# Transformer: single primary coil and multiple secondary coils

• barnflakes
In summary, a power transformer has a primary and a secondary coil with different turn ratios. If I have a coil with 10 turns and a primary voltage $$V_p$$ and primary current $$I_p$$ and a single secondary coil with 10 turns, I understand the power in the secondary will be $$P=V_pI_P$$ and hence voltage and current in the secondary coil will be the same. If I now bring a 3rd coil with 10 turns into the mix, what will be the voltage in and current in the secondary and tertiary coil? My intuition says that the voltage will be the same and equal to $$V_p$$ in the secondary and tertiary coil but the current will be halved $$barnflakes If I have a coil with 10 turns and a primary voltage$$V_p$$and primary current$$I_p$$and a single secondary coil with 10 turns, I understand the power in the secondary will be$$P=V_pI_P$$and hence voltage and current in the secondary coil will be the same. If I now bring a 3rd coil with 10 turns into the mix, what will be the voltage in and current in the secondary and tertiary coil? My intuition says that the voltage will be the same and equal to$$V_p$$in the secondary and tertiary coil but the current will be halved$$I_p/2 in both in order to conserve energy.

Is this correct? It makes me wonder how the third coil would "know" about the existence of the second coil and realize how to change its current in response to it.

The current on a coil (of the transformer) is not determined by the coil alone. The turn ratios will tell you the voltages, and then you have to calculate the currents based on the load.
With that you can sum up the power on the secondary, then determine the current on the primary (if the power is still within the specification of the transformer, and the currents are within the specification of the relevant coil).
If you have values out of spec that is a different story.

Fisherman199, Merlin3189 and anorlunda
barnflakes said:
If I now bring a 3rd coil with 10 turns into the mix, what will be the voltage in and current in the secondary and tertiary coil? My intuition says that the voltage will be the same
You need to include the source impedance of the signal source driving the primary winding, and the load impedance on each of the output windings, in addition to the turns ratios.

The load impedances transform back (via a version of the turns ratio -- can you say what that ratio is?) to the primary side, and load the signal source accordingly. To calculate the secondary output voltages and currents, you need to solve for the whole system working together.

And then when you are ready for the next layer of the model, you will include the effects of the leakage inductance (Lk) of each winding, versus the magnetizing inductances (Lm) that couple the B-field between the coils...

barnflakes said:
It makes me wonder how the third coil would "know" about the existence of the second coil and realize how to change its current in response to it.

Hmmm we are speaking of a traditional iron core power transformer here, not an air core signal transformer ?
And sinewave excitation ?

The third coil is oblivious to all other coils.
In transformers you sum the currents, see below

The primary coil establishes magnetic flux in the core.
The magnitude of that flux you can measure as volts per turn
and so long as primary voltage is constant so will be flux
That flux passes also through the secondary winding where it induces voltage
if any current can flow in the secondary,
the current there makes a second magnetic flux opposing that from the primary
reducing total flux
so more current will flow into primary to keep total flux constant.

Adding a second secondary doesn't change a thing .
Current in that secondary will also result in more primary current.

In an unloaded power transformers the product of amps X turns will be constant, and a very small number (in fact for an ideal transformer it'll be zero)
and when you load it the SUM [(primary amp-turns) - (Σsecondary amp turns)] will be the same small number (or zero if an ideal transformer)..

That approach will lead your brain to the right equations. Practice it.

Any help ?

old jim

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## 1. What is a transformer?

A transformer is a type of electrical device that is used to transfer electrical energy from one circuit to another through the process of electromagnetic induction. It consists of two or more coils of wire wrapped around a core made of magnetic material.

## 2. How does a transformer work?

A transformer works by passing an alternating current (AC) through the primary coil, which creates a changing magnetic field around the core. This changing magnetic field then induces a voltage in the secondary coils, which can be stepped up or down depending on the ratio of the number of turns in the primary and secondary coils.

## 3. What is the purpose of having multiple secondary coils?

The purpose of having multiple secondary coils is to be able to step the voltage up or down to different levels. This allows for more efficient transfer of electrical energy over long distances and also allows for different levels of voltage to be used for different applications.

## 4. Can a transformer have more than one primary coil?

Yes, a transformer can have more than one primary coil. This is known as a multi-winding transformer and is often used in applications where multiple sources of power need to be combined or where the voltage needs to be stepped up or down in multiple stages.

## 5. What are the advantages of using a transformer with multiple secondary coils?

Using a transformer with multiple secondary coils allows for more flexibility in terms of voltage levels and more efficient transfer of electrical energy. It also allows for isolation between different circuits, which can be beneficial for safety and reducing interference between different systems.

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