What happens if you connect a transformer the other way around?

Thread Starter

rambomhtri

Joined Nov 9, 2015
319
Hi, here's another quick question. Suppose you have a 125V device and your wall socket is 220V. You would buy a 220V to 125V transformer and that's it. BUT, I was thinking the other day:
What if you mistakenly connect the transformer reversed and connect the 220V end to the machine and the 125V to the wall socket?
Would the machine get 400V?
I've really never used a transformer, but a friend of mine needs one to use his american machine in Europe, and this question came to my mind the other day. I guess transformers have some security measurements to avoid this situation, besides labeling each end.
 

Tonyr1084

Joined Sep 24, 2015
4,914
Usually the primary (higher voltage) winding is also a finer wire than the secondary winding. That means the secondary winding is thicker and less resistance (per linear inch, foot, meter - whatever). Hooking a winding designed for 110 volts to 220 volts will tend to be catastrophic to the transformer. Fortunately when you buy a converter that you plug into a 220 VAC outlet and get 110 VAC - they're designed to handle the voltage. Depending on their size will depend on their wattage (or in the case of AC circuits - - - VA).

If you took a transformer and connected it reverse - IN THEORY you get the ratio upwards instead of down. Take a 120 VAC transformer that puts out 12 VAC. If you hook it up backwards (and the windings don't blow up) then the output would be 1200 VAC. That's a typical 10:1 step-down transformer. Wiring it backwards makes it become a 1:10 step-up transformer.

Again, the secondaries aren't designed for high voltage as an input. Good way to keep warm though.
 

dl324

Joined Mar 30, 2015
11,289
What if you mistakenly connect the transformer reversed and connect the 220V end to the machine and the 125V to the wall socket?
Consumer versions have the appropriate plug/receptacle to prevent people from making that mistake.
Would the machine get 400V?
Yes, but you could have issues if the insulation isn't rated for that voltage or too much current is drawn (maybe briefly).
I guess transformers have some security measurements to avoid this situation, besides labeling each end.
Manufacturers are more likely to assume you know what you're doing. There's only so much you can do for ignorant people.
 

be80be

Joined Jul 5, 2008
2,049
They make Buck transformers that can step up 120 to higher voltage I have one that can step up or step down.
It was used to fix a problem were the wire was sized wrong and the conduit to small to change the wire size.
so it had 480 on the primary and stepped that down to 240 .
The floor got cut out and we installed new conduit for bigger wire and I got too keep the transformer
It about to much to mess with tho it's almost a 100 pounds.
 
You can think of the transformer as a ratio like 1:2 or 2:1, but it also has a max voltage. The primary is usually the inner winding for efficiency reasons. The designed current can;t be exceeded either.

If you use a variable auto-transformer, the wiper needs to be fused at the max current at the designed voltage. So, if it is 0-120 V, 10A, don;t try to draw 6V at 30 A. It's 1200 VA or 6*30 or 180 VA. Fusing the input AND the wiper makes sense, but the wiper has to be fused.
 

AlbertHall

Joined Jun 4, 2014
10,243
If you apply a voltage higher than the rating for the transformer winding, the core will likely saturate, allowing a large current to flow limited mainly by the winding resistance, and thus zapping the transformer.
Or, with luck and a little forethought, it will blow the fuse.
 

Thread Starter

rambomhtri

Joined Nov 9, 2015
319
I think it was clear, but just in case, I was asking this because I don't want it to happen, not because "oh, great, now I can work with 400V". So, I wanted to know what security measurements do transformers have to avoid this mistake. I don't know, if both ends of the transformer have the same input, I think it's more than probable that a user will connect it reversed and unleash hell sooner or later.

The device is a drilling machine, and my friend, who is not savvy about science and electricity, used a universal plug and directly plugged the 120V to a 220V wall socket. He told me that there were sparks inside the machine, and immediately unplugged it. He told me it was broken, and after some investigation I suddenly saw in the label "120V".

What kind of damage can you make to a 120V drill if you connect it briefly to a 220V source?
I guess none.
 

DickCappels

Joined Aug 21, 2008
6,535
(some text removed for clarity)
What if you mistakenly connect the transformer reversed and connect the 220V end to the machine and the 125V to the wall socket?
Would the machine get 400V?
.
You understand the basic operation of a transformer. That is what it would try to do (give 400V on the output), but if you got 400V there would likely be some problems because transformers are not symmetrical between primary and secondary windings, particularly when loads are present.
 

Thread Starter

rambomhtri

Joined Nov 9, 2015
319
You guessed wrong. Even a fraction of a second may be long enough to fry a variety of appliances.
Yeah, but it's a drilling machine of 300W, no electronics involved, it's simply a motor that was supposed to work with 120V but suddenly received 220V. Would you damage a motor in that case? Not really? May be? Can't tell? For sure?
 

Thread Starter

rambomhtri

Joined Nov 9, 2015
319
You understand the basic operation of a transformer. That is what it would try to do (give 400V on the output), but if you got 400V there would likely be some problems because transformers are not symmetrical between primary and secondary windings, particularly when loads are present.
Yeah, of course, I guess it would survive. What can happen to a transformer connected reversed?
A cable would break due to the heat? Sparks? Smoke? Nuke? Hahahah
 

DickCappels

Joined Aug 21, 2008
6,535
Probably not, but the output voltage would probably be lower than would be expected and the load regulation not as good as expected because the "real" primary was designed to drive the magnetizing current of the core while the "real" secondary was not. But yes, it might get hotter than usual.
 

BobTPH

Joined Jun 5, 2013
2,505
The turns ratio is 2:1. The ratio of the inductances is the square of that or 4:1. So one effect is that the idle current, with no load, is 4x higher. This causes extra heating in the transformer and reduces the max load VA. With a light load, it would probably survive, with it full load it would likely not.

Now lets make it a 120V to 12V transformer. In this case, the turns ratio is 10 to 1 and inductance is
100 to 1. It likely would not survive the idle current in this case., the core would saturate and take even more current. If you are lucky, a fuse will blow before the transformer burns up.

Bob
 

dendad

Joined Feb 20, 2016
3,595
There is a pretty good chance the drill motor is damaged. Either with over voltage breaking down somewhere or over current burning out wires....
Get it tested before you use it for safety sake, or just get a new one.
 

Thread Starter

rambomhtri

Joined Nov 9, 2015
319
Ouch! Of course! That's right!
I can put a non removable male cable for the input 220V that goes into a wall socket, and the other end where you connect your 120V machine, a female cable, also non removable. That way there's no way for my friend to fail, right?
MaxHeadRoom, you once trolled the TV, now you're a genius!
 
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