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According to statistics, about 85% of the three-phase asynchronous motors used in production are electrical faults due to winding burnout, and about 15% are structural and other faults. The causes of winding burnout are mostly phase loss or overload operation. The winding is grounded and the winding is short-circuited between phases or turns. Secondly, there are mechanical reasons such as stator and rotor friction and broken bars. This article focuses on analyzing the causes of motor winding burnout from an electrical perspective, and proposes corresponding treatment methods.
Phase loss operation
-Fault phenomenon
The motor cannot start. Even if it can start without load, the speed slowly rises and there is a buzzing sound; the motor smokes and heats up, accompanied by a burning smell.
-Check results
Remove the motor end cover and you can see that 1/3 or 2/3 of the polar phase windings at the end of the winding are either zoomed or turned dark brown.
-Causes of faults and solutions
(1) The fuse circuit of the motor power supply circuit has poor contact or is mechanically damaged, causing a certain phase fuse to blow out.
(2) The three-phase fuses in the motor power supply circuit have different specifications, and the fuse with a small capacity burns out. The fuse should be replaced with a fuse of the same specifications according to the power of the motor.
(3) The switches in the motor power supply circuit (isolating switch, rubber cover switch, etc.) and the contacts of the contactor have poor contact (burned or loose). Repair and adjust the dynamic and static contacts to ensure good contact.
(4) A certain phase of the line is missing. Find the broken wire and connect it firmly.
(5) Welding between the motor winding wires leads to poor contact. Carefully check the motor winding connecting wires and weld them securely.
Overload operation
-Fault phenomenon
The motor current exceeds the rated value; the motor temperature rise exceeds the rated temperature rise.
-Check results
All three sets of windings of the motor are burned out; the bearings have no grease or the sand frame is damaged; the stator and rotor cores rub against each other, commonly known as sweeping.
-Causes of faults and solutions:
(1) When the load is too heavy, consider appropriately reducing the load or replacing the motor with a suitable capacity.
(2) If the power supply voltage is too high or too low, a three-phase power supply voltage stabilizing and compensation cabinet needs to be installed.
(3) The motor has been severely affected by moisture or corrosive gas for a long time, and the insulation resistance has decreased. According to the specific situation, overhaul or replacement of the enclosed motor with the same capacity and specifications should be carried out.
(4) The bearing is short of oil, dry grinding or the rotor mechanism is not concentric, causing the motor rotor to sweep, causing the motor current to exceed the rated value. First, the bearing wear should be carefully checked. If it is not qualified, a new bearing should be replaced; secondly, the bearing should be cleaned and an appropriate amount of grease should be injected. Then check the motor end cover. If the center hole of the end cover causes the rotor to be out of center due to wear, the end cover should be processed or replaced.
(5) The transmission part of the mechanism fails, causing the motor to be overloaded and burning the motor windings. Check for faults in the mechanical part and take measures to solve them to make them rotate flexibly.
Winding grounding
-Fault phenomenon
The motor cannot be started without load; the fuse in the motor power supply circuit is blown or the switch is tripped.
-Check results
There are burn marks on the stator slot winding and iron core, and there is a copper melting point; the winding in the slot and the iron core are broken down; the outer insulation of the winding lead wire is damaged.
-Failure causes and solutions:
(1) When repairing the motor, the bamboo wedges were inserted carelessly, causing the insulation of the notch winding to be damaged; the bamboo wedges have aged over time and have poor insulation. High-quality bamboo wedges should be selected according to the motor off-line process and insulated. Be careful not to scratch the wire with the bamboo wedge when taking it off the line.
(2) Motors that have been exposed to moisture for a long time or work in corrosive gases should be replaced with enclosed motors.
(3) In open-type motors, the winding insulation is damaged due to metal or metal chips entering the motor. In this regard, a protective net or protective plate should be installed around the motor.
(4) The rotor balance block is loose or falls off, scratching the motor winding insulation. The balance weight should be readjusted and fixed, and the damaged parts of the winding should be dealt with.
(5) If there is no lightning arrester or the arrester fails, a lightning arrester should be added or the arrester should be rechecked.
Short circuit between winding phases
-Fault phenomenon:
The motor cannot start; the fuse in the motor power supply circuit is blown or the switch is tripped; the motor windings smoke and have a burning smell.
-Check results
The multi-stranded wires at the phase-to-phase short circuit are burned out, and there are copper melting points around them.
-Causes of faults and solutions:
(1) For motors with scratched insulation on the surface of the wires or poor insulation at the winding ends when offline, the burned wires should be separated, cleaned and soldered, wrapped with insulation and flattened, lowered into the slot and then painted with insulating paint. and dried. If it cannot be repaired, it should be rewound according to the original data.
(2) The casings of the connections between windings and reference wires must be compatible with the insulation level of the motor windings. The insulation casings of the connections should be 15 to 25 mm longer than the solder joints.
Short circuit between winding turns
-Fault phenomenon:
The motor emits smoke during operation, the local temperature rises too high, and there is a burning smell.
-Check results
The inspection result shows that the three-phase current of the motor is unbalanced; several turns or one coil becomes a bare wire.
-Causes of faults and solutions
(1) When several turns or one coil is burned out, if the slot full rate is not high, threading and winding repairs can be performed.
(2) For the rest, please refer to item 4.
Timely detection and rapid elimination of electrical equipment faults can prevent accidents and ensure smooth production. In this regard, electrical equipment must be inspected and maintained regularly in accordance with regulations, and operating faults of electrical equipment must be accurately judged and handled to reduce equipment accident losses and ensure normal production.
