Reversing a transformer?

[jvalh]

Hello Experts,

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?

TIA

 

[Mint]

Hello Experts,

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?

TIA

Much safer to buy a step down transformer.

 

[Paul]

Hello Experts,

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?

TIA

http://www.electricityforum.com/electrical-transformers/step-down-transformers.html

"Single phase step down transformers 1 kva and larger may also be reverse
connected to step-down or step-up voltages.

(Note: single phase step up or step down transformers sized less than 1 KVA
should not be reverse connected because the secondary windings have
additional turns to overcome a voltage drop when the load is applied.
If reverse connected, the output voltage will be less than desired.)"

Another thing to be aware of, is transformers come in more than one configuration.
There are "isolating" transformers, where the secondary has no path to the primary.
And there are "Autotransformers", where one winding is shared for input and output.
They're not the same thing.

http://en.wikipedia.org/wiki/Autotransformer

http://en.wikipedia.org/wiki/Isolation_transformer

A unit with a 1KW or 1 KVA rating is pretty big.

http://www.hammondmfg.com/172.htm

*******

Now, the question would be, how do you *safely* test your handy work ?
Do you know what the full fury of electricity looks like ? How long
would it take for the breaker in the panel to respond, if you've made
a mistake ?

I'd simply purchase the proper adapter, in the country you're currently
residing in, confident that such a device will not invalidate the
homeowner fire insurance, and meets all local regulations.

Paul

 

[philo]

Hello Experts,

I have a little problem.
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?

TIA

It should work as long as you do not exceed the power rating of the
transformer...


Of course I'd test it with a light bulb and volt meter if possible...
depending on how lossy the transformer is you could get some variation on
what you'd expect

 

[jvalh]

It should work as long as you do not exceed the power rating of the
transformer...


Of course I'd test it with a light bulb and volt meter if possible...
depending on how lossy the transformer is you could get some variation on
what you'd expect

I haven't the time to experiment on this now but I have found a little
more info. On the left side, where the plug is, the copper coil is
completely used. On the other side, where the socket is, there is an
additional wire coming from the coil a bit away from the centre. Let's
call this a middle wire and now we have two pairs of wires on this side
of the transformer. A diagram shows 240Volts across one of the end wires
to the middle wire and 180V from the middle wire and the other outer
wire. I also noticed two soldered copper wires on the surface of the
copper coil.

I bet someone can decipher this messy description and explain the
reason for it.

What would this setup do to my original thought of swapping the two
sides of the transformer?

 

[Paul]

I haven't the time to experiment on this now but I have found a little
more info. On the left side, where the plug is, the copper coil is
completely used. On the other side, where the socket is, there is an
additional wire coming from the coil a bit away from the centre. Let's
call this a middle wire and now we have two pairs of wires on this side
of the transformer. A diagram shows 240Volts across one of the end wires
to the middle wire and 180V from the middle wire and the other outer
wire. I also noticed two soldered copper wires on the surface of the
copper coil.

I bet someone can decipher this messy description and explain the
reason for it.

What would this setup do to my original thought of swapping the two
sides of the transformer?

So does the diagram look like the one here ?

http://en.wikipedia.org/wiki/Autotransformer

or does it look like something in this PDF document ?

http://www.hammondmfg.com/pdf/186_187Bulletin.pdf

That one, shows various isolating transformers, with multiple input
or output options. (Center tapped outputs, multiple primary windings for
input from 120 or 240V etc.) In the examples in the PDF, the primary
is separated from the secondary. Nothing in the second document,
is an autotransformer.

The first bit of text I quoted, in my other posting, says that the
transformer can be reversed, if it is rated for more than 1 kilowatt.
The reason being, for lower wattage ratings, the turns ratio may have
been adjusted to compensate for loss, and operating the reverse way
around, doesn't give the exact right voltage. At 1 kilowatt or higher
for the transformer, loss should be pretty low, and the turns ratio
closer to 2:1 (for 230 to 115).

I don't know why anyone would be interested in a 180V tap on a 240V
winding.

Paul

 

[jvalh]

So does the diagram look like the one here ?

http://en.wikipedia.org/wiki/Autotransformer

or does it look like something in this PDF document ?

http://www.hammondmfg.com/pdf/186_187Bulletin.pdf

That one, shows various isolating transformers, with multiple input
or output options. (Center tapped outputs, multiple primary windings for
input from 120 or 240V etc.) In the examples in the PDF, the primary
is separated from the secondary. Nothing in the second document,
is an autotransformer.

The first bit of text I quoted, in my other posting, says that the
transformer can be reversed, if it is rated for more than 1 kilowatt.
The reason being, for lower wattage ratings, the turns ratio may have
been adjusted to compensate for loss, and operating the reverse way
around, doesn't give the exact right voltage. At 1 kilowatt or higher
for the transformer, loss should be pretty low, and the turns ratio
closer to 2:1 (for 230 to 115).

I don't know why anyone would be interested in a 180V tap on a 240V
winding.

Paul

Thanks Paul - always coming to the rescue

The trafo I have is rather primitive - no such thing as spade connectors
- everything is soldered.
On the low voltage side there are only two wires coming out. On the
other side there are also only two wires coming out and these provide
the higher voltage. On this high voltage side there are two end wires
soldered together on the surface of the copper coil. They are not
connected to anything else. I would think that these two wires are the
ends of two coils on this high voltage side. I guess I could make a
measurement between this soldered pair and the two wires on that high
voltage side. Most likely a current have to be flowing on this high
voltage side to get a reading?

