Transformer Construction and Components

Transformer Construction Principles

• A transformer consists of two or more coils wound on a core.
• Coils are linked by magnetic lines of force, transferring energy from one circuit to another.
• Transformers come in various sizes, from large, high-voltage units for power distribution to miniature ones for electronics.

Basic Transformer Components

• Primary coil or winding receives energy from an AC source.
• Secondary coil or winding receives energy from the primary and delivers it to a load.
• Core supports the coils and provides a path for magnetic lines of flux.
• Windings are coils of wire wound on a core material, with some using cylindrical or rectangular cardboard forms.
• Air-core transformers effectively have air as their core material.
• Transformers for low frequencies (60 Hz and 400 Hz) use low-reluctance magnetic cores, typically iron, and are called iron-core transformers. Most power transformers are iron-core type.

Core Characteristics

• Core composition relies on voltage, current, frequency, size constraints, and construction costs.
• Common core materials are air, soft iron, and steel, each suited to specific applications.
• Air-core transformers are used for high-frequency voltage sources (above 20 kHz).
• Iron-core transformers are typically used when the source frequency is low (below 20 kHz).
• Soft-iron-core transformers are useful when small size and efficiency are needed.
• Laminated steel cores helps dissipate heat and provide efficient power transfer, commonly found in equipment transformers.
• Steel laminations are insulated with non-conducting materials like varnish and formed into a core, with about 50 laminations per inch.
• Laminations reduce losses, and an efficient core offers the best path for flux lines with minimal magnetic and electrical energy loss.

Hollow-Core Transformers

• Hollow-core transformers are made of laminated steel with a hollow square in the center.
• Windings are wrapped around both sides of the core of hollow-core transformers.

Shell-Core Transformers

• Shell-core is a popular and efficient core type.
• Each layer of the shell core includes E and I shaped metal sections.
• The E and I sections are butted together to form the laminations, which are insulated and pressed together.

Transformer Windings

• Transformers have two coils called windings around a core.

• It operates when AC voltage is applied to one winding and a load device is connected to the other.

• The primary winding connects to the source and the secondary winding connects to the load.

• Primary winding and primary are interchangeable terms; also, secondary winding and secondary.

• Primary windings consist of many turns of relatively small wire wound in layers directly on a rectangular cardboard form.

• Insulating material is placed between winding layers for high-voltage applications.

• After the primary winding is complete and wrapped with insulating paper or cloth, the secondary is wound on top.

• The secondary winding is covered with insulating paper and E- and I-shaped iron core sections are inserted around the windings.

• Leads from the windings are brought out through a hole in the enclosure or via terminals on the enclosure.

Transformer Symbols

• Transformers also have four leads, two from the primary and two from the secondary, for source and load connections.
• Air-core transformers have unique schematic symbols, iron-core transformers include bars indicating the iron core.
• Additional connections at points other than the ends of the windings are called taps, a center tap connects to the winding's center.
• Induced voltage depends on each inductor's impedance and the magnetic coupling between the two windings.
• The core material and relative position of the windings determine the amount of magnetic coupling.

Transformers and Alternating Current

• Transformers transfer electrical energy from one circuit to another using electromagnetic induction without changing frequency, AC voltage is used for explanation.
• When AC is used, voltage and current levels can either be increased or decreased by using a transformer.
• The power used by the load is equal to the current in the load times the voltage across the load, or P=EI.
• If a load requires 2 A at 10 V (20 W) and the source can only deliver 1 A at 20 V, a transformer can decrease the voltage to 10 V and increase the current to 2 A, keeping the power the same.

No-Load Condition

• A transformer can supply voltages higher or lower than the source voltage through mutual induction.
• Mutual induction occurs when the changing magnetic field from the primary voltage cuts the secondary winding.
• A no-load condition exists when voltage is on the primary but no load is connected to the secondary, so no current flows in the secondary winding.
• When AC voltage is applied to the primary with the switch open, a small excitation current flows in the primary to create a magnetic field.
  • Factors determining this are the amount of voltage applied (Ea) and the resistance (R) of the primary coil, and its core losses,
  • The XL depends on the frequency of the excitation current.
• Most of the excitation energy maintains the magnetic field of the primary, only a small amount overcomes wire resistance and core losses.
• Excitation current flows at all times maintaining the magnetic field, but no energy transfers while the secondary circuit is open.
• With no load on the secondary winding, the primary draws little because it is highly inductive with high inductive reactance.
• The alternating current in the primary winding has a continually varying magnetic field that induces a back-EMF in each turn.

