First of all, we need to make clear a concept. The biggest difference between the main loop of DC contactor and AC contactor is not its load flow, but how to kill the arc?

The arc extinguishing of AC contactor is easier than that of DC contactor because of the existence of zero crossing automatic arc extinguishing of AC current.

The Uh in the figure is the arc generated after the contactor contact is broken. The volt-ampere characteristic curve of the arc is shown in the figure above. The two points in the curve are arc stable combustion points. Why?

According to Kirchhoff’s KVL law:

Uh = E-RIh-L (dIh/dt), (Equation 1)

As can be seen from Equation 1, if the arc current does not change, then dIh/dt=0, and the reverse electromotive force generated by the inductor is also zero, then:

Uh= E – RIh, (Equation 2)

From Equation 2, we can draw the DC load line of the arc. See the EK line in the figure. When arc current Ih=0, Uh=E; And when Uh=0, Ih=E/R. It can be seen that the resistance R is closely related to the slope of the DC load line.

Therefore, we try to increase the resistance R in the main circuit when the DC line is off, so as to reduce the arc voltage, so as to realize the DC circuit arc extinguishing. According to the arc volt-ampere characteristic curve, the K point of the horizontal coordinate moves to K point. But this method has nothing to do with contactors.

When we look at the arc volt-ampere characteristic curve, we notice that there is a dotted line curve above the curve, which increases the arc length and makes the arc volt-ampere characteristic curve be raised away from the DC load line, so as to realize arc extinguishing.

As we know, AC contactor has three poles, while DC circuit only needs one or two poles. Therefore, we can connect the three poles of the AC contactor in series to increase the arc length, so as to realize arc extinguishing.

Note the matching relationship SCPD between the contactor and the front circuit breaker when the short circuit current is turned off. If contact bonding or fusion welding does not occur to the contactor after the circuit breaker is opened, the mating relationship between the two is TYPE2, that is, the two-type mating relationship. Generally, the selection of contactors should be selected according to two kinds of matching relations.

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