This video contains the numerical on temperature rise time relation under heating cooling and ventilation of electrical machines to better understanding of concepts.

1. Temperature Rise-Time Relation
(Heating and Cooling Curves of Electrical Machines)
Numerical
on

2. Problem statement:
The temperature rise of a transformer is 25⁰C after one hour and 37.5⁰C after
two hour of starting from cold conditions. Calculate its final steady temperature
rise and the heating time constant. If its temperature falls from the final steady
temperature to 40⁰C in 1.5 hours when disconnected, calculate its cooling time
constant. The ambient temperature is 30⁰C.
Solution:
(i) Under heating condition, we know that the equation,
2
Putting the values,
____________(1)

3. &
3
Take the ratio of eq. (2) & (1),
______________
____________(2)

4. 4
Inserting this value in eq. (1) or (2), we get,

5. 5
(ii) Under cooling condition,
Cooling has started from the final steady temperature rise θm = 50⁰C
and in a time t=1.5hr the machine temperature fallen to 40⁰C. But it is still
40-30=10⁰C above ambient temperature.
Now, we know that

6. 6
(i) Under heating condition,
The final steady temperature rise is,
The heating time constant is,
(ii)Under cooling condition,
The cooling time constant is,

7. 7