site transformer fuses

[Alexander Smith 1Participant @alexandersmith1]

My friend recently blew the fuse on his 110v site transformer (powering a table saw). The saw is fine on my transformer so it’s not that. He replaced the fuse which blew again making me think the transformer is faulty but I wondered whether these transformers need a slow-blow type fuse to cope with inrush current. Does anyone know what type fuse he needs to try.

Thanks

Sandy

[MacolmParticipant @macolm]

This is quite a common feature of some site and similar transformers which presumably do not use best magnetic core material. What I presume happens is that at the instant of previous switch off, significant magnetisation remains. At the point of next switch on, if the direction and size of the instantaneous voltage happens to strongly augment the magnetisation to the point that the core momentarily saturates, the current is then limited only by the copper resistance (neglecting residual inductive effects). So it does not happen every time, and the simplest work around is indeed slow blow fuses.

There is little harm in the current spike, and indeed it is similar to the situation when any (non soft start) large motor is powered up. The duration of the large initial current is very short.

[noel shelleyParticipant @noelshelley55608]

Malcolm s point is well made. It is quite common for modern RCDs, RCBOs and MCBs to drop with this type of load. Whilst there may be a fault it is more likely to be the inrush current. As for the starting current of a motor it does NOT have to be a LARGE motor to draw an initial high current. I had a 180w motor that would stall dead on start, a 2Kw genny, yet the same genny would run a 750w drill all day. Different types of motors have very different starting characteristics and split phase motors can have an initial current 10 or even 12 times there run current. That is why a fuse heavy enough to take the starting current will not protect a motor and the need for a proper starter is best. Noel.

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, you don’t say if it’s the fuse in the plug top, or the fuse in the output of the transformer. If it blows with nothing plugged into the transformer output, then it’s quite possible the transformer is at fault. If it blows when the saw is used, it maybe that the load is too high for the transformer output, or it’s a cheap one and suffers from that what Malcolm has said. I’ve used 110 volt site transformers virtually all my working life, from small ones to very large ones of different makes, and I’ve two 2kw ones, a 3kw one and a 5kw one. and I’ve only ever know a fuse to blow very occasionally, and that’s while it was in use, and due to the fuse just giving up due to its age. I have never known one to blow during switching the power on to the transformer, and never when after replacing a blown one. The output power from the transformer should be capable to deliver more the the in rush of whatever is being used on it.

Regards Nick.

[Alexander Smith 1Participant @alexandersmith1]

it was the fuse in the transformer output which blew. Not likely to be the size of the saw motor as it had worked quite happily until then. May just have been on old fuse that gave up the ghost and I’m not sure whether Ken replaced it with a slow blow.

sandy

[Robert Atkinson 2Participant @robertatkinson2]

The output fuse should be an anti-surge type. Probably a ceramic bodied type. The transformer should be marked with the fuse rating and type.
A faulty transformer won’t blow the output fuse.

Robert.

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