Bell transformer wiring and Protection?

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ritelec

ritelec

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Jersey
Yesterday, I wired a laundry pull chain light outlet to come on with other basement lighting operating from a switch.. took a 14-2 from a switched lighting outlet to the 4" round box that the keyed pull chain fixture...

The existing 4" round has a 14 -2 in and out and two bell transformers on it (one old green one, one newer silver one)... the bell transformers were paralleled on the LV side.. ( ? )

I removed the keyed fixture from the circuit... redid the existing in and out 120 v and bell xformer connections.

As I'm drilling and pulling the new wire, I'm smelling a burning smell ??? I go to the box and can't touch the newer looking transformer as it's almost red hot... the green one is hot..

I disconnected the line to the old xformer and disconnected the lv connection... Finish up the work, check the door bell and no good... took a reading on the new transformer low voltage side and got 1.2v
I disconnected the line and low voltage to that transformer and rehooked up the line and low voltage to the older green transformer.. bells work.. I got out of there, but wondering about it.

I'm 99.999 % sure it was all hooked up the way I rehooked it up... why no red hot transformer before I touched it?

I was thinking that they have a couple loads so the paralleled secondary would increase the va for the secondary load.... true??

Playing around with bell transformers in the past, I would put a 9 v battery across the lv side and attach wires to the high voltage side and have someone hold the leads giving that someone a rap.

So when I disconnected any of the two transformers, If I didn't connect the lv of the other xformer, I'm thinking 120v would be induced on that other transformers HV side..

So I'm thinking of xformers that are hooked up in parallel, are voltages induced back on the primary from the secondary windings of the other transformers that are paralleled together.. ??

Does that have any affect on line power?

Also, with the amount of heat this thing was putting out, it's very scary that the only oc and gf protection is a breaker which wasn't tripping... ???
 

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Bell transformer wiring and Protection?

You probably reversed the connection of either the primary or secondary windings. Basically puts the secondaries in a shorted condition.

Did you make sure the exact same transformer wires were reconnected together?
 
the xformers are mounted to the box thru 1/2" kos.. line voltage in the box... the low voltage is outside the box, jumped from one to the other..
 
190112-2211 EST

ritelec you did not answer the question that retirede ask. Reread his question and try to completely and correctly answer it.

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OK... Thank you, got it, I think I see what your talking about .

I may have reversed polarity on one of the two transformers. as in attached picture lets say redrawing H on x former 2 to right side of primary coil and N to left of primary coil.. is that what you're saying?

If so I see how that makes a difference, but how did the original install or even me if it were an original install figure how the coils where laying.
 
ritelec said:
If so I see how that makes a difference, but how did the original install or even me if it were an original install figure how the coils where laying.
Click to expand...
With both primaries connected to the source, connect one wire of each secondary together and measure the voltage between the two unconnected wires; you'll read approximately double voltage one way, and approximately zero the other. The zero polarity is the one you want.

To me, the correct repair would be a single transformer with a higher current capacity.
 
thank you... good to know, wondering if there would be continuity between one primary and one secondary common?..
 
190113-0600 EST

ritelec:

The transformer you identify as new with just 1.2 V unloaded output voltage now is shot, internal short. With no external secondary load, and normal primary voltage, I would expect it to run hotter than it should as a result of the internal short.

I am surprised that electrician training and testing for a license does not provide knowledge on how transformers work and how to work with transformers. Part of this is how sine waves combine with each other. Other threads seem to indicate there is a lack of basic training in this sine wave area.

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Ok thank you.


Would basic training tell me that the older one which won the battle between the two which is still working and not getting hot still function properly or is it to now shot ?

By the way, I was supposed to be a rock star not an electrician. 40 years later.
 
190113-1122 EST

ritelec:

The older transformer may be OK, but you really don't know how much damage was done to it. At least it probably only shortened its life a little. On the other hand it might only last a week.

As LarryFine said, put in a new one of adequate size. A new one should last at least 50 years.

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retirede said:
You probably reversed the connection of either the primary or secondary windings. Basically puts the secondaries in a shorted condition.

Did you make sure the exact same transformer wires were reconnected together?
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Why would you even consider paralleling the secondaries?
 
gar said:
I am surprised that electrician training and testing for a license does not provide knowledge on how transformers work and how to work with transformers.
Click to expand...

In most states there is no training necessary to get a license, only field experience.
There are questions on the test about the correct wiring of transformer and many of those questions are probably answered wrong but not enough to fail.

If you do take formal training in your efforts to become an electrician there is normally some portion of the training that's dedicated to transformers. A person can work in the field for 20 years and never use what they have learned in class so they tend to forget a lot of what they once learned.
 
190113-1307 EST

There are many transformers wound with two 240 V primaries, and two 120 V secondaries. These are connected in various ways. Quite often the secondaries are in parallel. That is the equivalent to wiring two separate transformer secondaries in parallel.

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gar said:
190113-1307 EST

There are many transformers wound with two 240 V primaries, and two 120 V secondaries. These are connected in various ways. Quite often the secondaries are in parallel. That is the equivalent to wiring two separate transformer secondaries in parallel.

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Click to expand...

Agreed.
But in this case, it sounds like he had 2 different transformers of different size and probably manufacturer. Chances are the output voltages and internal impedance are somewhat mismatched which will result in some amount of current circulating through the secondaries.

Maybe not the end of the world, but not ideal. Given the small cost of doorbell transformers, I would have just bought a bigger one.
 
retirede said:
Agreed.
But in this case, it sounds like he had 2 different transformers of different size and probably manufacturer. Chances are the output voltages and internal impedance are somewhat mismatched which will result in some amount of current circulating through the secondaries.

Maybe not the end of the world, but not ideal. Given the small cost of doorbell transformers, I would have just bought a bigger one.
Click to expand...

Yes different sizes and makes.
:- ) I was there to get a light working off a switch... not rewire door bell transformers...
I've wired many transformers, non paralleled. Thank you for the education..if I ever run into the scenario again.. :)
 
ritelec said:
Yes different sizes and makes.
:- ) I was there to get a light working off a switch... not rewire door bell transformers...
I've wired many transformers, non paralleled. Thank you for the education..if I ever run into the scenario again.. :)
Click to expand...

Yes sir. I realize you were just tying to restore the original hookup.
 
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