We have a motor driving a vertical pump. The problem is there is no check valve at the discharge, so whenever the pump is stopped, due to the back flow the shaft rotated for about 40 seconds. The excitation to the motor is provided the power factor capacitor. The Power factor of the motor feeder is almost unity PF 0.98~0.99 when the the PF improvement capacitor is connected. The motor is started through reactor starter which has a bypass latched type VC. I also posted about this earlier in this forum
In order to over come this problem, we had put a separate Vacuum contactor for disconnecting capacitors from the circuit whenever the motor is stopped. Now the capacitors have started to fail. In one starter, all the three capacitors have failed and in another starter one of the capacitors got failed. We feel that this is due to switching transients of the capacitor VC. We have no other non linear loads. Could resonance between the motor and the capacitor still damage the capacitor as it is almost unity pf?