Bathroom 12v selv fan problems

[MattPower]

I've got a 150mm 12v fan (made by Vents)above the shower with the transformer well away from it at other side of room.
Trouble is, I've had 3 transformers and they keep blowing after a short time.
The first couple I just thought they weren't up to the job, but the last one I got (manrose lt12t) was supposed to be more gutsy and has lasted 2 months working fine, but then misses said she accidentally left it on for a couple of hours and now it has kicked the bucket as well!
The second one I had looked like it had overheated as the case was warped and a bit brownish.
Like I say I just thought it wasn't up to the job but now I'm wondering if the fan could be at fault?
What would cause this?
Could it be drawing too much current?
Short circuit or too little resistance in the fan? (I have got a multimeter but don't even know how much resistance to look for)
I made sure it was wired up correctly on each occasion and that there was 240v going to the right terminals(live, neutral and switched live)
Anything else I can check for before forking out for a new fan?
Thanks
Matt

 

[ban-all-sheds]

I've had a Manrose 12V timer transformer flush mounted into my bathroom ceiling for about 10 years.

It's slightly yellowed (gets a lot of direct sunlight), and there is slight discolouration around some of the vents, but no warping, and it's going strong. More than once it's been left on for hours.


What is the consumption of the fan vs the capacity of the transformer(s)?

 

[MattPower]

From memory it was something like 25/32A for fan and 35VA for transformer but this is where I got lost when I was trying to compare them...

 

[ban-all-sheds]

From memory it was something like 25/32A for fan and 35VA for transformer but this is where I got lost when I was trying to compare them...

Those surely cannot be current draws for the fan - 25A and 32A.

A 35VA transformer would be good for a fan of around 30W, 2.5A @ 12V.

 

[MattPower]

Yes it does sound a bit excessive. Perhaps it's 32w instead!
I tried finding it on the website I bought it but they don't do it anymore.
What does 35VA mean on the transformer?
How much current can that give out? (Max load)

 

[MattPower]

Ban all sheds thanks very much for your replys by the way
I will look again at the rated consumption of the fan

 

[ban-all-sheds]

35VA = 35 Volt-Amps, so as it's 12V that's a tad under 3A.

Doesn't quite equate to a 3A, or 35W, fan though - there's a difference between watts & VA for things like motors.

Pretty sure you'd be fine with a maximum fan load of 30W, 2.5A.

Q to all - might 25/32 be the W/VA rating? Does a PF of .78 seem reasonable for a small 12V motor?

 

[ericmark]

I would have thought a switched mode power supply rather than a transformer would have been used? In which case it should auto power down with an overload.

VA and Watts are the same with DC but will AC we have something called power factor. So with BAS example of 0.78 then 12 x 2.5 x 0.78 = 23.4 Watts but 30 VA. By adding a capacitor this can be corrected although unlikely it would be done with such a small motor. But I suppose a capacitor failing could in theroy cause an overload.

But first lets ask the simple question is the transformer in free air. If it can't cool then it will over heat I have found this brewing my beer in a fridge (turned off) since heat can't escape very well even the small amount of energy released by yeast can cause over heating.

I would always look for a power supply 10% bigger than the requirement the other question that comes to mind was the transformer designed to be inside the fan? Forced cooling would clearly help with overheating.

 

[ban-all-sheds]

I would have thought a switched mode power supply rather than a transformer would have been used?

From memory the Manrose is an actual transformer.

 

[DaftPunk]

I've never seen an ELV fan fed from an SMPS. It's always a true transformer. Maybe to do with the electrical seperation required for a proper SELV supply?

I would have expected the PF to be closer to zero, but it's an inductive load without any PF correction, so may well be the case.

 

[ban-all-sheds]

Don't you mean closer to 1?

 

[Taylortwocities]

If PF = 0, the load would have to be purely inductive or purely capacitive.
I don't think it can be….

 

[MattPower]

Thanks to all the people who gave helpful responses when I started this thread but now I'm gonna be a PITA and ask for help again!
I have now got a replacement transformer rather than buy a whole new fan but in order to prevent this one burning out is it possible to limit the amount of current the fan will draw by fitting a resistor inbetween the fan and transformer?
If so what resistance would I need to cap it at 2.5A @12v?

 

[Taylortwocities]

No. If you put a resistor in the circuitit will also reduce the voltage to the fan, so the fan will run more slowly.

If you keep blowing transformers, it sounds perhaps like there may be a problem with the fan itself or the wiring between the tranny and the fan?