Transformer Electric Machine Principles and Operation

Transformer Electric Machine Fundamentals

Transformers are static machines that transmit energy through an alternating magnetic field. They transfer energy from a system with a specific tension (voltage) to another system with a different tension. Depending on their purpose, they are classified as power transformers, communication transformers, or measurement transformers.

Classification Criteria for Transformers

There are several other criteria used to categorize these machines:

• Voltage adjustment: Step-up (elevators) or step-down (reductors) transformers.
• Phase: Single-phase or three-phase systems.
• Environment: Indoor or outdoor (weathering) installation.
• Cooling: Natural or forced refrigeration, using either a dry system or an oil bath.

Single-Phase Transformer Constitution

A single-phase transformer is primarily constituted by a ferromagnetic core and two windings (primary and secondary).

The Ferromagnetic Core

The core provides the magnetic coupling and permits the circulation of magnetic flux between the primary and secondary windings. It is constructed from silicon steel plates, each electrically insulated to reduce magnetic hysteresis losses and eddy currents.

The Windings (Coils)

These are circular coils made of copper wire which generate the magnetic flux through the core. They can be arranged in various configurations, such as symmetric, alternating, concentric, or battleship-cooling designs.

Principle of Operation

In an ideal single-phase transformer, there are no hysteresis losses, no magnetic flux leakage, and the resistance of the windings is zero.

Real Single-Phase Transformers Under No-Load

In a real transformer, we must consider various losses, including magnetic hysteresis, eddy currents, flux dispersion (φd), and the resistance of the primary (R1) and secondary (R2) windings. These losses imply that the no-load current (Io) is not out of phase by exactly π/2, but by an angle (fi)o.

• Active component: Ip = Io · cos(fi)o
• Reactive component: Im = Io · sin(fi)o
• Iron Loss (PFE): U1 · Ip

Reduction of the Secondary to the Primary

To prevent disproportionate vector diagrams, engineers use the method called reduction of the secondary to the primary.

Under this method:

• Primary vectors: These remain unchanged.
• Secondary tension vectors: Multiplied by the transformation ratio.
• Secondary intensity vectors: Divided by the transformation ratio.
• Impedances: Multiplied by the square of the ratio.
• Powers and Angles: These do not vary.
• Rotation: Secondary vectors are usually rotated 180º.

Losses and Performance in the Transformer

We distinguish between two primary types of losses:

1. Copper Losses (PCU): Losses occurring within the windings.
2. Iron Losses (PFE): These occur in the ferromagnetic core as a result of hysteresis and eddy currents; these are usually constant.