URT
URT
U ltra- R apid Fuse Links for the protection of semiconductor-rectifiers
Annex A: Example calculations
Annex A: Example calculations
Thermal influences A semiconductor cell has to be protected by a low voltage fuse link. The fuses are installed inside a temperature controlled cabinet with an air speed of about v L = 4 m/s, while the temperature in the cabinet is 80 °C. The fuses carry a load current of IL = 200A. Fuses of size 000 with bolted connections are preferred. According to the data sheets for these fuses the thermal constant " a " is given as 130.
a - T (ambient-
A1 = SQRT
= SQRT 130 - 80 = 0.71
130 - 30
a - 30
B1 = 1 + 0.05 * V L = 1 + 0.05 * 4 = 1.2 I min = I Load / ( A1 * B1 ) = 200 / ( 0.71 * 1.2 ) = 235A The rated current of the fuse link should be 235A or higher. Typically, one would take a rating of 250A. Cyclic Load Operation The designer calculates a regular load period through the thyristor in a given circuit. Load times and currents are shown in the load diagram. The fuse links will be connected in series with the semiconductor. As the preferred fuses of type "SQB", the coefficients for cyclic load operation "A2" and "B2" have to be taken from the fuse data sheet.
I 1 = 260A t 1 = 1020s I 2 = 340A t 2 = 300s I 3 = 0A t 3 = 420s From SQB-data sheet A2 = 0.75 B2 = 0.6
Calculation of the load current R.M.S. value:
2 *t
2 *t
2 *t
I 1
1 + I 2
2 + I 3
260 2 *1020 + 340 2 *300 + 0*420
3
I RMS = SQRT
= SQRT =
244A
t 1 + t 2 + t 3
1020 + 300 + 420
Calculation of the smallest fuse rated current
I min = I RMS / A2 = 244A / 0.75 = 325A
The rated current of the fuse link should be 325A or higher. Typically, one would take a rating of 355A. Review of the load steps and comparison with the time-current-curve of the rating 355A: I TCC >= I n / B2
portal: www.siballc.net e-mail: info@sibafuse.com
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