Series connected transformer

[Kajunbee]

In one of my experiments I salvaged a 120vac to 160vac transformer out of a power supply. When I connected it to 120 vac with no load it pulled about .35 amps. I then connected the primary and secondary in series. When I first applied voltage it pulled high current. I then swapped two of the wires . When I applied power again the current was around .01 amps. When I checked the voltage from one leg of the 120 v in to where the two windings connected I had say 55 volts. The other leg to center connection would be maybe 65 volts. Was wondering if any of you had ever tried this or if it might have any usefulness. I didn't try placing a load across these lower voltages so I don't know if it would give any useful power.

 

[Warpspeed]

It certainly does have usefulness.
By series connecting two windings you effectively increase the number of turns connected across the incoming supply.
That will make a very big difference to the flux swing in the core and the magnetizing current.

As you have noticed, the no load current falls from 350mA to only 10mA, a huge reduction. If you need a transformer for some special purpose and you want it to have absolutely minimum no load current, it is a very effective way to do it.

 

[Kajunbee]

By interconnecting the two windings does this basically make a multi tap transformer.

 

[Warpspeed]

Yes, or you could take the output from a third winding.

 

[Kajunbee]

Gives a couple more voltage options for the same transformer. Step up, step down or interconnect windings and get two more further stepped down voltages. thanks for the reply. Haven't heard from you in while. How are things down unda.

 

[KlausST]

Hi,

If you connect primary and secondary, then you might see it is additionaly tap, or you may see it as an autotransformer.
Where you may draw more output current than in isolated connection.

Klaus

 

[5inc]

Hi guys
I don't understand a few things about how you did these testings.
If I may ask, how many terminals does that transformer have? does it have center tap on both windings or only on secondary?

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also, when you say you connect primary and secondary in series, how do you do that? do wou mean you join both ends of secondary....?


or you join primary blue to secondary end, brown primary to other secondary end? if so, where do you place the ac mains? :drevil:

 

[Kajunbee]

This particular transformer had multiple voltage options on secondary. I was only connecting the winding on secondary that step the voltage up from 120v on primary to 160 v on secondary. All I did was connect the primary in series with that secondary winding. I do not know exactly how the transformer is wound or tapped.

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Sorry I didn't see the rest of your questions. The start of one primary is connected to one leg of ac power. The other end of primary is connected to start of secondary winding. Then the other end of secondary is connected to 120v neutral.

 

[5inc]

right, I see, and when you swapped 2 wires it was the end of primary with an other secondary end right?

(dark clouds around my head) I don't understand... i thouht transformers principle was a wire, both ends connected to mains (for example) wrapped around a core, induces a voltage on an other wire wrapped around the same core, from witch you test voltage on both ends.
That's more or less how i thought it worked, two wires that aren't phisically connected induce voltage between each other.

This case appears to be different, two wires wrapped around a core, but joined at one end, and into mains in the other end(making the winding so many times longer. it's a simple difference that i thought would turn the wire into something like a resistor... or something.

I guess I'll have to read about inductors and coils next.

 

[Kajunbee]

It's been a while back that I did this If I remember correctly all I did was swap either both primary or both secondary. You'll know if it's not right because it will draw more current. I think it has to do with the polaritys but that's just a guess. The moderators could say for sure.

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I had to research auto transformers and that seems to be a better description of it . Variacs work this same way except your not limited to specific voltages. You also do not have isolation.