Principle of three-phase asynchronous motor
When symmetrical three-phase alternating current is applied to the three-phase stator winding, the rotating magnetic field rotates clockwise along the inner circular space of the stator and rotor at the synchronous speed n 1. Because the rotating magnetic field rotates at the speed of n 1, the rotor conductor is stationary at first, so the rotor conductor will cut the stator rotating magnetic field and produce induced electromotive force (the direction of induced electromotive force is determined by the right-hand rule). Because both ends of the conductor are short-circuited by short-circuit rings, under the action of induced electromotive force, induced current in the same direction as induced electromotive force will be generated in the rotor conductor. The current-carrying conductor of the rotor is acted by electromagnetic force in the stator magnetic field (the direction of the force is determined by the left-handed rule). The electromagnetic force generates electromagnetic torque on the rotor shaft, which drives the rotor to rotate in the direction of rotating magnetic field.
Through the above analysis, it can be concluded that the working principle of the motor is as follows: when three-phase symmetrical alternating current is applied to the three-phase stator winding of the motor (the electrical angle difference of each phase is 120 degrees), a rotating magnetic field will be generated, which will cut the rotor winding, thus generating an induced current in the rotor winding (the rotor winding is a closed path), and the current carrier conductor will generate electromagnetic force under the action of the stator rotating magnetic field, thus forming electromagnetic torque on the motor shaft and driving it.
Fault analysis and treatment of three-phase asynchronous motor
The winding is an integral part of the motor. Aging, humidity, high temperature, erosion, foreign body invasion and external force impact will all cause damage to the winding. Motor overload, undervoltage, overvoltage and open-phase operation can also lead to winding failure. Winding faults are generally divided into winding grounding, short circuit, open circuit and wiring error. Now, the fault phenomena, causes and inspection methods are explained respectively.
A, winding grounding
Refers to grounding caused by winding and bushing tightness or insulation damage.
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.
Second, the 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, they are connected with low-voltage safe alternating current, and it is found that the group with small reading has 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.
Third, the winding is short-circuited.
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 mostly occur at the end of the winding, depending on whether there is bump or fracture and whether there is desoldering of the 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.
Fourth, the winding is wrong.
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 disconnected or reconnected with the new winding, 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. Structure of Three-phase Asynchronous Motor (I) Stator (Static Part)
1, stator core
Function: a part of the magnetic circuit of the motor and stator winding is placed on it.
Structure: The stator core is generally made of silicon steel sheet with a thickness of 0.35~0.5 mm, with insulating layer on the surface, and evenly distributed slots are punched in the inner circle of the core for embedding the stator winding.
Stator core groove types are as follows:
Semi-closed slot: the efficiency and power factor of the motor are high, but the winding is difficult to embed and insulate. Generally used for small low-voltage motors.
Semi-open slot: special-shaped winding can be embedded, which is generally used for large and medium-sized low-voltage motors. The so-called shaped winding means that the winding can be insulated in advance and then put into the slot.
Open slot: it is used to embed the forming winding, and the insulation mode is convenient. Mainly used for high-voltage motors.
2. Stator winding
Function: it is the circuit part of the motor, which is connected with three-phase alternating current to generate rotating magnetic field.
Structure: It consists of three windings with the same structure, with an interval of 120 electrical angle, arranged in a straight line. Each coil of these windings is embedded in each slot of the stator according to certain rules.
The main insulation items of the stator winding are as follows: (Ensure reliable insulation between the conductive part of the winding and the iron core and between the winding itself).
(1) Insulation to ground: insulation between the whole stator winding and stator core.
(2) Interphase insulation: insulation between stator windings of each phase.
(3) Turn-to-turn insulation: Turn-to-turn insulation of each phase stator winding.
Wiring in the motor junction box:
There is a terminal board in the motor junction box. Six terminals of three-phase winding are arranged in two rows. The number of three terminals in the upper row from left to right is 1(U 1), 2(V 1) and 3(W 1), and the number of three terminals in the lower row from left to right is 6(W2). All manufacturing and maintenance should be arranged according to this serial number.
3. Basic framework
Function: Fix the stator core and front and rear end covers, support the rotor, and play the role of protection and heat dissipation.
Structure: the frame is generally cast iron, the frame of large asynchronous motor is generally welded with steel plate, and the frame of micro motor is cast aluminum. There are cooling ribs outside the frame of the enclosed motor to increase the cooling area, and the end covers at both ends of the frame to protect the motor have ventilation holes, so that the air inside and outside the motor can directly convection, which is beneficial to heat dissipation.
(2) Rotor (rotating part)
1 rotor core, three-phase asynchronous motor;
Function: As a part of the motor magnetic circuit, the rotor winding is placed in the iron core slot.
Structure: The material used is the same as the stator, and it is made of 0.5 mm thick silicon steel sheets, and the excircle of the silicon steel sheets is stamped with evenly distributed holes for installing rotor windings. Usually, the back inner circle of silicon steel sheet is used to punch the stator core to punch the rotor core. Generally, the rotor core of small asynchronous motor is directly press-fitted on the rotating shaft, and the rotor core of large and medium asynchronous motor (rotor diameter is above 300~400 mm) is press-fitted on the rotating shaft with the help of rotor bracket.
2. Rotor winding of three-phase asynchronous motor
Function: the rotating magnetic field of the cutting stator produces induced electromotive force and current, forming electromagnetic torque to make the motor rotate.
Structure: divided into squirrel cage rotor and wound rotor.
(1) squirrel-cage rotor: The rotor winding consists of a plurality of guide rods inserted into the rotor slot and two circular end rings. If the rotor core is removed, the whole winding looks like a squirrel cage, so it is called cage winding. Small-sized cage motor adopts cast aluminum rotor winding, and motors above 100KW are welded with copper bars and copper end rings. Squirrel cage rotors are divided into three types: impedance rotor, single squirrel cage rotor, double squirrel cage rotor and deep slot rotor, and their starting torque and other characteristics are different.
(2) Wound rotor: The wound rotor winding is similar to the stator winding, and it is also a symmetrical three-phase winding, which is generally connected in a star shape. The three outlet heads are connected to the three slip rings of the rotating shaft, and then connected to the external circuit through brushes.
Features: The structure is complex, so the wound motor is not as widely used as the squirrel-cage motor. However, in order to improve the starting, braking and speed regulation performance of asynchronous motor, additional resistors and other elements are connected in series in the rotor winding circuit through slip rings and brushes, so it is used in equipment that requires smooth speed regulation within a certain range, such as cranes, elevators, air compressors and so on.
(3) Other accessories of three-phase asynchronous motor.
1, end cover: support function.
2. Bearing: connecting the rotating part and the fixed part.
3. Bearing end cover: protect the bearing.
4. Fan: cooling motor.