Motor skin temperature limits> 7

[electricpete]

For smaller motors (100hp) without any embedded temperature detectors, at what skin temperature do you become concerned?
Are there any thumbrules for estimating winding temperature from skin temperature and air discharge temperature?

I'm thinking of a Siemens Allis 100 hp 3600hp motor. It runs hotter than it's sister units when you feel the motor casing (center of the stator... not at the ends by the bearings). About 160F. Clampon current reading doesn't indicate an overload condition (90 amps on 480v system).

 

[electricpete]

Whoops. I meant 3600 rpm, (not 3600hp)

 

[don01]

hi electricpete,
Thermal conductivity and motor case temps is a real pain in the a***.
It really is hard to draw a comparision between the cases. I take they are similiar loads same speed and work done. This current is about 75KW at pf=1 my memory says this approximates 95hp. (pretty close to the motor rating?)
The 160F equates to about 80C this would indicate the winding temps are a lot higher. Check the class of insulation on the name plate. For rule of thumb halve the temp rating- is it close to the surface temp ? if so then call the seimens rep and clarify it.

I have seen a lot of F***in hot motors (measured the temp with my hand) that haven't had a problem so a chat with seimens would be the go.

let us know how it goes
Don

 

[electricpete]

Your ballpark rule of thumb is the motor surface temp shouldn't exceed approx 1/2 of insulation temperature limit? Is that in degrees C? ie for class B should not exceed (40C amb+80C rise+10C HSallowance)/2 =130C/2=65C?

 

[don01]

electricpete
I hadn't ever thought about it but I suppose the answer is yes. Sorry but i live in a metric world and the first thing we do whenever imperial measures are used is to say "that approximates x".
The more I think about it the more I would worry where you say it's hot in the middle not at the bearings. I would exoect the temp to grade from the fan to drive end with a bit of a fudge factor near the bearings. (soundz really scientific doesn't it)
I have made a note in my diary to ask a trusted motor rewinder what he would expect on Monday. no promises but I'll let you know
Rgards Don

 

[electricpete]

Thanks Don. This particular motor has inlet ventilation on both ends and outlet in the middle (open construction, not TEFC). So in this case it makes sense that the middle is the hottest. We have seen on occasion where the end will get hot due to bearing lubration problems, but not in this case.

 

[jbartos]

Suggestion: Try

http://www.sea.siemens.com/motors/

etc.
for more info and training

 

[electricpete]

jbartos - The man with a link or a reference for everything.
That's a pretty good site that Siemens has. I like the articles on motor vibration and troubleshooting. Nothing on skin temperature... but still a good site. I'll give you a star for that one.

 

[don01]

hi electricpete
Oh well the best laid plans of men and mice?? My motor rewinder said yep you got a problem but like me he is talking TEFC oh well s**t happens. Assumptions arn't a good thing hey?
Re jbartos. The man with a link or reference waddyameen "A" link .more like 5 !! somewhat frightening really but give the guy credit - nearly every thread. How does he do it? Mr (or mrs)jbartos don't stop it's awesome. I am for ever making little notes for later & thanks.

Regards Don

 

[rhatcher]

Hey electricpete,
This site will help out.

http://www.usmotors.com/Service/faq11.htm

From US Electrical Motors, the frame temperature is usually 15-20 degrees centigrade less than the average winding temperature. This seems reasonable to me and results in:

71C (surface temp 160F) + 20C (surface comp factor) = 91C average winding temperature and 101C maximum winding temperature (hot spot allowance).

You should be OK with your motor as Siemens does rate their motors to run at Class B temperature rise at full load.
This means (for example) that with a 40C ambient and a Class B (80C) rise, the surface temperature at full load would be

40C(ambient) + 80C(rise) + 20C(losses between core and surface) = 140C or 284F (!)

That is one good reason why touching an electric motor is not a good idea (imagine a Class F rise motor w/ a surface temp of 165C or 329F).

Your unit is running about 72 percent loaded , ie. 90 amps of the (estimated) 125 amps FL at 460V. Check your ambient and figure out the temperature rise of your motor. See if it seems reasonable for that load. I am not sure of any methods that would allow you to calculate or estimate the expected temperature rise for less than full load, but I will look around and see what I can come up with.

