Figure 5: Time between starts
2.3 Thermal Overload Function
Overloads can result in stator temperature rises which exceed the thermal limit of the winding
insulation. Empirical results suggest that the life of insulation is approximately halved for each 10ºC
rise in temperature above the rated value. However, the life of insulation is not wholly dependent on
the rise in temperature but on the time the insulation is maintained at this elevated temperature. Due
to the relatively large heat storage capacity of an induction motor, infrequent overloads of short
duration may not damage the machine. However, sustained overloads of a few per cent may result in
premature ageing and failure of insulation.
If a motor is considered to be a homogeneous body, developing heat internally at a constant rate and
dissipating heat at a rate directly proportional to its temperature rise, it can be shown that the
temperature at any instant is given by;
T = T
max
(1 – e
-t/τ
)
Where;
T
max
= Final steady state temperature,
τ= Heating time constant.
This assumes a thermal equilibrium in the form:
Heat developed = Heat stored + Heat dissipated
Temperature rise is proportional to the current squared:
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