The moment a high-voltage circuit breaker opens, an arc composed of metal vapor-reaching temperatures of over ten thousand degrees-forms between the contacts. If this arc goes uncontrolled, it can burn deep pits into the contact surfaces or even weld them shut. The strength of axial magnetic field (AMF) contacts lies in their ability to force the arc to disperse evenly rather than concentrating in one spot.
Making the Arc "Spread Out" Instead of "Clustering"
An axial magnetic field is applied along the axis of the contacts. Charged particles within the arc move in helical paths, distributing energy evenly across the entire contact face. Instead of remaining anchored to a single point, the arc root moves rapidly across the contact surface. In contrast, transverse magnetic field (TMF) contacts rely on the Lorentz force to drive the arc in a circular motion; while this generates significant movement, the arc remains relatively concentrated, making it difficult to mitigate the risk of anode spots. AMF contacts take a different approach: rather than prioritizing arc speed, they ensure the arc remains spread out, thereby minimizing ablation.
The Higher the Voltage, the More Advantageous AMF Becomes
High-voltage vacuum interrupters require large contact gaps and must withstand high recovery voltages. Large gaps often weaken the magnetic field's control over the arc. AMF contacts possess an inherent advantage here: the number of coil turns can be increased, and the slot angles of cup-shaped contacts can be adjusted to compensate for magnetic field attenuation-adjustments that are structurally difficult to achieve with TMF contacts. However, AMF contacts are not without drawbacks; the conductive path is longer, resulting in higher closing resistance and potentially impacting current-carrying capacity. The engineering solution is a dual-circuit design: current flows through the main conductive rod during closing, and is switched to the AMF coil only at the moment of opening to extinguish the arc-effectively separating the two functions.
While AMF contacts are not without limitations, in the context of high-voltage interruption, the balance they strike between arc control precision and contact longevity makes them a more advantageous choice than TMF contacts.
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