Patent ID: 7538508

Claim:
A device for thermal overload protection of an electrical device, particularly an electric motor (M), the device comprising means for measuring at least one load current supplied to the electrical device (M), means for calculating the thermal load on the electrical device on the basis of said at least one load current, and means (S 2 ) for interrupting a current supply (L 1 , L 2 , L 3 ) when the thermal load reaches a given threshold level, wherein the device comprises a processor system employing X-bit, fixed-point arithmetic, the system comprising means for scaling the current measured into unit values to a range of 0 to Y, wherein Y represents Y/100% of a nominal current, and means for calculating a time-to-trip, in accordance with formula τ = R * C * ln ⁡ ( a ) a = 1 - ( Θ trip - Θ i 2 - Θ ) wherein Θ trip =trip level for thermal load Θ=calculated thermal load τ=estimated time to moment when Θ reaches trip level Θ trip ΔT=interval for thermal load calculation R=cooling factor of electrical device C=trip-class factor i=measured current whereby the equation and its operands are programmed into the microprocessor system structured such that a result or a provisional result never exceeds the X-bit value.