Thermal relay

QuickField simulation example

The thermal relay is attached to the tank with a hot liquid. Relay winding conductivity depends on temperature. With temperature raise the conductivity, current and electromagnetic force fall down.
The tank is cooling down. Calculate the relay switching temperature and time.

Problem Type
An axisymmetric problem of nonlinear transient magnetics.

Geometry

Given
Magnetic permeability of core μ = 400;
Voltage U = 12 V;
Spring force f = 0.8 H
Initial temperature T₁ = 200 °C;
Ambient temperature T₀ = 20 °C;
Time constant t_s = 10 hrs;
Conductivity of copper σ - depends on temperature:

Solution
The temperature falls down as
T(t) = T₀ + (T₁-T₀)·exp( -t / t_s).
The integration time was chosen to be 3t_s, integration step t_s/5.
To calculate the electromagnetic force as a function of time transient magnetic problem is simulated. The temperature function T(t) was added to the material properties of 'winding' block.

Results
After 16 hours the temperature falls down to 56°C and relay switches off.

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