110v transformers

[solomongrundy]

My partner bought a 2nd hand 3kVA transformer to run a router rated at 1850W. When the tranformer was switched on it tripped Type B RCD and blew plug fuse immediately - both in workshop which is on a 16A trip and also in house (on 32A). When bought it was tested on a market stall socket and worked.
Is this a start up surge that is causing the tripping? The market may have a Type C or D fuse which would be tolerant of surge. Does that make any sense or not?

Does a 3kVA transformer require more current than a Type B will allow?

Could we get a 1.5kVA transformer to do job? We want to run router at workshop which is on 16A trip. Is this going to be possible?

Any info would be a great help.

 

[Lectrician]

A 3kw transformer should be fine ok on a C16 supply.

If the plug top fuse has blown, this does indicate a faulty transformer.

 

[Spark123]

Yep, the blown plugtop doesn't sound good to me either.

New ones are not that expensive either:
http://www.scaffold-towers.com/shop/index.php?app=gbu0&ns=prodshow&ref=BWE203010

 

[ericmark]

Transformers vary I had some 6KVA ones which needed a D40 and tripped that sometimes and yet I have also seen 3KVA which will work on a B16.
With such a variation it is impossible to say it is faulty and had I not had 6 of the 6KVA transformers all which reacted the same I would have considered it as faulty.
However the yellow box type are not expensive new and also not very good quality and you will find they use quite a bit of power even when they are not under load. And the ratting is often for intermittent use not continuous.
I don't know how a router is used if it is used in short bursts then a 1.5kva may be OK but I would say better to use at least a 2kVA and if used a lot then don't use yellow box type but get a continuous rated type of non portable design.

 

[solomongrundy]

Problem is its a B16 we need the transformer to run off not a C16.

If we go for a new trannie, what size would we need to run a 1850W router? Looking at screwfix which has 1.5 or 3 KVA- both at about £60.

 

[Lectrician]

Do you really need to ask?

Your router is 1.85kw......

 

[solomongrundy]

fair enough. I didn't need to ask that one. Thanks for all comments.

 

[ColJack]

KVA being analogous to KW is not apparently obvious to most people..

I've a 3KVA transformer that I've never had a problem with, no matter where I've plugged it in.. ( well ok, there was one time, but that turned out to be a screw through a ring main cable in a show flat, creating a N-E fault that didn't show up until the ring was loaded.. )

 

[sparkydude]

Try changing the fuse and dont just plug it in and switch on, gently move the switch to the on position and it should be ok. Mines the same, mine tripped a b32 MCB the other day when i didnt do the soft start method , and to boot managed to blow up the charger for the cordless phones downstairs, go figure LOL


Nick

 

[bernardgreen]

If all else fails invest in a good quality solid state zero crossing point relay to switch the supply to the transformer. These only switch on when the AC voltage is going through O volts as it changes polarity.

Fuses blow when voltage is instantly applied to the transformer by a mechanical switch closing. Until the magnetic field can build up to create a back EMF to match and oppose the applied voltage the current is determined only by the resistance of the transformer winding and can be 100 times the normal current. This is the in-rush current

By closing the switch only when the voltage is very close to zero the in-rush current is very low and then, as the voltage increases, the magnetic field also increases and creates the back EMF to reduce the current to normal.

 

[streetlighter]

My 3.3Kva transformer always trips a 16 amp type B breaker, when the MCB is reset it works fine.

 

[solomongrundy]

Bought a new screwfix 3kVA transformer and the router works fine on Type B 16A mcb.
Second hand transformer the problem. It did work fine on market stall which is probably on Type C or D fuse. Anyone need a transformer?
Thanks for info. Relay switch makes sense. Hopefully we are sorted now . Cheers.

 

[mayfel]

Sorry to bump an old thread but i have the same problem and was was wondering which solid state relay to buy? Googled it on Google Shopping and gave a list of various components, but they're not sealed. Do they have to be fitted inside the transformer box or are there any sealed ones you can fit inline? It's a brand new transformer so can't open it up cos it's void the guarantee. Any ideas? Cheers!

 

[electronicsuk]

Sorry to bump an old thread but i have the same problem and was was wondering which solid state relay to buy? Googled it on Google Shopping and gave a list of various components, but they're not sealed. Do they have to be fitted inside the transformer box or are there any sealed ones you can fit inline? It's a brand new transformer so can't open it up cos it's void the guarantee. Any ideas? Cheers!

I don't know if bernardgreen was referring to an out-of-the-box product, but if such a thing does exist, I've not seen it. It could be done with a suitably rated triac and MOC3062 zero-crossing phototriac as a trigger, but you'd need a little knowledge of electronics and a soldering iron to make it happen.

Even then, I don't know if this would guarantee that your MCB will hold in every time, as the inrush current is still going to be pretty high on a transformer of that size.

I had a similar problem with the socket circuit in my garage, and ended up putting it onto a C20. This would be subject to suitably low ELI, of course.

 

[Stoday]

I wouldn't act on bernardgreen's advice because his understanding of electro-magnetic physics is, erm — lets just say he fell asleep when his teacher covered that part of the syllabus.

Until the magnetic field can build up to create a back EMF to match and oppose the applied voltage the current is determined only by the resistance of the transformer winding and can be 100 times the normal current. This is the in-rush current

The back emf is generated by the change of magnetic field, so it's the building up that prevents an inrush current, and that starts immediatly. If the build-up continues too long, the core becomes fully built up and saturates. With no more field change, you would get a current transient. Or you would if the voltage had not reversed because the input is ac.

You can get an occasional transient when switching on because the core has been left partly magnetised last time the transformer was switched off. If the switch on occurs when the input increases the magnetisation, the core may saturate for a mS or so, causing a current transient.

By closing the switch only when the voltage is very close to zero the in-rush current is very low

With no load connected to the transformer, the current is at its highest when the voltage passes through zero. That's because the rate of change of the magnetic field is highest when the voltage is zero. But this current is limited by the back emf.