Transformer Interview Questions and Answers

This article provides a list of questions and answers related to transformers, useful for job interviews for transformer-related positions and for engineering students during vivas.

Question 1: What is a transformer and how does it work?

Answer: A transformer is an electrical device that transfers electrical energy between two or more circuits through electromagnetic induction. It consists of two or more coils of wire called windings, usually wound around a core made of ferromagnetic material. When an alternating current (AC) passes through one winding (the primary), it creates a changing magnetic field in the core, which induces a voltage in the other winding(s) (the secondary).

Question 2: What are the types of transformers based on construction?

Answer: Transformers can be classified based on construction into two main types:

• Core type transformers: In core type transformers, the windings surround a considerable part of the core.
• Shell type transformers: In shell type transformers, the windings surround the core like a shell.

Question 3: What are the applications of transformers?

Answer: Transformers are used in various applications including power distribution, voltage regulation, impedance matching, isolation, and signal coupling in electrical and electronic circuits. They are essential components in power generation plants, substations, industrial machinery, and electronic devices.

Question 4: What is the significance of the turns ratio in a transformer?

Answer: The turns ratio of a transformer is the ratio of the number of turns in the primary winding to the number of turns in the secondary winding. It determines the voltage transformation ratio between the primary and secondary sides of the transformer. For an ideal transformer, the turns ratio is directly proportional to the voltage ratio and inversely proportional to the current ratio.

Mathematically, if $N_p$ is the number of turns in the primary coil, and $N_s$ is the number of turns in the secondary coil, then the turns ratio, $a$, is given by:

$a = \frac{N_p}{N_s}$

Question 5: Explain the concept of transformer efficiency.

Answer: Transformer efficiency is a measure of how effectively a transformer converts electrical power from the primary side to the secondary side. It is defined as the ratio of output power to input power, expressed as a percentage. Transformer efficiency accounts for losses such as copper losses (due to resistance in the windings) and core losses (due to hysteresis and eddy currents).

The efficiency, $\eta$, can be calculated as:

$\eta = \frac{P_{out}}{P_{in}} \times 100\%$

Where $P_{out}$ is the output power and $P_{in}$ is the input power. This can also be written as:

$\eta = \frac{P_{in} - P_{losses}}{P_{in}} \times 100\%$

Where $P_{losses}$ represents the total losses in the transformer (copper losses + core losses).

Question 6: What are the different types of losses in transformers?

Answer: There are primarily two types of losses in transformers:

• Copper losses (also known as $I^2R$ losses): These occur due to the resistance of the windings, resulting in power dissipation as heat. The power loss in a winding with current $I$ and resistance $R$ is given by $I^2R$.
• Core losses: These consist of:
  • Hysteresis losses: Caused by the magnetic properties of the core material.
  • Eddy current losses: Caused by circulating currents induced in the core.

Question 7: How are transformer ratings specified?

Answer: Transformer ratings are specified based on several parameters including voltage rating (primary and secondary), current rating (primary and secondary), power rating (in VA or kVA), frequency, and impedance. These ratings indicate the maximum allowable values for voltage, current, and power that the transformer can handle safely and efficiently.

Question 8: What is impedance matching in transformers?

Answer: Impedance matching is the process of matching the impedance of the load to the impedance of the source to maximize power transfer. In transformers, impedance matching is achieved by selecting appropriate turns ratios and winding configurations to match the load impedance to the source impedance, thereby minimizing voltage reflections and maximizing efficiency.

If $Z_p$ is the impedance seen from the primary side and $Z_s$ is the impedance of the load connected to the secondary side, and $a$ is the turns ratio, then:

$Z_p = a^2 Z_s$

For maximum power transfer, $Z_p$ should be the complex conjugate of the source impedance.

Question 9: What is saturation in transformers and how does it affect performance?

Answer: Saturation occurs when the magnetic flux in the core of a transformer reaches its maximum limit, causing a disproportionate increase in magnetizing current and a decrease in inductance. Saturation can lead to distortion of the output waveform, increased core losses, and reduced efficiency. It is important to operate transformers within their linear region to avoid saturation.

Question 10: What safety precautions should be followed while working with transformers?

Answer: When working with transformers, it is essential to follow safety precautions such as:

• Ensuring proper grounding of the transformer.
• Avoiding overloading and short-circuiting the transformer.
• Using appropriate personal protective equipment (PPE) when handling high-voltage equipment.
• Following manufacturer’s instructions and safety guidelines.
• Regularly inspecting and maintaining transformers to prevent accidents and ensure reliable operation.