We know that when an electric motor and drive operate, heat is generated inside the motor. The amount of heat generated inside the motor should be known as accurately as possible. Hence, thermal modeling of the motor is essential. The materials of the motors and the shapes and sizes of the motors are not unique but the heat output does not change much depending on these characteristics. Therefore, a simple thermal model of any motor can be obtained by assuming that it is a homogeneous body. The main purpose of this modeling is to choose the appropriate motor rating so that the electric motor does not exceed its safe limits during operation.
At time ‘t’, let the motor have the following parameters.
p1 = heat produced, joules/second or watts
p2 = heat lost to cooling medium, watts –
W = Weight of active parts of the machine.
h = specific heat, joules per kilogram per oC.
A = cooling surface, m2
d = Coefficient of heat transfer, Joules/Sec/m2/oC
θ = average temperature rise oC
Now, if time, let the temperature rise of the machine be dθ,
Therefore, heat absorbed in the machine = (heat generated inside the machine – heat transferred to the surrounding coolant)
Where, dθ = p1dt – p2dt…………….(i)
Since, p2 = θdA…………….(ii)
Substituting (i) into (i), we get
Therefore, from the above equation we can find the temperature rise inside the working machine, which is very close to correct and if we allow for the variation of temperature risk with time during heating and cooling. Prepare graphs and thus thermal modeling of a motor is completed.