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

# Heating and cooling of a slot of an electric machine

electric machine heating, electric machine cooling, slot transient temperature, multiphysics analysis

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.

Geometry

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

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

 Heat Conductivity(W/K-m) Specific Heat(J/Kg·K) Mass Density(kg/m³) 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.

 Convection coefficient (W/K-m²),temperature of contacting air (°C) Loading Stopped Cooling duct 150 W/K-m², 40°C - Inner stator surface 250 W/K-m², 40°C -

Solution
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.

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