Connecting 2 Transformers in Series: Turn Ratio Equivalent

[tefavolc]

how can i connect 2 transformers in series and what is there equivlent turn ratio ?

 

[Fish4Fun]

tefavolc,

I cannot think of a good reason to do this, and I can come up with several good reasons NOT to do this, but assuming you had two 120V transformers, and you wanted to use them on a 240V line, it would go like this:

The turns ratio would be PT1 + PT2 : ST1 + ST2, but again, this is NOT common practice.

Fish

 

[Phrak]

Assume your first transformer has a turns ratio of N, and the second is M. In "series" the effective turns ratio is the product N*M.

Fish4fun took you literally--that you series the primarys across the supply voltage, and then also series connect the secondaries.

 

[tefavolc]

i use one transformer 12-220 and obtain 180 volt at its secoundry , i need to connect it to another transformer to step it up agine , when i connect its secoundry coil direct to the primary of the secound trans, it doesn't work i guss . any idea on how to connect it right ?

 

[uart]

i use one transformer 12-220 and obtain 180 volt at its secoundry , i need to connect it to another transformer to step it up agine , when i connect its secoundry coil direct to the primary of the secound trans, it doesn't work i guss . any idea on how to connect it right ?

1. What are the ratings of the first transformer?

2. What is the nature of the voltage source you're driving it from?

3. What are the ratings of second transformer?

4. Do you think we have ESP or do you just think that details don't matter?

 

[Studiot]

In relative terms, the low voltage side of a transformer has low impedance and the high voltage side has high impedance.

So you are connecting the low impedance coil of the second transformer across the high impedance coil of the first one, thereby partially shorting it out.

 

[uart]

In relative terms, the low voltage side of a transformer has low impedance and the high voltage side has high impedance.

So you are connecting the low impedance coil of the second transformer across the high impedance coil of the first one, thereby partially shorting it out.

Yes, but if the transformers are correctly rated for the voltages each is working with then there's no reason why it shouldn't work.

 

[Studiot]

The setup was stated to be not working and I answered the question as to why it might not be working.

 

[uart]

The setup was stated to be not working and I answered the question as to why it might not be working.

Ok. The point I was making is simply that we shouldn't blame the way the transformers are connected, but rather how the transformers are designed. Let's not point the OP in the wrong direction. I'ts likely that the only thing wrong with his setup is that the transformers are incorrectly designed for the voltages on which he is expecting them to operate.

 

[Fish4Fun]

tefavolc,

Maybe you should back up and tell us what you are trying to achieve. It is beginning to sound like you are attempting to build something like a Tesla coil or perhaps some type of high-voltage lighting ballast?

In any case, I now assume you have something like this in mind?

I further assume you are attempting to use "standard" 220V line transformers? This is going to prove problematic. You are using the "primary side" of the transformer as a secondary. Because the primary side was designed for a 220V line voltage with a very low impedance, the primary itself has a fairly high impedance, thus you cannot drive it properly with 12V.

In general transformers are rated by VA (Volt-Amps), with the rating reflecting the maximum power the transformer can transfer. In simplified terms, a 22VA transformer is designed for ~0.1A on the primary side. Assuming a 12V output, this would imply ~1.8A on the secondary side.

To drive this transformer in reverse, you would need to apply a 1.8A signal to the "secodary" (now technically the primary), and you should see ~220V on the "primary" (now technically the secondary) with a maximum current of ~0.1A.

But here comes the problem with connecting a second line transformer in series: You are now attempting to drive the low impedance side of a transformer from the high impedance side of another transformer. To simplify what is happening, let's consider this in terms of DC resistance:

220V = 0.1A * R => 2200ohms (DC approximation of Primary)
12V = 1.8A * R => 6.7ohms (DC approximation of Secondary)

When you connect the 6.7ohm secondary to the 2.2k primary, the resistance drops to: 1/((1/2200) + (1/6.7)) = 6.64 ohms ==> 6.64 ohms and the voltage drops to 6.64 ohms* 0.1A = 6.64V which, of course, means that the output of the second transformer will be less than the output of the first!

Obviously the actual AC case is more complicated than this simplified explanation, but this should demonstrate in simple terms what you are experiencing.

Fish

 

[Bob S]

You also need to prevent the transformer iron from being pushed into saturation by exceeding the design volt-seconds of the winding.
From Faraday's law:

V = -N·A·dB/dt

so

∫ V·dt = V·Δt = -N·A∫dB = -N·A·ΔB volt-seconds

or

ΔB = -V·Δt/(N·A) Tesla

So the peak magnetic flux density ΔB in the iron is proportional to the voltage V applied to the transformer winding
times the duration of the applied voltage half cycle Δt. The volt-second limit is independent of the load, and is dependent only
on the applied voltage and frequency. In short, you cannot apply 16 volts at 60 Hz to a
winding designed for 12 volts at 60 Hz, unless the transformer winding was designed for the increased volt-seconds..

See page 32 in http://ecee.colorado.edu/copec/book/slides/Ch13slides.pdf

Bob S

 

[tefavolc]

thnx for the help

 

[uart]

thnx for the help

So you were trying to connect the output voltage of 180 volts to the input 12 volt primary of the second transformer!

Hopefully what you've learned is that a transformer must be designed for a particular voltage and frequency.