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vjr0512 (Mechanical) (OP)
12 Mar 12 0:39
Hi all,

In alternator datasheet, i have seen the following

Thermal class = F or H
Temperature rise calss = B if Thermal Class F, F if H

My doubt is " If we have given the thermal class, it means that it will withstand for that respective temperature rise say example: if Thermal Class of F (155°C) is considered, then the temperature rise is 105°C (considering max ambient temp of 40°C) " Please clarify whether my understanding is correct.

Why both datas are required. Why not simply Thermal class.

Thanks in advance.
ScottyUK (Electrical)
12 Mar 12 2:37
On large machines it is common to run a Class F insulation system under Class B limits, or more recently a Class H system under Class F limits. By keeping the insulation cooler than strictly necessary the life can be signifcantly extended. The Arrenhius equation predicts that every 10°C reduction in temperature will double the service life, all other factors being equal.

If we learn from our mistakes I'm getting a great education!

gattie (Electrical)
12 Mar 12 2:51
You would have to check with Alternator Supplier what Class of Insulation B or H, was actually installed in your Alternator.
Allowable temperature rise for duty is then applied.   
GEORGE2311 (Electrical)
12 Mar 12 11:19
I agree, it is quite common to run an insulation system of class F under the restrictions of class B.


7anoter4 (Electrical)
12 Mar 12 11:27
I agree with Scotty. Since the Generator is usually subject to overload the winding life time could be very much reduced.
I think if the overload will be 25-30% the temperature of F designed insulation will reach the H permissible temperature.
Here is a extract from an article treating motor insulation[but, in principle, is the same as for generators also]
"If the temperature limit is adhered to, the winding life time for all insulation classes
can be estimated at 100,000 h. This corresponds to approximately 12 year of
continuous operation at rated power. Insulation ageing is a chemical process, which
is highly temperature-dependent as shown in Fig. 2.4.1. Due to heating up, part of
the insulation material evaporates, which leads to an increasing porosity and, as a
result of this, a decreased voltage resistance. The following rule applies: if the
operating temperature exceeds the highest permissible temperature by 10K, the life
span reduces by half. Short-term excessively high temperatures do not have a
considerable impact on a motor's life span. The continuous operating temperature,
however, must not exceed the highest permissible value."
Rockwell Automation Basics for practical operation Motor protection
Kiribanda (Electrical)
13 Mar 12 11:36
It is like a "bonus".The equipment is made with higher thermal class material "F",but designed with a maximum rise only to "B".  
raghun (Electrical)
13 Mar 12 22:52
I heard that limiting temperature rises one step lower than those applicable for the installed insulation thermal class has another reason too -
the insulation alongwith the stator conductor is bent to route the same in the core slots, as required during winding the stator in the factory.
This process of bending introduces stress in the insulation and thus derating of the insulation as above is important for assured life of the motor.
gattie (Electrical)
15 Mar 12 15:06
Your specification should state the class of Insulation required eg: class B or class H.

Final temperature at full load must not exceed the Insulation
max rating.

electricpete (Electrical)
15 Mar 12 15:44


Final temperature at full load must not exceed the Insulation
max rating.
That's not exactly right.
For Class F insulation specified at Class B rise, it must not exceed the class B temperature rise.


(2B)+(2B)'  ?

Hoxton (Electrical)
15 Mar 12 17:41
I think i have posted this before, but we run our generating sets with generators at class F rises, with the generator wound with class H insulation (400V generators).  

If you compare the generator efficiency, then you save about 1 percentage point in having a larger generator with class F rise, rather than a smaller generator with (hotter) class H rise.  "I squared R" copper losses, basically.

For base load - work out the cost of fuel of this 1% over 15 years; for standby, work out the capital cost saving of a smaller class H machine, over the more expensive class F machine

Of course, the Class F rise generator will suffer less temperature stress and will statistically last longer than the class H rise machine.

"Engineers design: accountants decide!"
ScottyUK (Electrical)
16 Mar 12 2:47
Hi Hoxton,

I hadn't looked at it from that perspective before. Guess that's why I'm not an accountant!

If we learn from our mistakes I'm getting a great education!

7anoter4 (Electrical)
16 Mar 12 7:07
Only for record :I am not sure one does design an electric machine less voluminous based on higher conductor insulation class-above all the manufacturer
declares rated load is based on the lower class. The force applied to the winding by electromagnetically field depends on product of magnetic flux
density [B] and the amperes per m[or per inch] of the stator circumference and that does not depend on insulating class -more amperes per m-less
gausses [or Wb/m^2].The stator inner diameter depend on this force-and in turn depends on rated power.
gattie (Electrical)
17 Mar 12 2:02
Sorry electricpete
I should have said: Your specification should state the class of Insulation required eg: class B, class F or class H.


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