Analysis and treatment of three-phase asynchronous motor winding faults

Analysis and treatment of three-phase asynchronous motor winding faults

Windings are components of motors. Aging, moisture, heat, corrosion, foreign matter intrusion, and external force impact can cause damage to windings. Motor overload, undervoltage, overvoltage, and phase-loss operation can also cause winding failures. Winding failures are generally divided into winding grounding, short circuit, open circuit, and wiring errors. Now let's explain the fault phenomenon, cause, and inspection method.
1. Winding grounding
Refers to the grounding caused by the insulation damage between the winding and the body or the casing.
1. Fault phenomenon
The casing is energized, the control circuit is out of control, and the winding short circuits and heat up, causing the motor to fail to operate normally.
2. Causes
The insulation resistance of the winding decreases due to moisture; the motor is overloaded for a long time; harmful gas corrosion; metal foreign matter intrudes into the winding and damages the insulation; insulation damage touches the core when rewinding the stator winding; the end of the winding touches the end cover and the base; the friction between the stator and the rotor causes insulation burns; the insulation of the lead wire is damaged and touches the casing; overvoltage (such as lightning strike) causes insulation breakdown.
3. Inspection method
(1) Observation method. Visually inspect the winding ends and the insulation in the wire slots to see if there are any damage or burnt marks. If there are any, it is the grounding point.
(2) Multimeter inspection method. Use the multimeter low resistance setting to check. If the reading is very small, it is grounded.
(3) Megaohmmeter method. Use different megohmmeters according to different levels to measure the insulation resistance of each group of resistors. If the reading is zero, it means that the winding is grounded. However, for motor insulation that is damp or broken down due to an accident, it is necessary to judge based on experience. Generally speaking, when the pointer fluctuates at "0", it can be considered to have a certain resistance value.
(4) Test lamp method. If the test lamp is on, it means that the winding is grounded. If sparks or smoke are found somewhere, it is the winding grounding fault point. If the light is slightly bright, the insulation is grounded. If the light is not on, but sparks appear when the test rod is grounded, it means that the winding has not been broken down, but is seriously damp. You can also use hardwood to tap the edge of the shell. When the light turns off and on at a certain point, it means that the current is on and off. This means that the current is the grounding point.
(5) Current burning method. Use a voltage-regulating transformer. After connecting the power supply, the grounding point will quickly heat up. The place where the insulation smokes is the grounding point. Special attention should be paid to the fact that the current of small motors should not exceed twice the rated current for no more than half a minute; for large motors, the current should be 20%-50% of the rated current or gradually increase the current. When the grounding point just smokes, turn off the power immediately.
(6) Group elimination method. For the grounding point inside the core and the burning is more severe, the burned copper wire is melted with the core. The method used is to divide the grounded phase winding into two halves, and so on, and finally find the grounding point.
In addition, there are high-voltage test methods, magnetic needle exploration methods, power frequency vibration methods, etc., which are not introduced here one by one.
4. Treatment methods
(1) If the winding is damp and grounded, it should be dried first. When it cools to about 60-70℃, it should be poured with insulating paint and then dried.
(2) When the insulation of the winding end is damaged, re-insulate the grounding point, paint it, and then dry it.
(3) When the winding grounding point 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.
2. Winding short circuit
Due to the insulation damage caused by excessive motor current, excessive power supply voltage fluctuations, single-phase operation, mechanical damage, poor manufacturing, etc., it is divided into winding turn short circuit, winding short circuit, winding pole short circuit and winding phase short circuit.
1. Fault phenomenon
The magnetic field of the ions is unevenly distributed, and the three-phase current is unbalanced, which intensifies 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 heat up rapidly and burn out.
2. Causes
The motor is overloaded for a long time, causing the insulation to age and lose its insulation function; insulation damage during wire embedding; the winding is damp, causing the insulation resistance to drop and causing insulation breakdown; the insulation material at the end and between the layers is not properly padded or damaged during shaping; the insulation of the end connecting wire is damaged; overvoltage or lightning strike causes insulation breakdown; the rotor and stator winding ends rub against each other, causing insulation damage; metal foreign matter falls into the motor and there is too much oil.
3. Inspection method
(1) External observation method. Observe whether the terminal box and the winding end are burnt. The winding will leave a dark brown color and smell after overheating.
(2) Temperature detection method. Run the motor without load for 20 minutes (stop immediately if abnormality is found), and use the back of your hand to touch the various parts of the winding to see if the temperature exceeds the normal temperature.
(3) Power-on test method. Use an ammeter to measure. If the current of a phase is too large, it means that there is a short circuit in that phase.
(4) Bridge inspection. Measure the DC resistance of each winding. Generally, the difference should not exceed 5%. If it exceeds, the phase with smaller resistance has a short circuit fault.
(5) Short-circuit detector method. If the winding under test has a short circuit, the steel sheet will vibrate.
(6) Multimeter or megohmmeter method. Measure the insulation resistance between any two phases of the winding. If the reading is extremely small or zero, it means that there is a short circuit between the two phases of the winding.
(7) Voltage drop method. After the three windings are connected in series, a low-voltage safe AC is passed through. The group with the smaller reading is found to have a short circuit fault.
(8) Current method. When the motor is running at no load, first measure the three-phase current, then measure and compare the two phases. If it does not change with the power supply replacement, the phase winding with the larger current has a short circuit.
4. Short circuit treatment method
(1) The short circuit point is at the end. The short circuit point can be separated by insulating material, or the insulating wire can be re-wrapped, painted and dried again.
