Page 31 - Application Guide Semiconductor Fuse Link

AC controllers (soft-starters, static switches)

These circuits are used to provide a phase-controlled a.c. output from an a.c. source, and are used in
soft starters or as static switches. The 3-phase circuit is shown in Fig.24. The principles of fusing are the
same as for general converters, except that there are no d.c. fuse rating requirements. Fuse selection
is usually made on the basis of an assumed line-to-line fault. In this case there are 2 fuses in series in
2
the fault path, and the let-through I t should be calculated at 0.65 times the a.c. line voltage.

a
Load
F2

F1

b
Load

3-phase supply

c
Load

Fig.24 Three-phase a.c. soft starter or static switch

In most soft-starter applications the fuse current rating is determined by the overload produced by
the starting current of the motor. Very often this overload is 300% for 10-30s and can occur several
times per hour. (Sometimes the overload is as high as 450%). Fuse selection is mainly infl uenced by
the time-current curve in the 10-30s region.

It may be desired to base fuse selection on an assumed 3-phase fault. In this case there is maximum
asymmetry in one of the phases, which causes melting of the fuse in that phase before the other
two. This fuse operates fi rst and clears alone with a voltage of 0.866 times the line voltage, which
2
will give a higher I t let-through.

Location of the fuses in series with the devices (F1) gives the smallest fuses, with the best possible
protection of the devices. Locating the fuses at F2 requires only 3 fuses rather than 6 and so may be
a more economical choice. However the r.m.s. line current is 2 times the device current so higher
ampere rated fuses are needed at F2, making device protection more diffi cult.

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