Hi Raghun,
According to Rudolf Richter Electrical Machine vol I
[Erster Band-zweite verbesserte Auflage Basel 1951]
The electric motor heating equation is as follows:
?=Theta
Theta=ThetaMax*(1-exp^(-t/T))+Thetao*(exp^(-t/T))
where Theta is the actual temperature ThetaMax is the maximum temperature reached by a machine
and Thetao is initial temperature.
T is the heating constant and t is the heating time.
So T=t/Ln(ThetaMax-Thetao)/(ThetaMax-Theta)
R.Richter took the same constant as cooling constant.
Theta=Thetai*(exp^(-t/Tcooling))
Where Thetai is the initial cooling temperature
According to Siemens Handbook for Electrical Engineer chpt.7.5.3 Tcooling=(1.4 -2) *Theating
Best regards