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THeat1: Heating and cooling of a slot of an electric machine

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 problem of heat transfer.

Geometry:
slot of an electric machine

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

Given:

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.

Loading

Stopped

Convection coefficient (W/K·m2)

Temperature of contacting air (°C)

Convection coefficient (W/K·m2)

Temperature of contacting air (°C)

Inner stator surface

250

40

-

-

Outer stator surface

20

20

20

20

Cooling duct

150

40

-

-

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

electric machine operation cycle temperature

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

Download Download simulation files (THeat1Ld.pbm for loading phase, THeat1S2.pbm for the stopped phase)

View movie Video.