Permanent Magnet Fast-Switching Vacuum Circuit Breakers: Why Is Demagnetization the Core Challenge? - Blog

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Permanent Magnet Rapid-Interrupting Vacuum Circuit Breaker

Permanent magnet fast-switching vacuum circuit breakers typically use a bistable permanent magnet operating mechanism, in which permanent magnets provide the holding force for both the closing and opening positions, and the coil only needs short pulse excitation to change the magnetic circuit state. This structure has few parts and fast response, but once the permanent magnet undergoes irreversible demagnetization, the holding force decreases, directly threatening switching reliability.

Causes of the demagnetization challenge

This magnetisation poses a significant challenge for three main reasons.

The first is the effect of the opposing magnetic field. When the coil is switched on and off, the magnetic field generated by the coil current is always opposite to that of the permanent magnet.Fast switching requires short-time high-current excitation, making the reverse magnetic field stronger. If the magnetic circuit is poorly designed and the local operating point of the permanent magnets lies below the "kink" in the demagnetisation curve, irreversible demagnetisation occurs.

Secondly, the superposition effect at high temperatures.When the airflow is obstructed, the direct current is interrupted, causing heat to build up at the contact points and on the electronic components, thereby raising the temperature of the housing. As the temperature rises, the magnetic hardness of the NdFeB permanent magnets gradually decreases; furthermore, at high temperatures, their ability to resist reverse magnetic fields becomes very weak, leading to increased mechanical power loss.

Thirdly, the consequences are serious. After demagnetization, the bistable holding force becomes insufficient, which may cause the closing position to fail to hold, incomplete opening, or even failure to operate or maloperation.

How to Solve the Problem

To address the problem of demagnetisation, it is therefore necessary to implement measures relating to materials, magnetic circuits and thermal management: select permanent magnet materials with high coercivity and high resistance to high temperatures; optimise the magnetic circuit so that the opposing magnetic field does not exceed a safe range; limit the current and activation time of the coils; and improve heat dissipation and temperature monitoring. Only by controlling the risk of demagnetisation can permanent magnet fast vacuum circuit breakers achieve both speed and reliability.