Re: Reversing a transformer?
- From: KR <kenreed1999@xxxxxxxxx>
- Date: Mon, 11 Apr 2011 07:22:50 -0700 (PDT)
On Mar 28, 5:26 am, jvalh <jv...@xxxxxxxxxx> wrote:
I have a little problem.
It involves a transformer that has on one side a blue and brown wire.
Two other wires, also blue and brown are soldered to those two
transformer wires and the other ends of those wires go into a plug. This
plug has a ground wire running back to the transformer body and gets
fixed there with a screw. At present, the plug on this side of the
transformer connects into a 120volt outlet. The desire was to get
240volts on the other side of the transformer - 1:2 ratio.
The other side of the transformer has a socket attached to a plate that
gets bolted to the transformer. This socket has two holes+slots for the
plug (round or flat prongs) of a device e.g. a 240volt coffee grinder or
a steamer that needs to be connected to the transformer. The top slot is
wider than the bottom one, meaning that some flat prongs will fit only
one way into the socket.
This has worked very well but now I am in a 240volt country and I want
to reverse the two sides of the transformer so that the 240volt local
supply will provide for my 120volt devices. This is done by de-soldering
two wires on each side of the transformer and reversing the sides.
Can anyone see problems with doing this?
If you have taken a transformer intended to be used in a 120v country
(60hz) to a european country
(50HZ) you will have problems with the frequency. This will result in
partial saturation of the transformer and overheating. Will also
reduce the output wattage available.
Another problem with wiring transformers backwards is that (what was
intended as) the primary winding will probably be able to deliver a
higher wattage than what is available on the sum of all the
secondaries. For example if the secondary windings are specified to
deliver a total of 100VA, the primary might be designed to deliver
(say) 115va from the mains in order to make up for the losses due to
inefficiency in the transformer (heat, magnetizing current etc) and
ensure that there is enough VA left over to keep the secondary VA up
to specification at full load.
If you now reverse this transformer, and use the 100w intended
secondary as the primary - (with the correct voltage of course) after
losses there might only be (say) 85VA available on the intended
primary part that is now being used as a secondary. This if
significantly below its original design spec.
Finally, how are your appliances going to cope with this "wrong" mains
frequency ? Things like Lights, heaters (resistive loads),
switchmode power supply devices like in modern electronics are going
to be fine, but loads with a transformer or other inductive load are
not going to like being run on 50HZ. The microwave for example has a
transformer, transformer may overheat and the microwave may produce
less power - things taking longer to cook, or on high setting not ever
being able to heat as high as before. AC motors will run slower, give
less power and tend to overheat if on a long time including mains AC
powered fans in equipment (universal brush motors or "brushless" DC
type fans should be ok ). In these cases the only practical thing to
do is replace the transformer in the appliance - or likely cheaper to
replace the appliance.
If you want to run a microwave/toaster, heater etc - you will need a
massive transformer, probably 1500VA + or so depending on the VA
rating of the appliances and if you want to use more than one at the
Finally, many of these transformers are "autotransformers" which means
that there is no isolation between primary and secondary. They are
easily identifiable by only having 3 wires rather than 4. Often will
all be on the same side of the transformer also
While the output of these is still 120v - if the primary or mains
plug is wired the wrong way around, then you can end up with a
potential of 240v AC between one of your 120v terminals and mains
ground. This may cause failure of supression / line filter caps not
designed for 240v operation that go from the line to mains ground in
the appliance, and if you happen to be working on something that is
connected to the secondary winding, and touch a live terminal you will
likely get a 240v shock instead of 120v.
A local peanut factory imported a load of mechanical handling, packing
and sorting equipment from the USA, and had a world of trouble with
slow operation, overheating motors and endless other troubles.
You could try Ebay UK
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