How can you detect phase loss electrically at the motor terminals?

How can you detect phase loss electrically at the motor terminals?

• A. Visual: Motor runs rough, vibrates excessively, overheats
• B. Electrical: Measure voltage between all three phases at motor terminals
• C. The current in each phase will be balanced
• D. One phase will have voltage but no current draw

Answer: (D)

When a three-phase motor loses a phase, the windings no longer form a balanced set. The supply may still put voltage on all three lines, but the open phase has no current path, so no current flows through that winding. Electrically detecting this at the motor terminals is done by checking both voltage and current in each phase: you’ll find voltage present on the open line but essentially zero current in that same line, while the other two phases carry higher, unbalanced currents. In a healthy motor, currents in all three phases are roughly equal, so a phase with voltage but no current is a clear sign of phase loss. Measuring only voltages between phases or assuming balanced currents won’t reliably reveal the open phase, and visual cues alone aren’t definitive.

When a three-phase motor loses a phase, the windings no longer form a balanced set. The supply may still put voltage on all three lines, but the open phase has no current path, so no current flows through that winding. Electrically detecting this at the motor terminals is done by checking both voltage and current in each phase: you’ll find voltage present on the open line but essentially zero current in that same line, while the other two phases carry higher, unbalanced currents. In a healthy motor, currents in all three phases are roughly equal, so a phase with voltage but no current is a clear sign of phase loss. Measuring only voltages between phases or assuming balanced currents won’t reliably reveal the open phase, and visual cues alone aren’t definitive.