Heat produced by electric motors, no matter what type (AC single or 3 ph, DC, Brushless DC, Reluctance, Stepper, Homopolar yada yada yada) is the power loss that has to be dissipated by the motor. If you can accurately measure the power supplied to the motor (electrical), and just as accurately measure the output power (mechanical) of the motor you will have the power (in watts) that is contributing to heat and not work. As already mentioned, I2R is often the main source of heat. In some cases eddy current, hysteresis, friction, windage etc. can be significant sources of heat. Some manufacturers provide thermal resistance (ºC/W) for their products and this can be of some use, but these values are always under predetermined conditions that usually do not relate to your application. I’m not exactly sure what you are asking, so I hope this will help. And remember, it's alway the last place you look.