Transformer

Construction and Working Principle of a Transformer

Definition:

A transformer is a static electromagnetic device that transfers electric power from one circuit to another without a change of frequency, raising or lowering voltage with a corresponding change in current, ensuring efficient power distribution and utilization.

Components:

• Core:

  • Typically made from laminated silicon steel to minimize eddy current losses.

  • The design minimizes magnetic hysteresis, enhancing efficiency.

• Windings:

  • Consist of two windings that are separated electrically but magnetically linked.

  • They are insulated from each other and the core to prevent electrical contact:

    • Primary Winding: Connected to the AC supply, it receives electrical energy, generating an alternating magnetic field.

    • Secondary Winding: Connected to the load, it allows the induced EMF to deliver power to the output circuit.

Types of Transformers:

• Step-Up Transformer: Increases voltage (V2 > V1). It is commonly used in high voltage transmission to reduce losses over long distances.

• Step-Down Transformer: Decreases voltage (V2 < V1). It is used in electronic devices and power supplies to lower the voltage to a safe level.

Working Principle: Electromagnetic Induction:

• When AC passes through the primary winding, it generates an alternating magnetic flux in the core. This flux induces an EMF in the secondary winding as per Faraday's law:

  EMF Induced,e = -N * (dΦ/dt);where N is the number of turns in the winding and Φ is the magnetic flux. If the secondary circuit is closed, a current flows and electrical energy is effectively transferred.

EMF Equation of a Transformer:

• For sinusoidal waveforms, the RMS value of induced EMF per turn can be calculated as follows:

  E = 4.44 * f * Φm * N;

  where:

  • Φm: maximum flux in webers (Wb)

  • f: frequency in hertz (Hz)

  • N: number of turns in the winding

• This equation illustrates that the induced EMF is directly proportional to the frequency and maximum flux and the number of turns. It means that to achieve a higher induced EMF, either the frequency of the alternating current can be increased, or the number of turns in the winding can be enhanced, or the maximum magnetic flux can be increased through better core materials or design.

Transformer on No-Load Condition:

• When the secondary terminals are open, the transformer operates with no load.

• No-Load Current Components:

  • Magnetizing Component: Sets up the magnetic flux and lags the supply voltage by 90°.

  • Working/Energy Component: This component overcomes iron losses, ensuring that a portion of the energy is consistently returned for magnetic stabilization.

Transformer on Load:

• Loading occurs when the secondary terminals connect to a load, which results in a current (I2) flowing through the circuit.

• Current Relationships:

  • I1 = I0 + I_load, where I_load sets up its own magnetomotive force (MMF) against the main magnetic flux, contributing to efficiency and power transfer considerations.

Efficiency and Conditions for Maximum Efficiency:

• Efficiency (η): Defined as the ratio of output power to input power, expressed as:

  η = Output Power / (Input Power + Total Losses);

• Maximum Efficiency Condition: This condition occurs when copper losses (I²R losses in the windings) equal iron losses (hysteresis and eddy current losses in the core).

Direct Load Test Procedure:

• Configuration: Connect the AC supply, and measure input/output voltages and currents at various load increments.

• Calculating Efficiency and Regulation:

  • Efficiency: Calculated as Efficiency = Output Power/Input Power;

  • Voltage Regulation: Determined by the formula:

  Voltage Regulation = (No-load Voltage - Full-load Voltage)/No-load Voltage * 100;indicating how much the voltage drops under load conditions.

Indirect Test Methods (O.C. and S.C. Tests):

• Open Circuit Test:

  • Purpose: Used to determine iron losses (hysteresis losses and eddy current losses) and the no-load current of the transformer.

  • Procedure: Under this test, normal voltage is applied to the primary winding while the secondary winding is left open. The power input measured in this condition represents only the iron losses since no load current flows through the secondary.

  • Measurement: Voltage and current are measured to find the no-load losses. The wattmeter reading, in addition to measuring voltage and current on the primary side, allows for the calculation of losses due to magnetizing current and iron losses, expressing them as a percentage of the rated power.

• Short Circuit Test:

  • Purpose: Conducted to determine full-load copper losses and to calculate the equivalent impedance of the transformer.

  • Procedure: In this test, the secondary winding is shorted, and a reduced voltage is applied to the primary winding until the rated current flows in the primary. This condition simulates the transformer under full load conditions, where losses due to winding resistance (I²R losses) are observed.

  • Measurement: Power is measured using a wattmeter connected to the primary side. The total power consumed during this test indicates copper losses, allowing for the assessment of transformer performance under maximum load conditions. The equivalent circuit parameters can also be derived from the voltage and current measurements taken during the test.

Parallel Operation of Single Phase Transformers:

• Need: Additional transformers may be required for overloads or as backup. Unfortunately, operational efficiency can be compromised due to incompatibility.

• Conditions for Successful Parallel Operation:

  • Same voltage ratio.

  • Equal impedances.

  • Proper polarity alignment required to ensure balanced load sharing and safety.

Auto-Transformers:

• Difference: They use a variable voltage principle utilizing a single continuous winding rather than separate primary and secondary windings.

• Applications: Widely used for starting induction motors, as variacs for voltage regulation, boosting voltages for particular applications, and integrating diverse power systems for consistency and optimization.