Due
to the excellent magnetic circuit that toroidal cores
create, and also due to the remanence that results from
the more square hysteresis loop that these cores
possess, high inrush currents can be encountered when
switching on large toroidal transformers. These are
higher than in laminated stack transformers and can last
for a few half-cycles of the mains voltage. This is
caused by the core saturating for a split second and is
quite normal. However, this means that larger toroidals
(1.5 KVA and higher) should not be switched on without
some precautions.
It is recommended that
slow-blow (type T) fuses be used in the primary circuits
of all transformers over 100 VA. For larger toroids,
either NTC thermistors or circuit breakers designed for
motors and transformers (with type D delay
characteristics, for example) should be incorporated.
Simple relay-switched resistor soft start circuits can
also be used effectively, and a delay of between about
30 to 300 mS will usually work effectively (some relays
themselves have pull-in delays of approximately this
time). Soft start circuits of this kind should be
implemented with, and never replace, the proper circuit
protection provided by fuses or circuit breakers.
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