For overheating, the quickest diagnostic tool is a clamp meter. Measuring the current in all three phases and comparing it with the rated current on the nameplate largely tells you whether the problem comes from the load, the supply or the cooling.

ObservationPossible cause
Current above nameplate in all three phasesOverload, mechanical strain, low voltage
Clear difference between phasesPhase imbalance, loose terminal connection
Current normal, motor still hotBlocked ventilation, clogged fan cover, high ambient temperature
Many starts in a short timeHeat accumulated by frequent starting
Long operation at low speed on an inverterShaft-mounted fan not sufficient

Also check that the star or delta connection matches the voltage on the nameplate and that the bearings are not binding. Loose terminal connections are a heat source in themselves. For current values, see the electric motor current table; for symptom–cause matching, motor failures and their causes; and for low-speed cooling, the motor with inverter questions page.