More heat means reduced insulation life, accelerated wear of metallic (and some non-metallic) parts, and increased likelihood of personnel injury. There is an optimization required between all factors of machine performance: transient and steady-state torque output, thermal "loading", electrical source variability, initial cost, operating cost, time-to-delivery, mass/weight, availability of spare parts, ease of use, etc.
At normal wither temperature when electrical current pass through electrical machines produce heat if generation heat equal to dissipation heat so electrical machines will be stable temperature at this condition but if machines work under over load so high current will pass so that heat generation will be over than dissipation heat and machines go to unstable at this time electrical installation in machines defected by temp rise.
Motor overload condition is mainly a result from abnormal use of the motor, harmonics or unbalanced supply voltages. They all increase the motor losses and cause additional heating. As the temperature exceeds the rated limits specified for the insulation class in question, the winding insulation deterioration accelerates. This will shorten the expected lifetime of the motor and may lead in some point to an electrical fault in the winding. Thus, the thermal overload protection can be considered being the most important protection function in addition to the short circuit protection of the motor. Finally, if conditions worsen, and protection fails, results may impact combustion to take place and hence potential fire outbreak in extreme conditions. So, it follows that the significance in ‘thermal loading’ is an important criterion moreover when choosing winding classification and temperature rise classification for VFD applications.