A new approach to field modelling

Main >> Applications >> Sample problems >> Heating and cooling of a slot of an electric machine

Heating and cooling of a slot of an electric machine

QuickField simulation example

Two armature bars laying in the slot produce ohmic loss. Cooling is provided by convection to the axial cooling duct and surfaces of the core.

Problem Type

Plane-parallel multiphysics problem of Steady-state heat transfer coupled to Transient heat transfer.

Slot of an electric machine During the loading phase the slot is heated by the power losses in copper bars Wedge Insulation Copper Copper Steel Cooling duct 69 mm 29 mm 20 mm 105 mm Ø 15 mm Ø 690 mm Ø 480 mm T = +20°C T = +40°C

All dimensions are in millimeters. Stator outer diameter is 690 mm. Domain is a 10-degree segment of stator transverse section.

Outer stator surface convection boundary condition: 20 W/K-m², 20°C.

Heat Conductivity
Specific Heat
Mass Density
Steel core 25 465 7833
Copper bar 380 380 8950
Bar insulation 0.15 1800 1300
Wedge 0.25 1500 1400

During the loading phase the slot is heated by the power losses in copper bars. The specific power loss is 360000 W/m³. When unloaded, the power loss is zero.
We suppose the temperature of contacting air to be the same for both phases of working cycle. In turn, the convection coefficients are different, because the cooling fan is supposed to be stopped when the motor is unloaded.

Air temperature and Convection coefficient
Loading Stopped
Cooling duct 150 W/K-m², 40°C -
Inner stator surface 250 W/K-m², 40°C -

We assume the uniformly distributed temperature of 20°C before the motor was suddenly loaded. The cooling conditions supposed to be constant during the heating process. We keep track of the temperature distribution until it gets almost steady state. Then we start to solve the second problem - getting cold of the suddenly stopped motor. The initial temperature field is imported from the previous solution. The cooling condition supposed constant, but different from those while the motor was being loaded.

Each phase of the loading cycle is modeled by a separate QuickField problem. For the cooling phase the initial thermal distribution is imported from the final time moment of the previous solution.

Temperature vs. time dependence at the bottom of the slot (where a temperature sensor usually is placed).

electric machine operation cycle temperature

See problems THeat1Ld.pbm for loading, THeat1S2.pbm for the stopped state.