Understanding Transformers: Components, Operation, and Types
Transformers
Static electricity is a machine designed to operate with alternating current. It has two windings, primary and secondary, that can transform electrical energy at certain voltage and current (VI) magnitudes into others with different values.
Constructive Elements
A transformer consists of the following parts: core, windings, cooling system, and bushings.
Core
The core is the system that forms the magnetic circuit. It is made up of silicon steel sheets subjected to a special treatment called Carlito, which reduces iron losses. The magnetic circuit consists of columns where the windings are mounted and yokes that join the columns. The spaces between these, where the windings pass, are the windows of the core. Depending on the relative position between the core and windings, transformers are classified as core-type or shell-type. The joints of the columns with the yokes are called junctions. The top yoke is opened to allow the coils and insulation to be inserted. The junctions can be butt (or flat) or overlapped (interlaced).
Windings
The windings form the electrical circuit of the transformer and are made of copper wires. The conductors are lined with sheet insulation. Depending on the arrangement of the high-voltage (HV) and low-voltage (LV) windings, they can be concentric or alternating.
Cooling Systems
Losses in a transformer are transformed into heat, contributing to the heating of the machine. To prevent this from affecting the lifespan of the transformer’s insulation, a good cooling system must be provided. In small machines, an external surface is sufficient. For higher power ratings, transformers are immersed in oil and provided with a tank.
Insulators and Other Components
The terminals of medium-voltage transformers are brought outside the tank through bushings made of porcelain, either air or oil-filled. The HV and LV bushings are distinguished by their height, with higher voltage bushings being taller. Another element found in high-powered transformers is the Buchholz relay, which protects the machine from dangerous overloads, insulation failures, and other issues.
Transformer Characteristics Plate
The characteristics plate is a silkscreened melamine board that displays data such as rated output, rated voltage, frequency, and equivalent impedance (%). It also indicates the internal wiring diagram, type of cooling, manufacturer, serial number, etc. The standard voltages are those for which the transformer has been designed and are determined based on testing and use. The nominal power always refers to the apparent power and applies to both the primary and secondary windings: [Sn = V1n.I1n = V2n.I2n]. The magnitude of the current determines the copper losses, which affect the heating of the windings.
Principle of Operation
Conventions of signs: The primary winding acts as a receiver (drawing current and power and developing an electromotive force (emf)), while the secondary winding behaves as a generator (providing current and power and being the seat of an induced emf).
Ideal conditions:
- The windings have negligible resistance, resulting in no Joule losses.
- There are no leakage fluxes; the magnetic flux is confined within the core.
When an alternating voltage V1 is applied to the primary winding, an alternating current flows, producing an alternating flux in the core. This flux induces an emf e1 in the primary winding, given by: e1 = N1 (dΦ / dt), where N1 is the number of turns in the primary winding and Φ is the magnetic flux.
e1 represents an emf opposing the voltage v1. When the secondary circuit is closed, a current i2 tends to flow.
Applying Kirchhoff’s Current Law (KCL) to the primary and secondary windings yields: v1 = e1 = N1 (dΦ / dt). Knowing that the flux is Φ = Φm sin(ωt) = Φm cos(ωt – 90°), it follows that: v1 = e1 = N1ωΦm cos(ωt). This shows that the voltages and emfs are advanced by 90° with respect to the flux.
The turns ratio, denoted by m, is defined as: m = E1/E2 = V1/V2 = N1/N2.
When the secondary circuit is open, the transformer is said to be unloaded or operating in an open-circuit condition. In this case, the input power is equal to the iron losses: [P0 = PFe = V1I0 cos(φ)].