As usual, thanks Paul

 

[Paul]

Thanks Paul - always coming to the rescue

The trafo I have is rather primitive - no such thing as spade connectors
- everything is soldered.
On the low voltage side there are only two wires coming out. On the
other side there are also only two wires coming out and these provide
the higher voltage. On this high voltage side there are two end wires
soldered together on the surface of the copper coil. They are not
connected to anything else. I would think that these two wires are the
ends of two coils on this high voltage side. I guess I could make a
measurement between this soldered pair and the two wires on that high
voltage side. Most likely a current have to be flowing on this high
voltage side to get a reading?

As usual, thanks Paul

You will get voltage, while the transformer is not loaded. It does
not require current to flow.

As you add load, the voltage should drop slightly. The amount of
drop is a function of the loss in the windings.

For example, I could put 1000 turns of number 40 wire on there
(thin, like human hair), and it would have no current carrying
capacity. At no load, the transformer would deliver the correct
volts (as seen by an AC multimeter reading). But as soon as I put
a load on it (like a 60W light bulb), the voltage would drop.

If I wound the transformer with 1000 turns of number 22 wire, that
is much thicker. Now, the transformer is really heavy, and a lot
larger, as I need room to put the 1000 turns. But the drop under
load is less.

When you see two coil ends come to the surface, and be soldered to a
tab, that is a "tap". It was put there for a reason, as it costs
money to add a tap. They *might* have added the tap, only for usage
when reversing the transformer for example. If the transformer
had a manufacturer name and part number, you might even be
able to dig up some information on it on the web.

Transformers can be dangerous toys to play with. Take, for
example, a neon sign transformer. Or the transformer in a
microwave oven. Those could potentially be lethal, or cause
burns. I realize the turns ratio of your transformer is
either 1:1 or 1:2 or 2:1 or the like, but please be careful.

I've only been thrown across my basement floor once by high
voltage, and it was while working on a transformer. It was
a valuable lesson, as it hasn't happened since. It's almost
like being teleported, because one minute you're on one side
of the room, and then you're some distance away, saying to
yourself "how did I get over here ?". I had no recollection
of the trip.

I've wound my own transformers. But I can't say I've had too
many successes as a transformer designer. I built a Tesla coil
as a kid, but the sparks were only an inch or two long (in the
Tesla business, that's a flop). At least there wasn't a fire.

Paul

 

[jvalh]

I think the bottom line is becoming obvious: The OP should
simply buy a new transformer meant for 240VAC. Transformers
are of the analog world and their design has to be "tuned" and it
is as much an art as a technology. Why futz around and take
risks with fire and electrocution when transformer design is just
basically choosing winding ratios and core material and geometry
followed by lots and lots of cut 'n try? Even if you succeed in
producing the correct volt-amp output, you'll still have a lot of
testing to check internal temperatures and status of the wire
insulation and how everything ages. Life *must* hold more
interesting and rewarding activities than that, even in its more
advanced stages.

*TimDaniels*

I have been looking for transformers in the usual computer shops but
could not find them. Where are they available in western Europe? -
Ireland or U.K.?


I also have a 5KVA 50/60 step down transformer and it's doing fine but I
would have to re-wire most of the house. That one I bought in Richmond,
California for $400.

 

[Paul]

I have been looking for transformers in the usual computer shops but
could not find them. Where are they available in western Europe? -
Ireland or U.K.?


I also have a 5KVA 50/60 step down transformer and it's doing fine but I
would have to re-wire most of the house. That one I bought in Richmond,
California for $400.

Are you looking for power solutions for computers ?

Or for appliances in general ?

If it's computers, have you checked to see if there is a
red slide switch on the back of the supply ? Some power supplies
are universal, and don't have a switch, while others use the
red slide switch to switch voltages. With either of those,
perhaps all you'd need is a different power cable.

Paul

 

[jvalh]

Are you looking for power solutions for computers ?

Or for appliances in general ?

If it's computers, have you checked to see if there is a
red slide switch on the back of the supply ? Some power supplies
are universal, and don't have a switch, while others use the
red slide switch to switch voltages. With either of those,
perhaps all you'd need is a different power cable.

Paul

Not for computers. I have long been aware of that slider button 120-240
on most power supplies. My situation is that I have just about as many
120 volt products as 240 volt - microwave oven, steamer, coffee grinder,
heaters, even a large refrigerator, soldering iron, power tools, etc. I
have a few situations when I can use for both 120 and 240v. I have been
yo-yoing between USA and Ireland for too long ;-)

Thank you Paul for your interest

 

[KR]

Hello Experts,

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?

TIA

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
same time.

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

http://shop.ebay.co.uk/?_from=R40&_...tepdown+transformer&_sacat=See-All-Categories

http://cgi.ebay.co.uk/US-UK-Voltage...ctronics_PowerAdaptors_SM&hash=item45f18e073d

 

[KR]

Hello Experts,

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?

TIA

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
same time.

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

http://shop.ebay.co.uk/?_from=R40&_...tepdown+transformer&_sacat=See-All-Categories

http://cgi.ebay.co.uk/US-UK-Voltage...ctronics_PowerAdaptors_SM&hash=item45f18e073d