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When D.O.L. (Direct On Line) starting a 3-phase squirrel-cage motor a high inrush current will be inevitable. That current can reach 10 to 12 times In (nominal current). That means; when a motor runs on nominal speed with a current of 5 A, the inrush current can reach 50 to 60 A. This is especially inconvenient for motors with higher power. These motors will have a very high startup current which could lead to power dips in the power distribution. To counteract that inrush current, several things can be done. Nowadays a VFD (Variable Frequency Drive) or softstarter can be used. Before VFD's or softstarters were available, Y/Δ circuits were used. For smaller motors a Y/Δ is very expensive and the inrush current is low enough for the power distribution to handle. Only when having a small motor with high load, the speeding up took longer and so did the inrush current. This could damage the contactor by overheating the contacts. As mentioned earlier, a Y/Δ circuit is too expensive for a small motor. The solution; a bridging contactor could be used. When starting the motor, 2 contactors are switched parallel so the inrush current would devide over the contactors. Once started, 1 contacter will be deactivated by either a manual switch or timer relay. For example: the motor mentioned earlier, can have a inrush current of 60 A. That 60 A would, in case of a D.O.L. starter, run through 1 contactor. The contacts may one day weld together due to heating up by the fairly long inrush current. When having a bridging contactor, 30 A will run through both contactors, reducing the heating of the contacts.
In the simulated circuit, the left diagram represents the D.O.L. starter and the right diagram represents the bridging contactor starter. Just close the switches of the control circuits and watch how the inrush currents behave. The bridging contactor will automatically be deactivated.
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