MattS87 View Post
    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?
    Lack of cooling is only a part of the problems you may have running a motor on a VFD.

    If running at full voltage and rated frequency you still have high voltage spikes that is hard on winding insulation if motor wasn't designed to handle it.
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  2. #12
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    Severe duty motor construction offers superior windings; Stator has epoxy varnish, etc...click on link below and see attached PDF 'Baldor Severe Duty Motors' - go to sht 5.

    https://www.baldor.com/brands/baldor...rs/severe-duty

  3. #13
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    As mentioned severe duty isn’t defined. IEEE 841 is but IMHO overkill for normal HVAC cooling tower service.

  4. #14
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    Quote Originally Posted by Russs57 View Post
    As mentioned severe duty isn’t defined. IEEE 841 is but IMHO overkill for normal HVAC cooling tower service.
    Aside from all the other bells and whistles;
    Severe duty, IEEE 841 design gets you a 3 piece seal that is betted into the end plate and rides on the shaft. It's compromised of metal and rubber compound and is almost impenetrable to dirt and water. EPC design gets you slingers that are metal and seated in the endplate which also ride the shaft. They vibrate so when dirt and dust lands on the shaft, the slinger bounces it away.
    This is mainly for dirty environments such as Chemical and Refinery facilities and may be overkill for Cooling Tower applications - but worth considering.

  5. #15
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    Inverter duty is more about the insulation used in the windings and some of the other issues addressed in premium motors. But the Severe Duty concept does not address the issues regarding standing /reflected wave voltage spikes and bearing fluting. Inverter duty down not inherently cover the bearing issues either, but most quality motor suppliers do that as part of their Inverter Duty designs.
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  6. #16
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    Nov 2014
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    The question I haven’t seen asked and answered is; How long did current motor last and was it the OEM motor?

    Lots of things happen, from lightning to POCO problems, that aren’t the motor’s fault but might appear to be at first glance. Ditto on fan imbalance and other mechanical problems.

    Things are built to a price point. What does interruption of service cost? If a crane is needed, what are costs to replace said motor? Sometimes it pays to buy a higher duty rated motor. One has to do due diligence and ascertain the motor was the culprit though.

    A good motor shop can often diagnose failure mode. It isn’t a bad place to start. I’d want to see bearing/end bell condition and look at fan and mounting hub condition. In my experience motors get an unfair share of the blame.

  7. #17
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    Quote Originally Posted by Jraef View Post
    Inverter duty is more about the insulation used in the windings and some of the other issues addressed in premium motors. But the Severe Duty concept does not address the issues regarding standing /reflected wave voltage spikes and bearing fluting. Inverter duty down not inherently cover the bearing issues either, but most quality motor suppliers do that as part of their Inverter Duty designs.
    Class F insulation and the ISR (Inverter spike resistant wiring) take away much of the risk in VFD capable Severe Duty motors like EM, ECP and ECP8. Not all drives cause electrical spikes or fluting on bearings. For example, ABB Drives consistently do not however, AB Drives consistently do. If bearing fluting is a major concern, a ceramic bearing can be installed in the ODE of the motor, which is usually suggested in any motor above 100 HP that will be solely powered by a Drive (Inverter Duty). The other option would be to install a shaft grounding brush system or an Aegis shaft grounding ring system on the DE of the motor. Of course the Aegis or Brush represents a sparking device and therefore, cannot be placed in a Division 2 location. Cooling Tower is generally unclassified so it shouldn't matter.

  8. #18
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    Thanks guys - this was on a state agency job so it was aaaalllll decided for me - 5K$ for a new motor - exactly what I took out, and 6K$ - for the crane and removal/re-install. Back up and running - another one in the books!!!

  9. #19
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    Two issues when running a motor on a VFD
    Low speed (under 30 hz typically) causes a loss of motor cooling - use invertor duty motor
    High voltage wave form from VFD can damage motor windings - use spike resistant windings. Most motors I have seen recently are invertor spike resistant.
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  10. #20
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    Quote Originally Posted by tom baker View Post
    Two issues when running a motor on a VFD
    Low speed (under 30 hz typically) causes a loss of motor cooling - use invertor duty motor
    High voltage wave form from VFD can damage motor windings - use spike resistant windings. Most motors I have seen recently are invertor spike resistant.
    I installed one one time that does run loaded at low speeds, we used an inverter duty motor with additional electric fan for cooling it.

    When first put that machine on VFD it burned out a motor about once a year, we had a spare motor so swapped and had other rewound. After about 4th time we got the VFD rated, external fan cooled motor, and has ran for maybe 10-12 years now. Bearing has gone out on cooling fan, but careful selection of motor overload has caught it. Wired so drive won't give a run command if cooling fan isn't running.
    I live for today, I'm just a day behind.

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