Possible causes:  
1、Poor power supply: Three phase is unbalanced with voltage fluctuation more than +/-10%; accordingly the unbalanced three phase currents will increase the losses and thus more heat will be generated;
2、Overload: Extreme overload and rotor/stator robbing will draw higher three-phase current than the rated value and thus produce too much heat;
3、Only one or two phases is working (are disconnected);
4、Winding failure, such as inter-turn short circuit;
5、Cooling circuit is blocked, or too much dust is accumulated.