(2) The short circuit is in the wire slot. After softening it, find the short circuit point and repair it. After putting it back into the wire slot, paint and dry it again.
(3) For each phase winding with less than 1/12 short-circuit turns, cut off all short-circuit lines when the number of turns is connected in series, connect the conductive parts 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, all of them must be removed and rewound.
3. Winding short circuit
Poor welding or the use of corrosive flux, which is not cleaned after welding, may cause pot welding or loosening; coil short circuit, short circuit and grounding faults may also cause the wire to burn when subjected to mechanical stress or collision. When one or more of the wires are short-circuited in the several wires that are burned in parallel, the temperature of the other wires rises due to the increase in current, causing the winding to heat up and break. Generally, it is divided into one-phase winding end break, inter-turn short circuit, parallel branch break, multiple wires in the middle of a break, and rotor cage break.
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.
2. Causes
(1) Breakage or manufacturing quality problems during inspection and maintenance.
(2) Poor welding of winding components, pole (phase) groups, and winding and lead wire terminals, overheating and desoldering during long-term operation.
(3) Windings are damaged or broken by mechanical and electromagnetic forces.
(4) Short circuits between turns or between phases and grounding can cause serious burning or melting of the winding.
3. Inspection methods
(1) Observation method. Most of the breakpoints occur at the ends of the windings. Check whether there is any folding or desoldering at the joints.
(2) Multimeter method. Using the resistance range, connect one meter rod to the center point of the "Y" shape for the "Y" type connection, and the other to the beginning of the three-phase winding in turn. The infinite phase is the breakpoint; after the "△" type connection is short-circuited, measure each set of windings separately. The infinite phase is the circuit breakpoint.
(3) Test lamp method. The method is the same as before. The phase that does not light up is the circuit breakpoint.
(4) Megaohmmeter method. The phase whose resistance tends to infinity (i.e. not zero) is the circuit breakpoint.
(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 winding with the smaller current has a partial short circuit fault.
(6) Bridge method. When the resistance of one phase of the motor is greater than that of the other two phases, it indicates that the phase winding has a partial circuit breakage fault;
(7) Current balance method. For the "Y" type connection, the three-phase winding can be connected in parallel and low-voltage and high-current AC can be passed. If the current difference in the three-phase winding is greater than 10%, the end with the smaller current is a circuit breaker; for the "△" type connection, first disconnect one of the contacts of the stator winding, and then pass low-voltage and high current phase by phase, and the phase with the smaller current is a circuit breaker.
(8) Broken cage detector inspection method. During the inspection, if the rotor cage is broken, the reading of the millivoltmeter should decrease.
4. Circuit breaker treatment method
(1) When the circuit is broken at the end, connect it and weld it firmly, wrap it with insulating material, put on an insulating tube, tie it up, and then dry it.
(2) If the winding is severely burned due to inter-turn, inter-phase short circuit and grounding, it should generally be replaced with a new winding.
(3) For the break point in the slot, emergency treatment is carried out for a small number of break points. The break point is found by group elimination method, and the broken part of the winding is connected and used after the insulation is qualified.
(4) For the broken cage of the cage rotor, welding, cold connection or strip replacement can be used to repair it.
IV. Wrong connection of windings
Wrong connection of windings causes incomplete rotating magnetic field, resulting in symptoms such as difficulty in starting, unbalanced three-phase current, and loud noise. In severe cases, if not handled in time, the winding will burn out. There are mainly the following situations: one or more coils in a certain pole phase are embedded in reverse or the head and tail are connected incorrectly; the pole (phase) group is connected in reverse; a certain phase winding is connected in reverse; multiple parallel winding branches are connected incorrectly; "△" and "Y" connection methods are wrong.
1. Fault phenomenon
The motor cannot start, the no-load current is too large or the imbalance is too large, the temperature rise is too fast or there is severe vibration and loud noise, the fuse is burned out, etc.
2. Causes
Mistakenly connecting the "△" type to the "Y" type; one phase of the three-phase winding is connected in reverse during maintenance; the tap position is not selected properly or the internal wiring is wrong during decompression starting; the winding is connected incorrectly when the new motor is offline; the old motor is judged incorrectly.
3. Maintenance method
(1) Ball method. If the ball rotates and rolls along the inner circumference of the stator, it means it is correct, otherwise the winding is connected incorrectly.
(2) Compass method. If the winding is not connected incorrectly, in a single-phase winding, when the compass passes through adjacent pole (phase) groups, the polarity should be opposite, and the adjacent pole (phase) groups of different phases in the three-phase winding are also opposite; 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 the voltmeter has no indication in both measurements, or has a reading once and no reading once, it means that the winding is connected in reverse.
(4) Other common methods include dry cell method, milliammeter residual magnetism method, motor direction method, etc.
4. Treatment methods
(1) If a coil or coil group is connected in reverse, the no-load current will be greatly unbalanced and should be sent to the factory for repair.
(2) If the lead wire is wrong, the head and tail should be correctly judged and reconnected.
(3) If the reduced pressure starter is connected incorrectly, it should be checked against the wiring diagram or schematic diagram and reconnected carefully.
(4) If the wiring is wrong after the new motor is offline or the new winding is reconnected, it should be sent to the factory for repair.
(5) When one phase of the stator winding is connected in reverse, the current of the reversed phase is particularly large. The fault can be found and repaired based on this feature.
(6) If the "Y" type is connected as a "△" type or the number of turns is insufficient, the no-load current will be large and should be corrected in time.