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Technical Notes

Guide To Transformers - Inrush Currents
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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