Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here


Motor temperature rise and ambient temperature

Motor temperature rise and ambient temperature

Motor temperature rise and ambient temperature


Our Sag mill motor is rated 1500HP with 4000V, 212A and 0.8 lead power factor, insulation class F and permissible temperature rise 80K. Our motor is actually used at 112% of nominal charge (1320kW, 0.943 lead P.F., 190A, 4220V). We want to know if we can still increase the load without damaging the motor or if the motor capacity is already reached.

In my mind, as it is a synchronous motor, I can go up to 125% of nominal load by adjusting P.F. to 1.0 instead of 0.8 lead. So  4000*212*1.73*1 = 1468kW.

Rated Input power is 4000*212*1.73*0.8 = 1175 kW

1468/1175 = 125% of rated load.

We could also continue overexciting the motor and maybe reach a little more if the temperature limits are respected…

But I want to be sure that we respect the allowable maximal temperature of the motor.

So if I look at the RTDs, what values should I setup my alarms and trip to avoid damaging the motor ? Is it based on actual ambient temperature + permissible temperature rise (80K), or is it based on motor class insulation maximal temperature (155°C for class F)

Ambient temperature of the mill is 20°C and not 40°C. Insulation class is F (155°C) and permissible temperature rise is 80K.

Does the critical temperature to avoid is 155°C – 10°C (hot spot) = 145°C since it is a class F insulation motor? Or does the critical temperature to avoid is 20°C + 80K = 100°C. So that over 100°C temperature rise within the motor will exceed 80K..
There is a huge difference between both so I really need to understand the way it works to adequatly protect the motor.

Do I have to prevent exceeding the 80K temperature rise within the motor whatever ambient temperature is (below 40°C..because I know there are factors to consider if I'm over 40°C ambient) or do I have to prevent exceeding the maximal allowable temperature based on motor class insulation, or both .. ??

Thanks !


RE: Motor temperature rise and ambient temperature

Max temp is determined by the insulation class. A large part of the heat generated in a motor is from the current. I would not exceed the max rated current. However going to a higher voltage will increase the losses in the magnetic iron. Normally I would not recommend exceed the nameplate rated voltage.

RE: Motor temperature rise and ambient temperature

Standards don't specify 'uprating' for lower ambient/inlet temperatures. Just because you have lower ambient temperature doesn't mean you can overload the motor (because you have temperature margin) since mechanical factors like shaft strength come into play at higher loads.

120 deg C trip is the norm for class F machines.

Never a good idea to overexcite a motor. More losses and harmonics.


RE: Motor temperature rise and ambient temperature

You should respect the (rise + the 'standard' ambient) NOT the insulation system temperature. You need some safety margin and just because you have some RTDs telling you temps of specific points they can't tell you the temps of the hot points. Plus other things like bearings, the iron, lamination adhesives, and increasing copper losses due to temperature alone will all be piling up.

The final word is the temperature and the ambient plus rise should not be exceeded. Otherwise I see no reason not to mess with any of the drive values - as long as you realize you're dramatically minimizing motor life by all this brinkmanship.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Motor temperature rise and ambient temperature

Thank you for your answers but I'm still not sure I got my answer..

My permissible temperature rise is 80K. The standard ambiant tempersture is 40degC. So 80+40=120 degC. So i should avoid exceeding 120degC based on 40degC ambient temperature. But if my ambient temperature is 20degC, does my maximal temperature remain 120degC or does it change to 80+20=100degC ?

Also, class F insulation àllow 155degC. So what happen if I exceed the 80K permissible temperature rise but stay within the 155-10(hot spot)= 145degC? As I understand the 25degC beteeen 120 and 145 is a safety margin but can the motor works in that safety margin whitout damaging itself?

RE: Motor temperature rise and ambient temperature

Absolutely your maximum temp remains at specifically 120C. If the actual ambient is lower you will be able to load the motor more that particular moment but on a 30C day you will have to reduce the motor load.

All the temp limits on the motor are absolute based not relative based. The 120C is the absolute limit for safe operation. I would expect the motor to be damaged at anything above 120C and that operation there would be technically irresponsible. I would remove any staff that put my $150K motor in that situation. It shouldn't be your position to bail out management's undersized motor.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Motor temperature rise and ambient temperature

Thanks for the answer Itsmoked. That's what I thought but I still find it confusing. Why don't just say on nameplate: max temp 120degC ..instead of 80K permissible temp rise. Anyway, I'll keep in mind that under 40degC ambient temp, temp limit is set to 120K.

Do you have a standard or a norms reference for that conclusion? Because I made some searches and I did'nt see it nowhere.. I've looked in Nema MG-1 and some others and stil can't find this written clearly

RE: Motor temperature rise and ambient temperature

It's likely contained in IEC 60034 and IEC60085, with the former being the rotating electrical machines standard and the latter being the electrical insulation - thermal properties standard.

RE: Motor temperature rise and ambient temperature

It's also just physics. Material aging is a function of absolute temperatures not relative temperature differences.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Motor temperature rise and ambient temperature

Thanks everyone! I found my answers in IEC 60034. The exact answers is based on the fact that the motor is indirect cooled and not direct cooled. So this way, the maximal temp rise can be ajusted if ambient temp is lower then 40degc but higher then 0.

RE: Motor temperature rise and ambient temperature

synchronous motor has a lowest stator current with power factor or cos φ 1 so you should try to come to this value.
It all depends on the load and the value of rotor current , which should not exceed a maximum value from nameplate .
With these settings heat of stator winding should be the most economical and all the current from the grid should be active.
Your motor now with leading power factor make reactive power and power factor correction of the grid .
Good luck !

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close