| US 7,538,508 B2 | ||
| Thermal overload protection | ||
| Janne Kuivalainen, Vaasa (Finland); and Peter Österback, Vaasa (Finland) | ||
| Assigned to ABB Oy, Helsinki (Finland) | ||
| Appl. No. 10/588,300 PCT Filed Feb. 01, 2005, PCT No. PCT/FI2005/000067 § 371(c)(1), (2), (4) Date Oct. 02, 2006, PCT Pub. No. WO2005/074089, PCT Pub. Date Aug. 11, 2005. |
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| Claims priority of application No. 20040155 (FI), filed on Feb. 02, 2004. | ||
| Prior Publication US 2008/0253041 A1, Oct. 16, 2008 | ||
| Int. Cl. H02P 1/04 (2006.01); H02P 3/00 (2006.01) | ||
| U.S. Cl. 318—471 [318/634; 318/782; 318/783; 361/25; 361/31; 361/106] | 20 Claims |
1. 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 (S2) for interrupting a current supply (L1, L2, L3) 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
 Θtrip=trip level for thermal load
Θ=calculated thermal load
τ=estimated time to moment when Θ reaches trip level Θtrip
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.
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