Continuous duty vs. Sever duty???

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sundowner

sundowner

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


SceneryDriver
 
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.
 
sundowner said:
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.
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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.
 
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
 
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).
 
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.
 
Jraef said:
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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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?
 
MattS87 said:
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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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.
 
Russs57 said:
As mentioned severe duty isn’t defined. IEEE 841 is but IMHO overkill for normal HVAC cooling tower service.
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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.
 
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.
 
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.
 
Jraef said:
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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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.
 
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!!!
 
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.
 
tom baker said:
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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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.
 
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