Registered Electrical Engineering Licensure Practice Exam

Reversing a three-phase motor requires two contactors that control the motor's connections to the power supply. Specifically, one contactor is used to connect the motor to power in one direction, while the second contactor connects the motor to power in the opposite direction.

To achieve this, each contactor is typically set up to switch two of the three phases, reversing the phase sequence supplied to the motor. For instance, if the phases are labeled A, B, and C, one contactor would connect A, B, and C in one configuration, while the other would switch to connect A, C, and B, effectively reversing the motor's rotation.

In addition to the two contactors, it is common practice to include an auxiliary contactor for additional safety and control functions, such as ensuring that both contactors cannot be engaged at the same time, which could result in a short circuit.

Thus, to adequately reverse a three-phase motor while maintaining safety operations, the requirement typically is four contactors: two for motor direction control and two auxiliary contactors for ensuring safe operation. This brings the total to four, making the given answer correct in identifying the number of contactors needed.