Production of Counter-EMF

• When alternating current flows through a winding, a magnetic field sets up around it.
• As the lines of flux expand outward, A counter-EMF is produced in the winding.

Inducing a Voltage in the Secondary Windings

• As excitation current flows, magnetic lines of force are generated and expanding outward, cutting across the secondary.
• Remember, voltage is made when magnetic lines cut across the secondary, hence the voltage across the primary causes voltage to be induced across the secondary

Primary and Secondary Phase Relationship

• A transformer off load (no load connected to the secondary) acts as an inductor.

• In inductive circuits voltage leads current (CIVIL) so voltage will lead the current by 90°.

• The same magnetic field generates the BEMF and the voltage in the secondary. If both primary and secondary are wound in the same direction the polarity of the secondary mimics the BEMF which is 180° relative to the applied voltage.

Coefficient of Coupling

• Coefficient coupling of a transformer depends on the portion of total flux lines cutting both primary and secondary windings
• Ideally, all flux lines generated should intersect. When this happens, coupling coefficient is unity and maximum energy is transferred.
• Practical transformers employ close winding spacing to provide high coupling coefficients. Flux lines generated by one winding that don't engage the other are called leakage flux.
• Leakage flux from the primary does not cut the secondary, the secondary voltage will be less.
• Leakage flux is like an inductor, and can be duplicated by assuming an leakage inductor is connected in series with the primary, dropping the voltage.

Turns and Voltage Ratios

• The total voltage induced into the secondary winding of a transformer it is determined by the ratio of its ratio and the amount of primary turns.

• If a transformer consists of 10 primary turns and 1 secondary turns, as the lines of flux generated by the primary expand and collapse, they intersect both.

• A transformer with primary of 10 turns, with a single turn that induces 1v on each means that the secondary will output at 1V

• In a transformer that has a 10 turn primary and 2 turns secondary, total voltage across the secondary will be at 2V, due to the total voltage being 1V per each turn in the winding.

• Because the CEMF in the primary equals the applied volts, the value of voltage induced to the secondary is always proportional.

V/V = N/N S/P = S/P

Turns Ratio, an Example

• A voltage ratio of 5:1 is the same as a turn ratio, 10:2, or even 400:80 etc.

• Turns voltage and ratios are also used in cases where a turn and voltage ratio may be provided.

• A step down transformer is written as 4:1. A transformer who has less turns at the primary will generate voltage that's greater than the primary.

• A transformer who has greater voltage in the secondary is called a step up transformer. It can also be written as 1:4.

• The ratio of the amount of the voltage in the primary winding is to the Vs, which is in the secondary winding

Mutual Flux

• Flux that is common to the primary and secondary windings is known as mutual flux.
• Energy transfer occurs from the primary winding to the secondary winding from mutual flux.
• Currents in the primary and secondary windings produce flux when source current flows in the primary winding.
• Voltage induced into the secondary winding causes current flow. The current produce a flux field about the secondary in opposition to the flux field of the primary.
• The CEMF is reduced and more current is drawn from the source, with less flux surrounding the primary.
• More lines of flux are generated by additional current in the primary, re-establishing numbers of total flux lines.

Turns and Current Ratios

• The quantity of flux developed is proportional to the magnetizing force.
• The magnetomotive force produced by 1A of current is defined as the ampere-turn.
• Since the same flux is found by the primary and secondary windings, the ampere-turns are the same.
• In terms of voltage, less turns steps down voltage, as well as steps up the current, as the inverse ratio.

Primary and Secondary Winding Power

• If voltage is doubled in the secondary, current is halved. Conversely, voltage is doubled in the primary, halved at the secondary, allowing all power transmitted to the source for the transformer load.
• In a turn ratio of 20:1, if the primary has an input of 0.1A at 300 volts, the primary power is 30W, and due to all is delivered, the secondary has 30W as well.