Patent ID: 7555366

Claim:
A device for thermal overload protection of an electrical device, particularly an electric motor, the device comprising a current meter configured to measure at least one load current supplied to the electrical device; a processor system configured to calculate a thermal load on the electrical device on the basis of said at least one load current, and a switch device disconnecting a current supply when the thermal load reaches a given threshold level, said processor system employing 32-bit fixed-point arithmetic and being configure to scale the measured current into unit values to a range of 0 to Y, wherein Y represents Y/100% of a nominal current and is a real number greater than 0, and to calculate the thermal load using a mathematical equation that, together with its operands, is programmed into the processor system structured such that a result or a provisional result never exceeds the 32-bit value, wherein the mathematical equation is Θ k = Δ ⁢ ⁢ T * i 2 C + ( 1 - Δ ⁢ ⁢ T R * C ) * Θ k - 1 wherein Θ k =current calculated thermal load Θ k−1 =previous thermal load ΔT=interval for thermal load calculation R=cooling factor of electrical device C=trip-class factor i=measured current.