Seeking help regarding motor phase loss

If one phase of the winding is open-circuited, during startup the magnetic field generated by that winding can be divided into two rotating magnetic fields of equal magnitude but opposite directions; as a result, the torque produced by the rotor is also of equal magnitude but opposite direction. In other words, the torque is zero, and therefore starting is not possible

The motor at the site makes a lot of noise, and there’s an imbalance in the current. No way – can’t they even detect a missing phase?

This is definitely not a problem related to a missing phase in the motor. Additionally, the phrase “no matter how small the output is” is unclear, and it’s also not known what type of pump you have. Try to answer your question by guessing: a centrifugal water pump uses an outlet valve to control the load; the greater the opening of the valve, the higher the current, with the lowest value when the valve is completely closed. But note that over time, the pump body will heat up. Check whether the valve head of the outlet valve has fallen off ? ? And others? ? ? For reference

Another question: One netizen replied that I don’t understand what it means to have a missing phase, so I’m asking. It seems that as long as there is a missing phase, the pump will stop working

If there is a phase loss in the power supply while the motor is running, it will continue to rotate, but the current will increase, the motor will heat up, and noise may be heard from the motor. Regarding the motor you mentioned, it depends on the specific conditions under rated current; a detailed analysis is needed. You can measure the voltage at the motor terminals to determine whether there is a phase loss

Thank you for your explanation, Haiyou. I’m a bit confused now: if a motor lacks a phase, it shouldn’t be able to start. But how can it continue to operate if a phase is missing while it’s already running? Won’t it skip directly?

The motor is in operation when the power supply loses a phase; the motor will continue to rotate, but the current increases, causing the motor to overheat and generate noise. It is necessary to check whether the associated current protection device triggers

That’s indeed the case; the description is exactly the same. Afterwards, the current drawn by the backup pump remains high, though the noise level is not high

1. Excess current is not necessarily due to a missing phase. Low or high voltage, improper motor selection, reduced motor load capacity, mechanical wear, and other factors can all cause motor overcurrent. 2. If one phase of the motor is missing, the three-phase rotating magnetic field becomes a magnetic field that generates forward torque and a magnetic field that generates reverse torque; in this case, the motor operates in a pulsating state. This is manifested as a continuous \"buzzing\" sound from the motor. 2. The reasons for motor phase loss can be divided into the power supply side and the motor itself. Common issues on the power supply side include poor switch contacts, disconnected power lines, use of fuses for protection, and transformer failures ； Regarding the motor, issues include loose screws on the motor’s wiring box, poor welding at the internal connections of the motor, and broken wires in the motor windings.