Page 37 - Application Guide Semiconductor Fuse Link

Fuse selection examples

System data :
460 V
a.c
Max AC line voltage 460 V, 60Hz
Ambient temperature 45 °C
500 V
Current loading 250 A d.c. d.c
AC s.c. current 10 kA
F1
DC working voltage 500 V (regenerative load)
DC load time constant 30ms

Occasional overload 500% for 0.1s Fig.29 3-phase bridge
Repetitive cyclic load 141.4% for 1 hour, then 1 hour off.
Natural cooling
Semiconductor devices I t withstand 68kA s at 8.33 ms; Peak inverse voltage 1.5kV
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The fuses must have an a.c. voltage rating of at least 460V and a d.c. voltage rating with L/R = 30
ms of at least 500V. Referring to the product datasheets these values are :

VAC VDC at L/R=30ms

A50QS 500 464
A70QS 700 635

The d.c. voltage rating of the A50QS is too low, so this type cannot be used. The A70QS however
may be suitable. The thermal coeffi cients for this fuse type are

a = 150 For a 1hour ON/OFF cycle the fuse will cool down
completely between cycles so the lowest value of A’2
= 25
o
must be used (see section 5)
B1 = 1.25
= 0.85
C1
Cf3 = 0.75 A’2 = 0.6

First estimate the required continuous current rating
150 - 45

-

correction for ambient temperature A1 = 150 - 25 = 0.917
correction for non-standard terminal contacts C1 = 0 .85

correction for load variations A’2 = 0.6

For the 3-phase bridge the r.m.s. current in the leg at F1 is 0.577 times the average d.c. current, i.e.
0.577 x 250 = 144.25A.

In’ = In x A1 x C1 x A’2 144.25

In x 0.917 x 0.85 x 0.6 144.25

This gives In 308.4A. Choose the next higher size in the series, which is 350A. To check now
2
that this fuse will protect the thyristor, the let-through I t will be evaluated at a voltage of 0.65 x 460
= 299V. Referring to the product datasheet for the A70QS350 we obtain :
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