The medium size segment can be found in the industry and automation as well as in drive trains for electric vehicles; therefore mostly induction motors and synchronous motors with permanent magnets.
Motor with squirrel-cage rotor
At the beginning of the industrial age, power density of medium machines was much less; so that most of the existing surface of the casing was sufficient to remove the losses; for smallest and small machines, this is still a good practise case. Cooling fans enable an extension of the casing surface and a better heat transfer per convection. An active cooling with a fan, with a water mantel will enable much better heat transfer per forced convection.
When the stator of a motor is equipped with a water mantel than the current density can be doubled for low speed rotors and low iron losses and even tripled for high speed rotors and high iron losses. For motors with a magnet excitation of the rotor, the thermal challenge is no more the temperature of the insulation of the stator winding, it is the temperature of the magnets. Accordingly the estimation of the magnet losses is of the utmost importance.
Predefined driving cycles
determine the boundary conditions and the speed sequences. The main engine
of an electric car and all auxiliary engines must be able to drive their own
driving cycles without temperature damage. The engine starts from a cold
state, it than produces heat, which it initially hold into the heat
capacities of the components and further into the cooling media and into the
environment. If the components have a high heat capacity, they will slowly
warm up and slowly cool down. If the warm-up phase is relatively short, then
a much higher current density can set than if the motor is in continuous
The next picture shows the maximum winding temperatures of a motor with permanent magnets. The two blue curves for 24.3 A continuous operation; the two pink-red curves for 3000 seconds heating followed by 1000 seconds cooling with 26 A and the violet curves for 1000 seconds heating followed by 3000 seconds cooling with 44A.
Temperatures of the stator winding during continuous operation and cycles