why do some oil filled coils take a resistor

by:Haiyan     2023-06-20

Why Do Some Oil-Filled Coils Take a Resistor?


Oil-filled coils are essential components in electrical power systems, providing the necessary magnetic field to induce voltage in transformers and other devices. While oil-filled coils are generally efficient and reliable, some variations require the presence of a resistor for optimal functioning. In this article, we will explore the reasons behind the use of resistors in certain oil-filled coils and delve into their significance in electrical applications.

Understanding Oil-Filled Coils:

1. The basics of oil-filled coils:

Oil-filled coils, also known as oil-immersed coils, consist of copper or aluminum windings insulated with enamel coatings. These windings are immersed in transformer oil, which not only provides electrical insulation but also aids in heat dissipation. The insulation and cooling properties of the oil contribute to the coil's overall performance and longevity.

2. Achieving optimal performance:

To ensure the efficient operation of oil-filled coils, designers consider several factors, such as the coil's inductance, impedance, and thermal characteristics. Additionally, they must account for potential issues like excessive harmonic distortion, voltage fluctuations, and over- or undercurrent conditions. To address these concerns, resistors can be incorporated into the coil design.

Importance of Resistors in Oil-Filled Coils:

3. Managing inrush current:

One key reason for using resistors in oil-filled coils is to manage inrush current during the initial energization of the coil or transformer. When an electrical power system is turned on, a sudden rush of current can occur due to the energization of capacitors and the saturation of magnetic circuits. This inrush current, if left uncontrolled, can damage the electrical system.

To mitigate the effects of inrush current, resistors are connected in series with the coil. By limiting the current magnitude, the resistors protect the coil from thermal and mechanical stress that can lead to premature failure. Once the coil or transformer is fully energized, the resistors can be bypassed through the use of switches or relays.

4. Damping transient oscillations:

Another crucial role of resistors in oil-filled coils is to dampen transient oscillations. When sudden changes occur in the system, such as during switching operations or fault conditions, transient oscillations may be induced. These oscillations can generate high-frequency currents and voltages, potentially causing damage to the coil windings.

By introducing resistors, the energy of these transient oscillations can be dissipated, preventing voltage spikes that could harm the insulation or trigger electrical breakdown. This protects the coil and associated equipment, ensuring smooth power transmission and stable operation.

5. Limiting harmonic distortion:

Harmonic distortion refers to the presence of additional frequency components in the system that are multiples of the fundamental frequency. Such distortions can lead to undesirable effects like increased losses, overheating, and electromagnetic interference. Resistors can play a crucial role in limiting harmonic distortion within oil-filled coils.

By carefully selecting and connecting resistors in the circuit, harmonic currents can be attenuated or canceled out, ensuring that the electrical system operates within acceptable limits. This results in improved power quality and helps prevent damage to sensitive equipment connected to the coil.


In conclusion, resistors are utilized in certain oil-filled coils to manage inrush current, dampen transient oscillations, and limit harmonic distortion, among other benefits. When engineers design electrical power systems, they consider the unique characteristics of oil-filled coils and incorporate resistors strategically to optimize their performance and protect the equipment in which they are employed. By understanding the reasons behind the usage of resistors in oil-filled coils, engineers can ensure safe and efficient operation of electrical systems that rely on these essential components.

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