What happens to the current when an inductor is suddenly disconnected?

When an inductor is suddenly disconnected, the current undergoes significant changes due to the inductor's characteristic of maintaining a constant current. Here is a detailed explanation:

1. Basic Characteristics of an Inductor

The basic characteristic of an inductor can be expressed by the following formula:

V=L（dI/dt）

where:

• V is the voltage across the inductor,

• L is the inductance of the inductor,

• I is the current through the inductor,

• dI/dt is the rate of change of the current.

This formula indicates that the voltage across the inductor is proportional to the rate of change of the current. If the current changes rapidly, a high voltage will be generated across the inductor.

2. When an Inductor is Suddenly Disconnected

When an inductor is suddenly disconnected, the current cannot immediately drop to zero because the inductor resists sudden changes in current. Specifically:

Current Cannot Change Instantly

Reason: The inductor stores magnetic field energy, and when the current tries to stop abruptly, the inductor attempts to maintain the original current.

Result: The inductor generates a high transient voltage at the point of disconnection to try to keep the current flowing.

Transient Voltage Spike

Voltage Spike: Due to the inability of the current to change instantly, the inductor produces a high transient voltage at the point of disconnection. This voltage spike can be extremely high and may damage other components in the circuit.

Energy Release: This high voltage causes the stored magnetic field energy in the inductor to be released rapidly, often in the form of an arc.

3. Practical Effects

Arc Discharge

• Arcing: At the point of disconnection, the high voltage may cause an arc discharge, leading to sparks or arcs.

• Damage: Arcing can damage switches, contacts, or other circuit components.

Voltage Spike

Protective Measures: To prevent damage from voltage spikes, a diode (known as a flyback diode or freewheeling diode) is often placed in parallel with the inductor, or other forms of transient voltage suppressors (such as varistors) are used.

4. Solutions

Flyback Diode

• Function: A flyback diode provides a low-impedance path for the current when the inductor is suddenly disconnected, preventing the generation of high voltage spikes.

• Connection: The flyback diode is typically connected in reverse parallel with the inductor. When the inductor is disconnected, the diode conducts, providing a path for the current to continue flowing.

Transient Voltage Suppressor

• Function: A transient voltage suppressor (such as a varistor) rapidly clamps the voltage when it exceeds a certain threshold, absorbing excess voltage energy and protecting other components in the circuit.

• Connection: The transient voltage suppressor is typically connected in parallel with the inductor.

Summary

When an inductor is suddenly disconnected, the current cannot immediately drop to zero due to the inductor's characteristic of maintaining a constant current. This results in a high transient voltage at the point of disconnection, which can cause arcing and damage circuit components. To protect the circuit, a flyback diode or a transient voltage suppressor is often used to prevent the generation of voltage spikes.