USCG Exam PracticeElectrical measuring devices and instrumentation
Before connecting a replacement motor to a three-phase bus, a technician uses a phase-sequence indicator. The primary purpose of this check is to:
- A. Verify that the supply frequency matches the motor's nameplate rating
- B. Confirm the insulation resistance of the motor windings is acceptable
- ✓ Ensure the phase rotation of the supply matches the motor's designed rotation directionCorrect
- D. Measure the line-to-line voltage to confirm it is within the motor's voltage tolerance
Why C is correct
A phase-sequence indicator confirms the order in which the three phases reach their peak values. Connecting a three-phase motor with the wrong phase sequence will cause it to rotate in the reverse direction, which can damage driven equipment. Frequency, insulation, and voltage are verified by other instruments.