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80 centigrade cooling air over induction motor

80 centigrade cooling air over induction motor

80 centigrade cooling air over induction motor

(OP)
The scenario: Fans over a radiator field. Sucking heated air upwards from the radiators below. Water in radiators is 80+ centigrades. Geographically sometimes close to equator sometimes in 'Siberia'. The induction motors are cooled by the air streaming from radiators over motors.

Questions: How much would you derate the fans in the former situation compared to rating in 'Siberia'? What would the life expectancy be if not derated? What would the typical failure mode be?

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: 80 centigrade cooling air over induction motor

My guess is if the machine is loaded near nameplate rating and the ambient temperature is quite a bit higher than 40C, you can expect an increasing in winding failure rates and bearing failure rates.

That's probably not going too far out on a limb. The only thing I have to go on is that it reminds me of a transformer fan.  We have had moderately high failure rate on our transformer fans maybe a ratio of 2 bearing failures for every one winding failure (swag).   I don't know to what extent external temperature, motor loading, etc played a role in our failures.
 

=====================================
(2B)+(2B)'  ?

RE: 80 centigrade cooling air over induction motor

I think you can figure the surface of the motors at ~ 5-10 C above the 80 C coming off the radiator water.  With more electrical experience than I, you can start increasing the temperature inside the motors from there with respect to what the temperature rise of the insulation might be.

The problem as I see it is that the motors were probably designed to reject heat to an atmosphere around them of no more than 40C max.  The internal delta T is calculated and designed accordingly.  With the atmosphere around them at twice that, the internal temperature gradient is much higher than the original design.  Not twice, that is just the outside temperature difference, but much more than twice.

That's my two centavos worth.

rmw

 

RE: 80 centigrade cooling air over induction motor

Install winding RTD's and monitor the temp under load. Max recommended for class F is 140 deg C.

Muthu
www.edison.co.in

RE: 80 centigrade cooling air over induction motor

I seem to remember an old rule of thumb for transformers.
Each 10 Degrees above design temperature causes a 50% reduction in life expectancy.
Are you able to work backwards from typical failure times and tweak this figure or is this a new installation?
Failure modes:
I would expect insulation failure from elevated temperatures to be the electrical failure mode.
I suspect that there may be greases available that will safely withstand the elevated temperatures. It's easy to say;
"Use a high temperature grease!"
but I can't recomend a specific grease. Possibly a link to one of the automotive or mechanical fora may yeild some good suggestions.
How is your tropical vacation going? How much snow will be waiting for you to dig out when you get home?
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: 80 centigrade cooling air over induction motor

If these are dry radiators instead of evaporative coolers, it might be possible to invert the installation and place the fans, motors, and gearboxes on the bottom pushing upward.  Not only would machinery lifetimes be improved, but the fans would be more efficient in the cooler air, and it would also be possible to service the machinery while the radiator field was in service.

RE: 80 centigrade cooling air over induction motor

Some more thoughts:
1 – It is possible to guesstimate winding temperature from current measurements, ambient air temp, nameplate info (FLA, insulation class, temperature rise sometimes also listed for large motors but we can swag that if not). Then you can compare to insulation rated temperature use Bill's Arhhenius relation to guesstimate change in lifetime
2 -  It is certainly possible to determine average winding temperature by recording dc resistance over time after shutdown.  It involves plotting on log resistance (or temp) vs linear time scale and drawing a straight line back to t0.  You are probably familiar with this method, but if you want more info, I can provide it.

=====================================
(2B)+(2B)'  ?

RE: 80 centigrade cooling air over induction motor

Failure mode of a small selection of PSC motors over a/c condenser fans in this subtropical climate comprises first  dryout failure of sleeve bearings, then winding overheat from reduced speed and excessive slip, then stall or failure to restart, at which time the problem becomes detectable and the motor becomes junk.

However, I sized a big radiator array last year, and I could swear the input data included local ambient, so I'd guess the array manufacturers have a handle on derating.  ... but I can't imagine any commercial incentive for them to reveal what they know, since they sell the arrays as systems.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: 80 centigrade cooling air over induction motor

We used to use fan motors in lumber drying kilns. Some kiln manufactures now design the kilns so that the motors are in a more forgiving ambient and either shaft drive or belt drive the fans. Both drive approaches have their own issues with the temperature.
The motors were derated by the motor manufacturer and were enormous. Still burnouts were common both from heat and from moisture.
You know more about motors than most of us, so I assume that you are looking for an overview of motors in hot locations. I hope this anecdote helps.
It sounds like you are stuck with a design that sucks.  

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: 80 centigrade cooling air over induction motor

(OP)
To all.

I haven't visited the site yet. Talked it over with the producer yesterday and it turns out that there are DGBBs (no sleeve), motors (designed for horizontal operation) vertically mounted with a rather heavy fan on top and fan sucking hot air from the radiator over the motor. Fan excerts downward force on rotor and bearings that are not designed for that extra axial load.

The manufacturer of the radiator field is a well-known player but I have a suspiscion that he didn't take all facts into account this time. The motors do not seem to be 'enormous' as Bill says.

The motor protections have been set to their maximum, which is a bit above rated current. No NTC/Pt100/RTDs in the motors.

All the warnings (other thread) have made me think. And I am now trying to arrange things slightly differently. I will probably train someone younger and tougher to do the measurements and observations and support him via internet/e-mail and phone. Could be quite efficient. And, hopefully, not inviting Montezuma and his warriors.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

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