Last month we began our discussion of three-phase systems by covering the basics. We talked about the difference between single-phase and three-phase, how three-phase offers a more efficient means of getting a large motor spinning and therefore most commonly used in large motor applications. We discussed the role of transformers and balancing in three-phase systems. This month we turn our attention to the interface between incoming three-phase power and the pump - the pump control panel.
A pump control panel has four major components which provide four basic functions in a three-phase system. First, the disconnect is the means of disconnecting the pump and its control equipment from incoming power. Second, the contactor switches the pump on and off as directed by a control device (pressure switch, float switch, timer, etc.) Third, fuses or circuit breaker are the primary protection for short circuit electrical faults, and fourth, overload devices protect equipment from being damaged by overload faults. Figure 1 shows a full voltage start pump control panel. Figure 2 is a schematic for the same panel.