A fused combination motor starter controls a 3-phase motor. Line side voltage is 480 volt. There is a control transformer to reduce the voltage to 120 for the controls. The starter is controlled by a start/stop station. The start button is pushed, and the motor runs slow and then trips the C overload. What would be a possible reason for this fault?

A fused combination motor starter controls a 3-phase motor. Line side voltage is 480 volt. There is a control transformer to reduce the voltage to 120 for the controls. The starter is controlled by a start/stop station. The start button is pushed, and the motor runs slow and then trips the C overload. What would be a possible reason for this fault?

• A. Control transformer miswired
• B. Phase C overload is bad
• C. Motor starter coil is bad
• D. Phase B fuse is bad

Answer: (D)

The situation tests what happens when one of the three supply phases is lost in a 3‑phase motor circuit. If a fuse in one line (phase B) is bad, that phase is open and the motor is fed only from the remaining two phases (A and C). A two‑phase supply can still run the motor, but with much less torque and a tendency to draw abnormal current in the energized phases. That unbalanced condition causes overheating in the remaining phase circuits, so the overload protection associated with that phase can trip. In this case, the motor runs slowly and the phase C overload trips, which fits with an open phase B diverting current and causing overload on phase C. Other possibilities don’t align as well with the symptoms: miswiring the control transformer would mainly affect the control circuit rather than the motor’s line currents; a bad motor starter coil would typically prevent the motor from energizing properly; and a defective phase C overload would not specifically point to a line fault causing the motor to run slow before tripping. The most consistent root cause is an open B supply due to a bad fuse.

The situation tests what happens when one of the three supply phases is lost in a 3‑phase motor circuit. If a fuse in one line (phase B) is bad, that phase is open and the motor is fed only from the remaining two phases (A and C). A two‑phase supply can still run the motor, but with much less torque and a tendency to draw abnormal current in the energized phases. That unbalanced condition causes overheating in the remaining phase circuits, so the overload protection associated with that phase can trip. In this case, the motor runs slowly and the phase C overload trips, which fits with an open phase B diverting current and causing overload on phase C.

Other possibilities don’t align as well with the symptoms: miswiring the control transformer would mainly affect the control circuit rather than the motor’s line currents; a bad motor starter coil would typically prevent the motor from energizing properly; and a defective phase C overload would not specifically point to a line fault causing the motor to run slow before tripping. The most consistent root cause is an open B supply due to a bad fuse.