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What are the common faults of motor windings?

Date:2023-08-24   Author:XINDA MOTOR
The winding is an integral part of the motor. Aging, moisture, heat, corrosion, foreign matter intrusion, and external impact will all cause damage to the winding. Motor overload, under-voltage, over-voltage, and lack of phase operation can also cause winding failure. Winding faults are generally divided into winding grounding, short circuit, open circuit, and wiring error.

Fault phenomena, causes and inspection methods of motor windings.

1. Winding grounding

Refers to the grounding caused by the insulation damage between the winding and the iron core or the casing.

1. Fault phenomenon

The casing is electrified, the control circuit is out of control, and the winding is short-circuited to generate heat, resulting in the failure of the motor to operate normally.

2. Causes

The insulation resistance of the winding is damp; the motor is overloaded for a long time; harmful gas is corroded; metal foreign matter invades the inside of the winding and damages the insulation; when the stator winding is rewinded, the insulation is damaged and touches the iron core; the end of the winding touches the end cover frame; ; The insulation damage of the lead-out line collides with the shell; overvoltage (such as lightning strike) causes insulation breakdown.

3. Inspection method

(1) Observation method

Visually inspect the winding end and the insulator in the slot to see if there is any damage or scorched traces, if any, it is the grounding point.

(2) Multimeter inspection method

Check with the low-resistance range of the multimeter, if the reading is small, it is grounded.

(3) Megger method

According to different grades, different megohmmeters are used to measure the insulation resistance of each group of resistors. If the reading is zero, it means that the winding is grounded. However, if the motor insulation is damp or broken down due to an accident, it needs to be judged based on experience. Generally speaking When the pointer swings at "0", it can be considered to have a certain resistance value.

(4) Light test method

If the test light is on, it means that the winding is grounded. If sparks or smoke are found somewhere, it is the winding ground fault point. If the light is dimly lit, the insulation has a grounding breakdown. If the light is not on, but sparks also appear when the test rod is grounded, it means that the winding has not yet broken down, but is seriously damp. You can also use hardwood to tap lightly on the edge of the seam of the shell. When you tap a certain place and wait for it to turn off and turn on, it means that the current is on and off, and this place is the grounding point.

(5) Current burning method

With a voltage regulating transformer, after the power supply is connected, the grounding point will heat up quickly, and the place where the insulating material smokes is the grounding point. Special attention should be paid to the fact that the small motor should not exceed twice the rated current, and the time should not exceed half a minute; the large motor should be 20%-50% of the rated current or gradually increase the current, and immediately cut off the power when the grounding point just smokes.

(6) Group elimination method

For the grounding point inside the iron core and the burning is severe, the burnt copper wire is fused with the iron core. The method used is to divide the grounded one-phase winding into two halves, and so on, and finally find out the grounding point.

In addition, there are high voltage test method, magnetic needle exploration method, power frequency vibration method, etc., which will not be introduced here.

4. Processing method

(1) If the winding is damp and grounded, it should be dried first. When it is cooled to about 60-70°C, it should be poured with insulating paint and then dried.

(2) When the insulation at the end of the winding is damaged, re-insulate at the grounding point, paint it, and then dry it.

(3) When the grounding point of the winding is in the slot, the winding should be rewound or some winding components should be replaced.

Finally, different megohmmeters should be used for measurement to meet the technical requirements.

Second, the winding short circuit

Due to excessive motor current, excessive fluctuations in power supply voltage, single-phase operation, mechanical bruises, poor manufacturing, etc., insulation damage is caused, and the sub-windings are short-circuited between turns, short-circuited between windings, short-circuited between poles and phase-to-phase.

1. Fault phenomenon

The uneven distribution of the magnetic field of the ions and the unbalanced three-phase current make the motor vibrate and noise aggravate when it is running. In severe cases, the motor cannot start, and a large short-circuit current is generated in the short-circuit coil, causing the coil to heat up rapidly and burn out.

2. Cause

The motor is overloaded for a long time, so that the insulation aging loses its insulation effect; the insulation is damaged when the wire is embedded; the insulation resistance of the winding is damp and the insulation resistance is reduced, resulting in insulation breakdown; the end and interlayer insulation materials are not properly matted or damaged when they are shaped; the end connection wire insulation is damaged ; Overvoltage or lightning strikes cause insulation breakdown; rotor and stator winding ends rub against each other to cause insulation damage; metal foreign objects fall into the interior of the motor and there is too much oil.

3. Inspection method

(1) External observation method. Observe whether the junction box and the end of the winding are burnt. After the winding is overheated, it will leave dark brown and smell bad.

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

(3) Electricity test method. Use an ammeter to measure, if the current of a certain phase is too large, it means that there is a short circuit in this phase.

(4) Bridge inspection. Measure the DC resistance of each winding, generally the difference should not exceed 5%, if it exceeds, the phase with the smaller resistance has a short circuit fault.

(5) Short-circuit detector method. If there is a short circuit in the winding under test, the steel sheet will vibrate.

(6) Multimeter or megohmmeter method. Measure the insulation resistance between any two-phase windings. If the reading is extremely small or zero, it means that there is a short circuit between the two-phase windings.

(7) Voltage drop method. After the three windings are connected in series, the low-voltage safe alternating current is connected, and it is measured that the group with a small reading has 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. If it does not change with the power supply, there is a short circuit in the one-phase winding with the larger current.

4. Short circuit treatment method

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

(2) Short circuit in the wire slot. After softening it, find out the short-circuit point and repair it, put it back into the trunking, and then paint and dry it.

(3) For each phase winding with short-circuit turns less than 1/12, cut off all the short-circuit lines when connecting the turns in series, and connect the conductive parts to form a closed loop for emergency use.

