One of the features of ship power systems is the presence of different power quality disturbances, such as frequency and voltage root-mean-square (rms) value deviations, unbalance, and voltage waveform distortions. The disturbances (including their synergy) cause an increase in the windings temperature of marine induction machines that are exposed to overheating, even if the voltage parameters fulfill the requirements of ship classification societies. Effective protection of marine induction machines against overheating requires an appropriate method of power quality assessment. This paper proposes a new power quality factor, the value of which is proportional to the windings temperature of induction machines supplied with voltage of lowered quality. The power quality factor has been experimentally verified. The results of the thermal tests based on the dedicated laboratory stand to validate the new power quality factor are presented. The time characteristics of the power quality factor based on voltage monitoring on ships are shown.