In a motor start/stop circuit, which of the following would be a plausible root cause if the motor runs slowly and trips the overload after pressing the start button?

In a motor start/stop circuit, which of the following would be a plausible root cause if the motor runs slowly and trips the overload after pressing the start button?

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

Answer: (A)

When a motor starts but then runs slowly and the overload trips after you press start, the control circuit voltage is the likely limiting factor. The control transformer is supposed to provide the correct low voltage to energize the starter coil and pull the contactor in solidly. If the transformer is miswired, the coil may receive the wrong voltage (or be undervolted), so the contactor doesn’t pull in fully. That means the motor doesn’t get full line voltage, so it develops only a small amount of torque, runs sluggishly, and draws more current than normal. The overload relay then trips because of the prolonged high current at a reduced speed. Check by measuring the control voltage at the coil and verifying the transformer's wiring against the schematic to ensure proper secondary voltage and correct connections to the coil and supply. If the voltage is correct and the wiring is proper, other issues like a missing phase or a faulty coil would be investigated next, but they tend to produce different symptoms, such as no start or immediate abnormal current patterns.

When a motor starts but then runs slowly and the overload trips after you press start, the control circuit voltage is the likely limiting factor. The control transformer is supposed to provide the correct low voltage to energize the starter coil and pull the contactor in solidly. If the transformer is miswired, the coil may receive the wrong voltage (or be undervolted), so the contactor doesn’t pull in fully. That means the motor doesn’t get full line voltage, so it develops only a small amount of torque, runs sluggishly, and draws more current than normal. The overload relay then trips because of the prolonged high current at a reduced speed.

Check by measuring the control voltage at the coil and verifying the transformer's wiring against the schematic to ensure proper secondary voltage and correct connections to the coil and supply. If the voltage is correct and the wiring is proper, other issues like a missing phase or a faulty coil would be investigated next, but they tend to produce different symptoms, such as no start or immediate abnormal current patterns.