Transformer Technology Overview

Double Wound Core Type Transformer

  • Definition: A Double wound core type transformer features multiple windings; specifically, a primary winding and a secondary winding.

    • Primary Winding: The section around which the supply is wound.

    • Secondary Winding: The section that outputs the induced electricity.

Principles of Operation

  • Electromotive Force (EMF) Induction:

    • A changing electric current in the primary coil generates a changing magnetic field, which in turn induces a current in the secondary coil.

    • This is one of two methods to induce EMF:

    1. Changing current in a coil creating magnetic fields.

    2. Relative motion of a current-carrying conductor within a magnetic field.

Turns and Induced EMF

  • The transformer operation is described by the Turns Ratio which affects the induced EMF:

    • Formula for Turns Ratio:

    • If $N_p$ = Number of turns in primary winding,

    • $N_s$ = Number of turns in secondary winding,

    • The turns ratio (TR) is given by:
      TR = rac{Ns}{Np}

Changing Magnetic Flux

  • AC Voltage on Primary Winding:

    • When AC voltage is applied , it initiates:

    1. A changing magnetic flux in the iron core.

    2. An induced EMF in the secondary winding

  • This interaction is essential for the transformer to function effectively, represented as: Vp/Vs = Np/Ns = Is/Ip

  • where Vp and Vs are the primary and secondary voltages, Np and Ns are the number of turns in the primary and secondary coils, and Ip and Is are the primary and secondary currents, respectively.

Efficiency of Transformers

  • Assuming 100% efficiency, the relationship between power input and output is:

    • Power input = Power output - meaning the power in the primary winding is equal to the power in the secondary winding.

  • Current Considerations:

    • Ip = Primary current,

    • Is = Secondary current,

    • Note: The current in the secondary winding is at the top of the equation to avoid confusion.

Eddy Currents and Their Effects

  • Laminated Cores:

    • Core transformers utilize laminated (layered) cores instead of solid metal to minimize Eddy currents.

    • Eddy currents cause resistive losses which can transform energy into heat, thereby reducing efficiency (Joule heating).

    • Laminating the core restricts Eddy currents and improves overall efficiency.

  • Material Composition:

    • Core Material: Soft iron is commonly used as a ferrous material to enhance mutual inductance.

    • Winding Conductors: Insulated, low-resistance conductors are used to prevent short circuits and reduce copper losses.

Back EMF vs. Eddy Currents

  • Back EMF:

    • Not to be confused with Eddy currents, back EMF is defined as:

    • The induced voltage in the primary circuit that opposes the applied voltage due to self-induction.

Types of Transformers

Step-up Transformer

  • Characteristic: Output voltage is greater than input voltage.

  • Configuration: More turns on secondary winding than on the primary.

  • Example:

    • Input: 120V AC, Current: 10A,

    • Output: 240V AC, Current: 5A.

    • Turns: Primary 900 turns, Secondary 1800 turns with a turns ratio of 1:2.

Step-down Transformer

  • Characteristic: Output voltage is lower than input voltage.

  • Configuration: Fewer turns on secondary winding than on the primary.

  • Example:

    • Input: 240V AC, Current: 5A,

    • Output: 120V AC, Current: 10A.

    • Turns: Primary 1800 turns, Secondary 900 turns.

Additional Transformer Types

  • Power Transformers:

    • Commonly used in power distribution, found in:

    1. Step-up transformers from power stations (25 kV–400 kV).

    2. Step-down transformers in substations (132 kV to 11 kV and 11 kV to 400 V).

    3. One-to-one transformers for safety devices, like shaver sockets.

    • Insulated laminations help minimize eddy currents.

  • Current Transformers:

    • Used for accurate current metering, such as Class X CTs.

    • Important to ensure proper coupling for accurate readings; must be connected on the secondary side to prevent large voltage discharges when open circuit.

  • Isolation Transformers:

    • Intended for safety and separation from primary supply, available for both step-up and step-down uses.

    • Commonly used in plug sockets, with additional insulation to minimize shock hazards.

  • Voltage Transformers:

    • Comprised of two windings on a common core used for voltage regulation.

  • Earthing Transformers:

    • Used to provide a neutral point for delta-connected power transformers, suitable for various earthing systems.

Types of Transformer Cores

  • Core Type Transformer:

    • Channels magnetic flux through one path, resulting in potential leakage flux at corners.

  • Shell Type Transformer:

    • More efficient due to two paths for magnetic flux circulation, therefore minimizing losses.