The failure of squirrel cage two- and three-phase asynchronous motors can be divided into two parts: mechanical and electromagnetic.
Mechanical failure
The mechanical part is mainly due to the gap fit of the bearing, rotor shaft, end cover, bearing cover and other components and poor working conditions caused by the motor current increase, heating, abnormal sound, rotor jamming and other phenomena, which is closely related to the installation and daily maintenance of the motor. As long as we pay attention to follow the relevant installation and use procedures during the assembly and installation process, it can be solved.
Electromagnetic faults
Electromagnetic faults are more complex, mainly including:
1. Winding grounding
Winding grounding means that the stator winding of the motor is damaged due to moisture or external force, resulting in the insulation value of the winding and the stator core and rotor of the motor being reduced or even zero. If this fault occurs, the motor casing cannot be started normally when it is charged, the temperature of the machine body rises, or even burns out. The DC resistance of each phase winding can be detected by a megohmmeter, if the resistance value is small, the winding can be judged to be damp, and if the resistance value is equal to zero, the winding has been completely grounded.
There are several reasons for this:
(1) The motor is not used for a long time or is not properly maintained, resulting in moisture in the windings and reducing the resistance. It should be tested for insulation resistance before operation, and it can not be used when it is less than 0.5 megaohms, and it should be tested regularly during operation, and it should be dried in time once it is found that the insulation resistance is reduced.
(2) The motor operates in a corrosive gas environment for a long time with overload, overvoltage, overcurrent, and phase loss, resulting in insulation aging and performance degradation. The motor should be readjusted to achieve good working condition.
(3) The motor winding is burned out.
(4) When repairing the motor, due to improper operation, the insulation layer of the magnet wire and the lead wire is damaged or the local slot insulation is damaged, and the slot insulation is not well mated or the end coil is too long, too large and too small.
2. Short-circuit fault
This fault is generally divided into phase-to-phase short circuit, turn-to-turn short circuit (or coil-to-coil short circuit), and pole-phase group short circuit.
(1) Phase-to-phase short circuit
The interphase short circuit is caused by the aging, breakdown and moisture of the interphase insulation layer at the end of the winding or in the double-layer winding groove or the long-term overvoltage, overload and phase loss operation of the motor.
When the fault occurs, the power fuse melts quickly, and the motor can not be started. Use a megohmmeter or multimeter resistance to measure the DC resistance value between each phase, if it is lower than the specified value, check whether there are traces of scorch between the internal windings. If it is not serious, treat the damaged point with insulating materials, pad the interphase insulating paper, tie the ends, coat the paint for drying, and measure again before operation to ensure that the resistance value is not less than 0.5 megaohms. If the winding has been seriously burned, it is necessary to re-embed the wire and replace the winding.
(2) Inter-turn short circuit
An inter-turn short circuit is a short circuit that occurs due to damage to the insulation of the conductor itself, resulting in contact between adjacent conductors.
If there is a short circuit between turns, the three-phase current is unbalanced, there is obvious electromagnetic noise, and the short-circuit current makes the winding quickly heat, smoke and even emit a scorching smell.
The causes of this failure are as follows:
The end of the winding is mechanically damaged, the coil insulating paint layer is scratched when the wire is embedded, the insulation is aging, cracking, falling off due to long-term overload and overvoltage operation, and the winding is discharged after being wet. The DC resistance value of each phase winding can be measured with a multimeter resistance file or bridge, if it is much smaller than the normal value, the end cover needs to be opened to check the winding. If there is a serious short circuit and the coil has burned out, it should be re-embedded according to the actual situation.
(3) Pole-phase short circuit
Most of them are caused by the insulating sleeve of the winding connection wire is not covered. Generally, it mostly occurs in the repaired motor, and the winding can be heated and softened, and then the sleeve can be re-sleeved or isolated with composite insulating paper to deal with it.
3. Circuit breaker fault
The fault generally occurs at the end of the motor winding of each terminal or lead wire, is due to poor welding, corrosive flux is not cleaned and caused by false soldering or loosening, and the winding by mechanical stress or mechanical collision caused by coil breaking.
This fault is the most common when repairing, as long as the process standards are strictly followed and the maintenance quality is strengthened, the problem is not difficult to solve.
4. Phase-out operation
Phase-missing operation is the operation of a three-phase motor in a state where the one-phase winding does not work, which is also called two-phase operation. The two-phase voltage cannot start the motor, and it only occurs when the phase is suddenly out of phase during operation. It will cause the motor to heat up and burn out in a short period of time, and the power fuse will blow.
The main causes of this failure are:
Poor contact of the switch contact, too small contact pressure of the contactor and a phase break in the power supply line: a phase connection in the three-phase lead line is not real or disconnected, etc.: the motor winding is broken: the winding indirect line is soldered, etc., which will cause phase loss operation. If it is a power supply or lead failure, use a multimeter to find out the cause and repair it. If there is a problem inside, open the end cover, find the fault, re-weld, wrap, dip paint drying, and use it after testing it is normal. If the winding has been severely damaged, it should be replaced with a new one.
5. Overload operation
The characteristics of overload operation are dull motor working sound, speed drop, weakness, motor temperature rise is too high, and even smell the burning smell or smoke of insulating paint and increase the indication current.
The main reason for the occurrence of overload operation is the unreasonable configuration of the motor and load.
When choosing a motor, you should understand the performance of the motor, understand the minimum power required for the load, and reasonable configuration can avoid unnecessary losses caused by overload operation.