With respect to the temperature difference between this motor and the others:
- how much are we talking about?
- is everything else the same? This includes:voltage, current, load, ambient temp, and duty cycle. Note that a motor that is frequently started and stopped will run hotter than one running steady state.
- has the motor been rewound?
- is it dirtier (older) than the others? Note that most stator cores have cooling ports (vents) in them. These can become clogged up with dirt over time despite the cleanliness of the outside of the motor. The ports can also become clogged up with varnish during a rewind if the winder isn't careful.

I hope this helps

 

[electricpete]

Thanks rhatcher. That's good info (star-worthy). This unit runs around 160F and the sister units run 100-120F in an ambient of approx 85F. So the rise at the surface is 2-3x bigger.

I suspect that as you indicate the windings are not in danger. But I will investigate a little more.

 

[don01]

hi rhatcher
what have I done wrong this time?
I take the motor volts (480) then the amps (90) the I multiply by root 3 (1.73) to get 74.736 kw input. approx conversion to hp is kw times 1.25 = hp .
Assumed this is pf =1 then 93.4 hp. at shaft. To run at say pf =.8 then hp rating on name plate is reduced ??
Don't think I'm saying your wrong if I stuff up (too often I know) I just wanna figure out how.

Re hand measuring of case temp I was making a tongue in cheek reference to the high temps of cases.

Regards
Don

 

[electricpete]

don - I think you're closer to the true nameplate data (which of course neither of you had access to).

Motor nameplate indicates 100hp, 112A FLA.

I = P / [sqrt(3) V pf Eff]
= P / [sqrt(3) V pf Eff] * [1000VA / 1.341HP]
I/amps = [1000/sqrt3*1.341] * P/hp / [V/v pf Eff]
= 432 P/hp / [V/v pf EFF]
pf*EFF = 432 P/hp / [V/v I/amps]

Plug in I=112, P=100hp, V= 460 volt

pf*eff = 432* 100 / (460*112) = 84%
Thus combined effect of power factor and efficiency is 84%
ie perhaps 95% efficient, 88.5% pf?

Motor has s.f. of 1.1.5 but I believe that is relevant to the above calcs.

Note that in my notation P/hp is "Power in horsepower". I/amps is "I in amps" etc. I find this makes it much easier to do unit analysis.

 

[rhatcher]

Don01,

First, my statement: "That is one good reason why touching an electric motor is not a good idea..." was in no way a reference to your statement: "I have seen a lot of F***in hot motors (measured the temp with my hand)...". I apologize if you (understandably) took that as a comment directed at you or about your post. It was pure coincidence that my train of thought led me to that statement. By the way, I touch motors all the time (very carefully!). Anyway, sorry for the misunderstanding.

Second, with respect to the estimated load on the motor, it wasn't my intent to contradict your previous estimation. In fact, you did a better job at that part than I did. I was guess-timating and was sloppy in my method. I estimated FLA at 125A and 90A/125A= 72%. That is the wrong approach. The proper way to look at it is your approach using input power. I got the efficiency and power factor data from the siemens web site for a 100 hp, 3600 rpm ODP motor.

75kw input * .94 efficiency * .91 power factor = 64kw output
Conversion of 746 watts/hp yields 86 hp.

Again, I apologize if you took my post as a snub because nothing could be farther from the truth. I have enjoyed and learned much from your posts as well as those of electricpete, our friend jbartos, and others. As well, I welcome any and all feedback as I have been known to screw up here and need guys like you to keep me straight.

For Electricpete:

Your ambient is about 30C and your estimated winding temperature is 91C, giving a 61C temperature rise at an estimated 85% power output. The motor is rated for a temperature rise of 80C (Class B) at full load. On one hand, this seems ok. However, my gut feeling right now is that this may be a little high for that load. As well, there is the fact that this motor is running 20-30C hotter than the sister units. I have been very short on time lately but will continue to try to come up with a way to estimate temperature rise versus load so we can see if this is reasonable.

In the meantime...Are you sure that everything else is the same between this motor and the sisters? Also, you are measuring the hottest temp in the middle of the stator(which is normal). Is the temperature consistant all the way around the middle or is one particular area much hotter than the rest?

 

[don01]

hi all
late post from me but to all no offence was taken or meant I did think I'd done it again and couldn't figure out how.

I put astar on rhatcher and a note in my diary ( it's so rare that these guys are "sloppy" that I automatically think I missed. Thanks to all.
Don

 

[jbartos]

Suggestion: Please, visit Siemens motor application manual (NEMA)

http://www.sea.siemens.com/motorsbu/product/Application Manual/section 5.pdf

for more info on temperature and classes (click on "Temperature Rise&quot.