Difference between DC contactor and AC contactor?


With a contactor, you want to break the current safely and get rid of the arc that forms as quickly as possible. Since an arc occurs due to potential difference, the arc breaks itself in an alternating current (AC) application when the voltage crosses zero. In the case of direct current (DC), the current is constantly above zero and is therefore more difficult to break.

In the switching sequence, you want to increase the voltage in the arc to a higher voltage than that of the

the supply voltage

. This eventually eliminates the arc and stops conducting current. There are four ways to increase the voltage of the arc:

  1. Divide the arc into several smaller arcs.
  2. Extend the arc.
  3. Narrow the arc.
  4. Reduce electrical conductivity (cool the arc).

The most common way to achieve this is by using

magnetic blasting


An AC contactor is usually designed to break all three phases simultaneously, so there are three connection poles. However, since it is easier to break the alternating current, the design does not need to maximize the above-mentioned factors to increase the voltage in the arc. This means that the design of an AC contactor is simpler but takes up a lot of space.

One way to develop a DC contactor is to take an oversized AC contactor and build the poles so that it is single-pole but breaks the same pole three times. This is a technique that works, but the design is not optimal as the contactor becomes large and wears out quickly if it breaks under load.

A robust DC contactor is usually designed to break one pole and optimized for that. But since breaking AC is more difficult, all four factors need to be maximized to increase the voltage. This is done here with a permanent magnet and an open arc chamber that has ceramic parts to split the arc. In addition, a design to pull it out and make it longer and narrower, as well as cooling it.

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