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

3. Winding open circuit

Due to poor welding or the use of corrosive flux, if it is not cleaned after welding, it may cause pot welding or loosening; when the coil is subjected to mechanical stress or collision, short circuit, short circuit and ground fault can also cause the wire to burn. When one or several wires in the wires are short-circuited, the temperature of the other wires will rise due to the increase of current, causing the winding to heat up and break the circuit. Generally, it can be divided into broken wire at the end of one phase winding, short circuit between turns, open circuit at parallel branch, multiple wires and one broken circuit burned, and rotor cage broken.

1. Fault phenomenon

The motor cannot start, the three-phase current is unbalanced, there is abnormal noise or vibration, the temperature rise exceeds the allowable value or smoke is emitted.

2. Cause

(1) Breakage or manufacturing quality problems during inspection and maintenance.

(2) The components of the winding, pole (phase) groups, and the terminals of the winding and lead wires are poorly welded, and they are overheated and desoldered after long-term operation.

(3) The winding is damaged or broken by mechanical force and electromagnetic field force.

(4) Turn-to-turn or phase-to-phase short circuit and grounding cause severe burning or fusing of the winding.

3. Inspection method

(1) Observation method. Most of the breakpoints occur at the ends of the windings, and check whether there is any bump and whether the joint is desoldered.

(2) Multimeter method. Using the resistance file, connect one meter stick to the center point of the "Y" shape for the "Y" connection method, and connect the other to the first end of the three-phase winding in turn, and the infinite one phase is the breakpoint; " After the short-open connection of the △" type connection method, measure each group of windings separately, and the one with infinity is the break point.

(3) Light test method. The method is the same as before, and the phase that does not turn on is an open circuit.

(4) Megger method. A phase whose resistance value tends to infinity (that is, not zero) is the break point.

(5) Ammeter method. When the motor is running, use an ammeter to measure the three-phase current. If the three-phase current is unbalanced and there is no short circuit, the one-phase winding with a smaller current has a partial short-circuit fault.

(6) Bridge method. When the resistance of one phase of the motor is larger than that of the other two phases, it means that the phase winding has a partial open circuit fault;

(7) Current balance method. For the "Y" type connection method, the three-phase windings can be connected in parallel, and the alternating current of low voltage and high current can be connected. If the current difference in the three-phase windings is greater than 10%, the end with the smaller current is open circuit; for "△" For the type connection method, first disassemble a contact of the stator winding, and then pass in low voltage and high current phase by phase, and the phase with small current is open circuit.

(8) Inspection method of broken cage detector. When checking, if the rotor cage is broken, the reading of the millivolt meter should decrease.

4. Disconnection processing method

(1) When the open circuit is at the end, weld it firmly after connection, wrap it with insulating material, put it on an insulating tube, tie it up, and then dry it.

(2) If the winding is seriously burnt due to inter-turn, inter-phase short circuit and grounding, etc., it should generally be replaced with a new winding.

(3) For those with a small number of breakpoints in the slot, do emergency treatment, use the group elimination method to find out the breakpoints, and use them after the winding broken parts are connected and the insulation is qualified.

(4) The broken cage of the cage rotor can be repaired by welding, cold jointing or bar replacement.

Wrong connection of windings will cause incomplete rotating magnetic field, resulting in difficulty in starting, unbalanced three-phase current, loud noise and other symptoms. In serious cases, the windings will be burned out if not dealt with in time.

Main conditions: One or several coils in a certain pole phase are embedded reversely or the head and tail are connected incorrectly; the pole (phase) group is connected reversely; the winding of a certain phase is connected reversely; "Wrong connection.

1. Fault phenomenon

The motor cannot start, the no-load current is too large or the imbalance is too large, the temperature rises too fast or there is violent vibration and noise, and the fuse is blown.

2. Causes

The "△" type is mistakenly connected to the "Y" type; during maintenance, one phase of the three-phase winding is reversed; the decompression start is due to improper selection of the tap position or wrong internal wiring; when the new motor is offline, the winding connection is wrong; The judgment of the motor coming out is wrong.

3. Overhaul method

(1) Rolling ball method.

If the ball rotates and rolls along the surface of the inner circumference of the stator, it is correct, otherwise the winding may be wrongly connected.

(2) Compass method.

If the windings are not wrongly connected, in a one-phase winding, when the compass passes through adjacent pole (phase) groups, the polarity indicated should be opposite, and in three-phase windings, adjacent pole (phase) groups of different phases are On the contrary; if the polarity direction does not change, it means that one pole (phase) group is reversed; if the direction is uncertain, there is a reversed coil in the phase group.

(3) Multimeter voltage method.

According to the wiring diagram, if there is no indication on the voltmeter during the two measurements, or there is a reading once and no reading once, it means that the winding is reversed.

(4) The common ones are dry battery method, mA meter residual magnetism method, motor steering method, etc.

4. Processing method

(1) If one coil or coil group is reversed, the no-load current will have a large imbalance, and it should be returned to the factory for repair.

(2) If the lead-out line is wrong, it should be reconnected after correctly judging the beginning and end.

(3) If the decompression start is wrongly connected, it should be carefully checked and re-wired according to the wiring diagram or schematic diagram.

(4) If the wiring is wrong after the new motor is offline or reconnected with a new winding, it should be sent to the factory for repair.

(5) When one phase of the stator winding is reversed, the current of the reversed phase is particularly large, and the fault can be found and repaired according to this characteristic.

(6) If the "Y" type is connected to "△" type or the number of turns is not enough, the no-load current will be large, and it should be corrected in time.