Reasons and solutions for short circuit faults in motor windings

Reasons and solutions for short circuit faults in motor windings

Winding short circuits are caused by insulation damage due to excessive motor current, large fluctuations in power supply voltage, single-phase operation, mechanical damage, poor manufacturing, etc. They can be divided into winding turn short circuits, winding pole short circuits, and winding phase short circuits.

Fault phenomenon

The uneven distribution of ion magnetic field and the imbalance of three-phase current intensify the vibration and noise of the motor during operation. In severe cases, the motor cannot start, and a large short-circuit current is generated in the short-circuit coil, causing the coil to quickly heat up and burn out.

cause

Long term overload of the motor causes insulation aging and loss of insulation function; Insulation damage caused by wire embedding; Moisture in the winding causes a decrease in insulation resistance, resulting in insulation breakdown; End and interlayer insulation materials are not properly padded or damaged during shaping; The insulation of the end connecting wire is damaged; Insulation breakdown caused by overvoltage or lightning strike; Friction between the rotor and stator winding ends causes insulation damage; Metal foreign objects falling into the interior of the motor and excessive oil contamination.

Inspection Method

(1) External observation method. Observe whether the junction box and winding ends are burnt, and whether the winding remains dark brown and has a foul odor after overheating.

(2) Temperature detection method. Run without load for 20 minutes (stop immediately if any abnormalities are found), and use the back of your hand to touch whether each part of the winding exceeds the normal temperature.

(3) Electrical experiment method. Measure with an ammeter. If the current in a certain phase is too high, it indicates that there is a short circuit in that phase.

(4) Bridge inspection. Measure the DC resistance of each winding, and the difference should generally not exceed 5%. If it exceeds, the phase with lower resistance may have a short circuit fault.

(5) Short circuit reconnaissance method. If there is a short circuit in the tested winding, the steel plate will vibrate.

(6) Multi meter or megohmmeter method. Measure the insulation resistance between any two phases of the winding. If the reading is extremely small or zero, it indicates that there is a short circuit between the two phases of the winding.

(7) Voltage drop method. After connecting the three windings in series and applying low-voltage safe AC power, it was found that the group with the smallest reading had a short circuit fault.

(8) Current method. When the motor is running without load, first measure the three-phase current, and then measure and compare the two phases by swapping. If it does not change with the power supply swap, the winding of the phase with higher current will have a short circuit.

processing method

(1) The short-circuit point is at the end. Insulation materials can be used to separate short-circuit points, or insulated wires can be re wrapped, painted, and re dried.

(2) Short circuit in the online slot. After softening it, find the short circuit point and repair it. Put it back into the cable tray and then paint and dry it.

(3) For each phase winding with less than 1/12 of the short circuit turns, all short circuits should be cut off when the number of turns is connected in series, and the conductive parts should be connected to form a closed circuit for emergency use.

(4) When the number of turns at the short-circuit point of the winding exceeds 1/12, it should be completely removed and rewound.