Winding grounding
Refers to the grounding caused by insulation damage between winding and iron core or bushing.
1, failure phenomenon
The shell is charged, the control circuit is out of control, and the winding is short-circuited and heated, which leads to the motor not operating normally.
2. the reason
The winding is damp, which reduces the insulation resistance; Long-term overload operation of motor; Harmful gas corrosion; Metal foreign bodies invade the winding and damage the insulation; When rewinding the stator winding, the insulation is damaged and touches the iron core; The winding end contacts the base of the end cover; Friction between stator and rotor leads to insulation burn; Lead insulation damage and shell collision; Overvoltage (such as lightning strike) leads to insulation breakdown.
3. Inspection method
(1) observation method. Visually inspect the insulation of winding ends and slots, and observe whether there are any signs of damage and blackening, and if there are any, it is the grounding point.
(2) Multimeter inspection method. Check with a low-resistance multimeter. If the reading is small, it will be grounded.
(3) Megger method. According to different grades, choose different megohmmeters 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 accidentally broken down, it needs to be judged according to experience. Generally speaking, when the pointer swings irregularly at "0", it can be considered as having a certain resistance value.
(4) Test lamp 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 grounding fault point. If the light is dimmed, there is a ground fault in the insulation. If the light is off, but sparks appear when the test bar is grounded, it means that the winding has not yet broken down, but it is seriously wet. Hardwood can also be struck at the edge of the shell, hitting a place, indicating that the current is off, and that place is the grounding point.
(5) Current burn-through method. When a voltage regulating transformer is connected to the power supply, the grounding point will heat up rapidly, and the place where insulation smokes is the grounding point. It is especially important to note that the small motor should not exceed twice the rated current for less than half a minute; The large motor is 20%-50% of the rated current or gradually increase the current, and the power will be cut off as soon as the grounding point smokes.
(6) Grouping elimination method. For the grounding point in the core wire and serious combustion, the burning copper wire fuses with the core wire. The method adopted 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 and power frequency vibration method. I won't introduce it here.
4. Processing method
(1) If the winding is grounded due to moisture, it should be dried first. When it is cooled to about 60-70℃, it should be dried after being poured with insulating paint.
(2) When the winding end insulation is damaged, re-insulate it on the ground, paint it, and then dry it.
(3) When the winding grounding point is in the slot, the winding should be rewound or some winding elements should be replaced.
Finally, different megohmmeters are used for measurement to meet the technical requirements.
Winding short circuit
Insulation damage is caused by excessive motor current, excessive power supply voltage change, single-phase operation, mechanical collision, poor manufacturing and other reasons, including turn-to-turn short circuit, winding short circuit, inter-pole short circuit, winding phase-to-phase short circuit and so on.
1. Fault phenomenon
The uneven distribution of ion magnetic field and unbalanced three-phase current aggravate the vibration and noise of the motor during operation. In severe cases, the motor can't be started, but a large short-circuit current is generated in the short-circuit coil, which leads to the rapid heating and burning of the coil.
2. the reason
The motor is overloaded for a long time, which makes the insulation aging lose its insulation function; Insulation damage caused by wire embedding; The winding is damp, which reduces the insulation resistance and leads to insulation breakdown; During the molding process, the insulating materials at the ends and between layers are not filled or damaged correctly; The insulation of the end connection wire is damaged; Insulation breakdown caused by overvoltage or lightning strike; Friction between rotor and stator winding ends leads to insulation damage; Foreign metal objects fall into the motor, resulting in excessive oil pollution.
3. Inspection method
(1) external observation method. Observe whether the junction box and winding end are burnt, and the winding still has dark brown and odor after overheating.
(2) Temperature detection method. Run at no-load for 20 minutes (stop immediately if any abnormality is found), and touch with the back of your hand whether all parts of the winding exceed the normal temperature.
(3) charged experimental method. When measuring with ammeter, if a certain phase current is too large, it means that there is a short circuit in this phase.
(4) Bridge inspection. When measuring the DC resistance of each winding, the general difference should not exceed 5%. If it exceeds 5%, the phase with small resistance has a short circuit fault.
(5) Short circuit detector method. If the winding is short-circuited, the steel plate will vibrate.
(6) Multimeter or megger method. Measure the interphase insulation resistance of any two-phase winding. If the reading is very small or zero, it means that there is a short circuit between the phases of the two-phase winding.
(7) Pressure drop method. After the three windings are connected in series, low-voltage safe alternating current is applied, and the group with small reading is found to have short circuit fault.
(8) Current methods. When the motor is running at no load, measure the three-phase current first, and then measure the two-phase comparison when it is changed. If it does not change with the change of power supply, the single-phase winding with large current is short-circuited.
4. Short circuit treatment method
(1) The short circuit point is at the end. The short-circuit points can be separated by insulating materials, or the insulated wires can be repackaged, repainted and dried again.
(2) The short circuit is in the wire slot. After softening, find out the short circuit point for repair, put it back in the tank, and then paint it to dry.
(3) For each phase winding with short-circuit turns less than112, when the turns are connected in series, the short circuit is completely cut off, and the conductive parts are connected to form a closed loop for emergency use.
(4) When the number of turns at the winding short-circuit point exceeds112, all windings shall be dismantled and rewound.
