Study Of On-Line Compensation Unit Effect In Suppressing Contact Rebounds

High repulsive force value between contact members arising mostly at contact closure and/or in transient under short circuit conditions is a real hazard for power contact switch. It results in fast contact welding for vacuum circuit breakers of a low voltage. It is related not only to higher value of the fault currents but also to much smaller dimensions of vacuum chambers for the low voltage applications, different contact unit parameters and required shape of the contact members as well. The ring shaped contacts which are necessary to force a rotating arc under influence of a radial component of a magnetic field make existence of a contact bouncing much more real. The rebounds are found also in vacuum contactors equipped with flat contacts in a case of misalignment of the mating contact surfaces. To overcome this problem usually an increased contact force value is applied in practice. It is not a right way since it results in increased dimensions and increased energy consumption of a switch driving system, and it makes the contact vibrating system much more unstable under closure.In the paper a new structure of the developed vacuum circuit breaker with the on-line dynamic contact force compensation by means of the contact current is presented. The on-line compensation system is composed of two especially shaped, movable, current conducting plates which are fastened to the electromagnetic drive and the vacuum chamber respectively. They displace each other under operation with isolation clearance. The principle of the compensator is discussed, which is proved by experiments. As a result, the contact bounces under closure and/or under inrush current in transients are efficiently suppressed when the compensator as well as switch structure parameters are selected properly and the dynamic resistance of the closed contact is found to be even decreased. Due to the reverse threshold of the compensator under...