# Working Principle of Transformer

A transformer is used to transfer AC energy from one circuit to another. The two circuits are coupled by a magnetic field that is linked to both instead of a conductive electrical path.

This transfer of energy may involve an increase or decrease in voltage, but the frequency will be the same in both circuits. In addition, a transformer doesn’t change power levels between circuits. If you put 100 VA into a transformer, 100 VA (minus a small amount of losses) comes out.

The average efficiency of a transformer is well over 90 percent, in part because a transformer has no moving parts. A transformer can be operated only with AC voltage because no voltage is induced if there is no change in the magnetic field. Operating a transformer from a constant DC voltage source will cause a large amount of DC current to flow, which can destroy the transformer.

# Working Principle of Transformer

Transformer works on Faraday’s law of mutual induction. The law states that, when a magnetic flux linked with a coil changes, an electromotive force is induced in the coil.

Above figure illustrates a simplified version of a single-phase (1ϕ) transformer. The transformer consists of two electrical conductors, called the primary winding and the secondary winding. The primary winding is fed from a varying alternating current, which creates a varying magnetic field around it.

According to the Faraday’s law, the secondary winding, which is in this varying magnetic field, (i.e. flux linking with it is changing) will have a voltage induced into it.

In its most basic form a transformer is made up of the:

• Core, which provides a path for the magnetic lines of force.
• Primary winding, which receives energy from the source.
• Secondary winding, which receives energy from the primary winding and delivers it to the load.
• Enclosure, which protects the components from dirt, moisture, and mechanical damage.

The essentials that govern the operation of a transformer are summarized as follows:

If the primary has more turns than the secondary, you have a step-down transformer that reduces the voltage.

• If the primary has fewer turns than the secondary, you have a step-up transformer that increases the voltage.
• If the primary has the same number of turns as the secondary, the outgoing secondary voltage will be the same as the incoming primary voltage. This is the case for an isolation transformer.
• In certain exceptional cases, one large coil of wire can serve as both the primary and secondary. This is the case with autotransformers.
• The primary volt-amperes (VA) or kilovolt-amperes (kVA) of a transformer will be equal to that of the secondary less a small amount of losses.

## Voltage and Turns Ratio of Transformer

The ratio of turns in a transformer’s primary winding to those in its secondary winding is known as the turns ratio and is the same as the transformer’s voltage ratio.

For example, if a transformer has a 10:1 turns ratio, then for every 10 turns on the primary winding there will be 1 turn on the secondary winding. Inputting 10 V to the primary winding steps down the voltage and will produce a 1-V output at the secondary winding.

The exact opposite is true for a transformer with a 1:10 turns ratio. A transformer with a 1:10 turns ratio would have 1 turn on the primary winding for every 10 turns on the secondary winding. In this case, inputting 10 V to the primary winding steps up the voltage and will produce 100 volts at the secondary winding. The actual number of turns is not important, just the turns ratio.

A transformer turns ratio test set can directly measure the turns ratio of single-phase transformers as well as three-phase transformers. Any deviations from rated values will indicate problems in transformer windings and in the magnetic core circuits.

The voltage ratio of an ideal transformer (one with no losses) is directly related to the turns ratio, while the current ratio is inversely related to the turns ratio:

A transformer automatically adjusts its input current to meet the requirements of its output or load current. If no load is connected to the secondary winding, only a small amount of current, known as the magnetizing current (also known as exciting current), flows through the primary winding.

Typically, the transformer is designed in such a way that the power consumed by the magnetizing current is only enough to overcome the losses in the iron core and in the resistance of the wire with which the primary is wound.

If the secondary circuit of the transformer becomes overloaded or shorted, primary current increases dramatically also. It is for this reason that a fuse is placed in series with the primary winding to protect both the primary and secondary circuits from excessive current.

The most critical parameter of a transformer is its insulation qualities. Failure of a transformer, in most instances, can be traced to a breakdown of the insulation of one or more of the windings.

For a purely resistive load, according to Ohm’s law, the amount of secondary winding current equals the secondary voltage divided by the value of the load resistance connected to the secondary circuit (a negligible coil winding resistance is assumed).

## Transformer Power Rating

Just as horsepower ratings designate the power capacity of an electric motor, a transformer’s kVA rating indicates its maximum power output capacity. Transformers’ kVA ratings are calculated as follows:

The maximum power rating of a transformer can be found on the transformer’s nameplate. Transformers are rated in volt-amperes (VA) or kilovolt-amperes (kVA). You may recall that volt-amperes is the total power supplied to the circuit from the source, and includes real (watts) and reactive (VAR) power.

The primary and secondary full-load currents usually are not given. If the volt-ampere rating is given along with the primary voltage, then the primary full-load current can be determined using the following equations: