Can someone explain: I have two mains transformers; one has 6 and 12V output, the other 12 and 24V. In both cases, the primary winding is tapped so that mains voltage is applied across a greater or lesser number of turns so as to produce the different outputs. Both transformers when switched to the higher output - ie. mains is applied to a lesser number of turns in the primary winding - they buzz violently and draw about 10A from the mains even though there is no output load! There can't be anything wrong

Tim Burbridge, January 2008

Oh dear ...it's late at night ... it's the third sentence not the second! Sorry

Nyge, April 2011

Sorry - read it thro again and second sentence is wrong reasoning - please ignor it.

Nyge, April 2011

The primary winding has been incorrectly connected. A transformer correctly wired will draw next to no current when when no load is applied because it acts as a self inductance and the back emf will only differ from the applied emf due to an efficiency loss. The buzz of the core indicates a high magnetic flux rather than lose core and the 10 amps confirms this. A device drawing 10 amps at 240 volts consumes as much heat as a two bar elctric fire!
I would say that since you have multitap coils on the input that you have connected two in series OR parallel such that the current flowing in them act out of phase to produce the flux in the core. So as the current goes thro the sinusoidal profile the magnetic flux generated by the primary coils acts in opposition and cancels - this is why no back enf is produced in either coil - the applied emf is then limited by the resistance of the windings which will be low.
Don't test this out using mains ... it can be done using a 1.5 volt battery and some leds making use of back enf generated when the battery current is disconnected ... but that is another storey.
If you want to know the storey come back.
WOULD SAY YOUR TRANSFORMER IS NOW DANGEROUS AS THE SHERLACK USED FOR INSULATION IS DAMAGED!

Nyge, April 2011

If it's drawing 10 amps from the mains I would worry.

Have you measured the output voltage?

Have you connected the secondary winding to the mains in error?

Is there a short on the primary winding?

John J, July 2009

Transformers usually have a fixed primary i.e. it never needs to be changed - and a fixed secondary with "taps" along its length - you want 12v from a 24v secondary then use the half way point in the secondary winding - quarter way along = 6v and three quarters 18v...

The primary is a load on the input - if you only use half the primary the load is half (sounds obvious) and the current will double. Your transformer is trying to tell you something I think - it is saying, "please connect my primary winding properly."

High current is normal if the load is low - Transformers have metal plates that physically rub against one another when stressed - that's the cause of the buzzing.

Richard Buxton, September 2008