THeat1: Heating and Cooling of a Slot of an Electric Machine

Changing temperature field in the stator tooth zone of power synchronous electric motor during a loading-unloading cycle.

Problem Type:
The plane-parallel problem of heat transfer with convection.

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

Given:

  1. Working cycle
    We assume the uniformly distributed temperature 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 –cooling 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.
  2. Material Properties
    The thermal conductivity values are the same as in the Heat1 example. For transient analysis the values of specific heat C and volume density are also required:
      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

  3. Heat sources and cooling conditions
    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 are zero. We suppose the temperature of contacting air to be the same 20 °C 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)

    Loading Stopped
    Inner stator surface 250 20
    Outer stator surface 70 70
    Cooling duct 150 20

Solution:
Each phase of the loading cycle is modeled by a separate QuickField problem. For the loading phase the initial temperature is set to 20°C, 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).

Please see following problems in the Examples folder:

THeat1Ld.pbm - loading phase,
THeat1S1.pbm - the stopped phase.