Use and Maintenance of Electric Motors

 Use and Common Faults of Three-Phase AC Asynchronous Motors

       The function of a three-phase AC asynchronous motor is to convert electrical energy into mechanical energy, and it is widely used in our company's production processes. Correct and reasonable use, operation, and maintenance of motors play an important role in ensuring stable production. Daily maintenance, regular inspections, early detection and timely handling of abnormal conditions during operation, and careful analysis of accidents and corresponding countermeasures after they occur are all essential technical work to reduce and avoid motor faults and accidents during operation and to improve motor operating efficiency. However, to ensure that motors are not damaged in cases of phase loss, mechanical faults, overload, short circuits, etc., and to ensure safe motor operation, both electrical personnel and process operators need to master basic knowledge about safe motor operation, detect and eliminate hidden dangers in motor operation as early as possible, and nip accidents in the bud. To this end, I would like to share my insights on the safe and stable operation of motors gained from my work in electrical maintenance, hoping to contribute to the company's production and stable equipment operation. There may inevitably be shortcomings and inaccuracies in this article; please point them out and provide valuable suggestions.

       During motor operation, the following should be observed and inspected:

       1. Motor operating current.

       Generally, the motor's operating current is about 70-95% of the rated current. Pay attention to the motor's normal operating current. If the operating current changes significantly, exceeds the rated current, or the three-phase current difference exceeds 10%, investigate the cause and inform the maintenance electrician. The main causes of motor overcurrent include: 1) overload; 2) low supply voltage; 3) phase loss during startup or operation; 4) excessive resistance due to mechanical friction, jamming, etc.; 5) frequent motor starts, which prolongs starting time and increases starting current.

       2. Motor operating temperature

        2.1 Motor stator temperature

      According to motor insulation classes, Class A insulation is 105°C, Class E is 120°C, Class B is 130°C, Class F is 155°C, and Class H is 180°C. In actual motor operation, excessive temperature can cause motor damage, shortened service life, reduced insulation strength, and increased losses. Generally, it should be controlled within 100°C.

       The main causes of increased stator temperature include: 1) overload; 2) low supply voltage; 3) phase loss operation; 4) damp motor windings; 5) frequent motor starts; 6) poor heat dissipation due to dust accumulation on stator coils, or excessive dust on the motor housing. For motors in poor working environments with high dust, it is generally recommended to extract the core and blow out dust for inspection every 1-2 years; 7) fan damage; 9) high ambient temperature, etc.

      2.2 Motor bearing overheating

      Small and medium-sized three-phase asynchronous motors typically use rolling bearings or sliding bearings. For motors that have been operating stably for a long time, the causes of increased bearing temperature include: 1) incorrect lubricant grade, deteriorated lubricant, or insufficient oil quantity leading to poor lubrication; 2) for motors with cooling devices, insufficient or blocked cooling medium flow causing poor cooling effect and temperature rise; 3) concentricity deviation or shaft bending; 4) excessive bearing assembly clearance, bearing wear, or damage. Bearing temperature criteria: rolling bearings generally ≤70°C, sliding bearings ≤65°C. Under special circumstances, it can be appropriately increased: rolling bearings ≤95°C, sliding bearings ≤75°C.

      3. Motor sound

     Under normal operation, the motor sound is uniform and stable. When the following situations occur, the motor will emit abnormal sounds:

    3.1 When bearing damage causes the stator and rotor to rub, a harsh "scratching" sound will be produced.

     3.2 When the motor operates with phase loss, a loud "humming" sound will be emitted. The motor speed will slow down and the current will increase. In this case, if the load is not a potential energy load, you can cut off the power and then close the switch again to see if it can start normally. If it cannot start, it may be due to phase loss in the motor power supply.

      3.3 When the load is too heavy or the mechanism is jammed, and the motor cannot rotate, a "humming" sound will be emitted.

      3.4 When the bearing is severely short of oil, a "hissing" sound can be heard from the bearing housing; when the bearing is damaged, abnormal sounds like "rumbling" can be heard from the bearing housing. You can hear these sounds by placing a suitable hollow metal tube or the tip of a screwdriver against the bearing cover and putting your ear to the metal tube end or the screwdriver handle.
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