The paradigm shift in the operation of the adjustable speed drives operating under frequent start and stop instead of long continuous operation induces thermo-mechanical stress. This intermittent operation changes the operating temperature of the machine frequently, which results in uneven expansion/contraction of different materials. This difference causes stress which accelerates the ageing of the insulations. So far, the effect of thermal stress on the ageing phenomenon is well-known in the literature. However, the study of ageing process under thermo-mechanical stress is scant. To circumvent the unpredictable downtime due to the stress, continuous monitoring of insulation health is attracting more attention. This work aims to study the effect of thermomechanical stress on insulation by exposing 7 stators of 2.83 kW servomotor to thermal cycles under temperature-controlled environment. The degradation of insulation is continuously observed by measuring the groundwall capacitance. Results show that the capacitance varies along with the temperature, and both its average and the peak-to-peak variation decay with aging. Based on the experimental investigation, a mathematical model representing the ageing phenomenon of the winding insulation is proposed. The proposed model is based on the assumption that the observed changes in the insulation capacitance due to thermal aging are the result of internal chemical reactions which obey the Arrhenius law.