Winding open circuit
Failure to clean after welding may cause pot welding or looseness due to poor welding or corrosive flux; The short circuit, short circuit and grounding fault of the coil under mechanical stress or collision will also burn out the conductor. When one or several wires are short-circuited, due to the increase of current, the temperature of other wires rises, causing the winding to heat up and open. Generally, it can be divided into single-phase winding end break, turn-to-turn short circuit, parallel branch open circuit, multiple wires burned out by one-time open circuit, and rotor broken cage.
1. Fault phenomenon
The motor cannot be started, the three-phase current is unbalanced, there is abnormal noise or vibration, the temperature rise exceeds the allowable value or smoke.
2. the reason
(1) Damage or quality problems during overhaul and maintenance.
(2) The winding elements, pole (phase) groups, windings and leads are poorly welded, and they are overheated and desoldering after long-term operation.
(3) The winding is damaged or broken by mechanical force and electromagnetic force.
(4) Turn-to-turn or interphase short circuit and grounding lead to serious burning or fusing of windings.
3. Inspection method
(1) observation method. Breakpoints often occur at the end of winding, depending on whether there is bump or fracture, and whether there is desoldering of joint.
(2) Multimeter method. Using a resistor block, one rice wire is connected to the center point of Y-shaped connection, the other one is connected to the head end of three-phase winding in turn, and the infinite phase is the breakpoint; After the "△" wiring is short-circuited, measure each group of windings separately, and infinity is the disconnection point.
(3) Test lamp method. The method is the same as before, and the unlit phase is an open circuit.
(4) Megger method. The phase where the resistance tends to infinity (i.e. non-zero value) is the breaking point.
(5) ammeter method. When the motor is running, measure the three-phase current with an ammeter. If the three-phase current is unbalanced and there is no short circuit phenomenon, the winding with smaller current has some short circuit faults.
(6) Bridging method. When the resistance of one phase of the motor is greater than that of the other two phases, it indicates that the winding of this phase has a local open circuit fault;
(7) Flow balance method. For "Y" connection, three-phase windings can be connected in parallel to introduce low-voltage and high-current alternating current. If the current difference in the three-phase winding is greater than 10%, the low current end is open. For "△" connection, first disconnect a contact of stator winding, and then connect low voltage and high current in turn, in which the phase with low current is an open circuit.
(8) Inspection method of broken cage detector. During the inspection, if the rotor damages the rotor cage, the reading of the millivoltmeter should be reduced.
4. Open circuit treatment method
(1) When the terminal is disconnected, it should be welded firmly after wiring, covered with insulating material, covered with insulating tube, bound and dried.
(2) If the winding is severely burned due to turn-to-turn and interphase short circuit and grounding, a new winding should be replaced.
(3) Emergency treatment is carried out for a few breakpoints with breakpoints in the slot, and the breakpoints are found by grouping exclusion method, which will be used after the winding is disconnected and connected with insulation.
(4) The broken cage of cage rotor can be repaired by welding, cold joining or changing bars.
Wrong winding connection
Wrong winding connection leads to incomplete rotating magnetic field, which leads to difficulties in starting, unbalanced three-phase current, loud noise and other symptoms. In severe cases, if not handled in time, the winding will be burned out. There are mainly the following situations: one or more coils of a certain pole phase are embedded reversely or the head and tail are wrongly connected; Pole (phase) group inversion; Inverted winding;
Multi-path parallel winding branches are wrongly connected; The connection between "△" and "Y" is incorrect.
1, failure phenomenon
The motor can't start, the no-load current is too large or unbalanced, the temperature rises too fast or vibrates violently, the noise is loud, and the fuse is blown.
2. the reason
Misconnect the "△" type to the "Y" type; During maintenance, one phase of the three-phase winding is connected end to end; Decompression start-up is improper tap position selection or internal wiring error; When the new motor goes offline, the winding wiring is wrong; The old motor's judgment is wrong.
3. Maintenance methods
(1) ball method.
If the ball rotates and rolls along the inner circumferential surface of the stator, it is correct, otherwise the winding is wrongly connected.
(2) Compass method. If the windings are not wrongly connected, when the compass passes through adjacent pole (phase) groups in one-phase winding, the polarities should be opposite, and the adjacent pole (phase) groups in different phases in three-phase winding should also be opposite; If the polarity direction remains the same, it means that one pole (phase) is connected backwards; If the direction is uncertain, there are reverse coils in the phase group.
(3) Multimeter voltage method. According to the wiring diagram, if the voltmeter does not indicate twice, or there is one reading and one reading, it means that the winding is connected backwards.
(4) Dry battery method, milliammeter remanence method and motor steering method are also common.
4. Processing method
(1) If a coil or coil group is connected reversely and the no-load current is unbalanced, it should be repaired in the factory.
(2) If the outgoing line is wrong, it should be reconnected after the head and tail are judged correctly.
(3) If the decompression starts incorrectly, carefully check the wiring diagram or schematic diagram and reconnect.
(4) If the new motor is offline or reconnected with new windings, it should be sent to the factory for repair.
(5) When the stator winding is connected reversely, the reverse one-phase current is particularly large, so the fault can be found and repaired according to this characteristic.
(6) Connect "Y" into "△" or the number of turns is not enough, and the no-load current is large, so it should be corrected in time.