Energy, Power systems

Transformers

In this post I will explain how transformers work. Thanks to transformers it is possible to transmit electric energy in alternate current for long distances, receiving a voltage level and transmitting another level.

Working principle

A transformer consist in two coils linked by a nucleus made of blades of ferromagnetic material.

The primary coil receive current and creates a magnetic field.

Since the coil receive alternate current, the magnetic flux change direction and intensity constantly.

The magnetic flux generated in the primary coil go through the nucleus and arrive in the secondary coil, since the flux is variable there is a induced voltage in the secondary coil’s terminals.

The equation of Faraday’s Induction Law tells that the average value of induced voltage depends on the number of turns N and the magnetic flux’s variation speed \frac{\partial \phi }{\partial t}.

\varepsilon_{m} =-N \frac{\partial \phi }{\partial t}

The relation between the voltages E and the number of turns in the primary and secondary.

\frac{E_{p}}{E_{s}}=\frac{N_{p}}{N_{s}}

Why the transformer’s nucleus is in blades and not a massif block? The magnetic flux’s variation creates a induced voltage in the ferromagnetic nucleus and creates a current called Foucault’s current, This current in a massif block which have great area causes a great power loss because the conductor have a very low resistance and a big area which produces a big current which produce a magnetic field which disrupt the flux.

To reduce this parasite current, the nucleus must be divided in blades and insulate one from another to reduce this effect, this way many small currents are generated in blades and the loss is lower. The insulation between the blades is usually made of varnish or oxide layer.

Commercials transformers

Coils and nucleus aren’t enough to make a transformer. A power transformer generate too much heat because the current is very high, therefore it needs to have a refrigeration system usually by oil. The nucleus and the coils stay immersed in the oil.

The cooling tubes by side serve to heat exchange, the hotter oil goes to the top, enter in the tubes above and lost heat and goes to the bottom part. more above we have terminals with porcelain insulators. Many transformers have an oil tank to accommodate the oil volume exchange in the thermal convection.

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About Pedro Ney Stroski

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