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| Can a transformer alone blow fuses ? |
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| Hextejas:
I have this toroid connected via this wall receptacle and 9 times out of 10, I will blow a 3.15 or 4.0 slow blow fuse. There is nothing connected to it and it is new. I had 1 good power-on this morning and I stopped after 8 blown fuses. I thought that I would need to have some load connected in order to blow a fuse. Maybe not. |
| Matje:
Larger toroids have a problem with high inrush current in the moment of powering on, your 500VA one definitely counts as a large one. You will need to limit the inrush current, search for "inrush limiter" or "slow start" or similar. |
| Benta:
Not unusual. I had a 1 kVA toroid isolation transformer that would blow the fuses 10 out of 10 times. A controlled start circuit can help. |
| Kleinstein:
For such a size an type of transformer it is pretty normal to blow a fuse if turned on just be a hard switch to it's nominal voltage. The problem with these torrid core transformers it that the core rather easy runs in to saturation. In this case there is very little inductive impedance to limit the current. So peak current could go well over 50 A than. This is enough to blow 16 A or similar circuit breaker. Saturation is reached relatively easy in this type because the residual magnetization is often relative large and the core material (grain orientated FeSi) is used up to a relatively high field. Depending on the exact timing to turn on saturation will happen or not. So there is a chance not to blow the fuse, but only some 10%-25%. So normally one needs a kind of soft start circuit with this type of transformer ( about from 100 VA up). |
| Benta:
Yes, it's a major problem that most modern transformers are designed to operate very close to the core saturation point. One solution is to choose a transformer with a higher primary voltage than needed (and corresponding secondary voltage as well, of course). This unfortunately makes it physically larger. Another solution is switching on the transformer in a controlled way using SCRs or Triacs. In this case it should be switched on appr. at peak mains voltage. Lastly, upgrading your fuse box and cabling is possible, but normally costly (electrician needed). Edit: by a major coincidence, another thread is touching the same subject. Nice little circuit, should work: https://www.eevblog.com/forum/projects/yet-another-mains-active-soft-start-is-this-design-decent/ |
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