Forum Comments

Preventive Maintenance
In Milling
Tiang Hock Lee
Honorary
Honorary
Jun 17, 2022
Hi Low Wei Loong, you have asked a relevant question which I think all mill Engineers ought to know. An electric motor is unlikely to burn out without reason or cause. In most cases, it will demonstrate or show symptoms before it is burnt and breakdown. Therefore it is possible to detect these symptoms to avoid or save a motor from burnt if proper routine maintenance is carried out periodically. The following are the symptoms and monitoring parameters to detect Motor failure: (Not in order of importance) Insulation resistance, IR Vibration Heat/Temperature Noise Free running amp. No load amp Normal running amps Overload relay protection-regular testing recommended Insulation Resistance, IR This is basically the state of health indicator of a motor measured with a Megger Meter. Every motor when installed new or after rewinding must be meggered phase to phase to establish acceptance of the motor for installation. A general rule-of-thumb is 10 Megaohm or more is acceptable as GOOD. IR Value, Megaohm Insulation Level 5 ~ 10 Abnormal 10 ~ 50 Good 50 ~ 100 Very Good > 100 Excellent Over time in operation, the insulation of a motor may weaken due to heat, dirt and quality of wire used. Periodic monitoring of IR could help to prolong the Motor life. On falling IR, servicing the motor by cleaning and vanishing of the coils could often restore the IR value and prolong the life of a motor. If it is a fatigue failure, the IR of a motor is likely to drop gradually overtime to fail. Mill should maintain a record of these motor IR readings for monitoring and predict failure. This is especially vital for big and critical motors of the mill. Vibration Vibration is another symptoms that could lead to a motor failure. Motor vibration are normally caused by defective bearings or misalignment or unbalanced driven fans. Periodical checking could detect and correct this problem on time to avoid uncalled for breakdown or failure of motor. A vibration detection meter is used for this inspection. Heat/Temperature There are many factors that could lead to rise of motor temperature, such as overloading, dirty motor both internal and external, lower IR generating excessive eddy current, etc. An Infrared Thermometer is normally used to measure the motor temperature. Again, a normal temperature value for the motor under normal operating condition needs to be established and recorded for subsequent reference. Noise This often occurs when there is a sudden change such as a bearing failure, loosen footing, imbalance of driven fan or snapped/loosen beltings. This will normally attract attention easily. Free Running Amp Free running amp of a motor refers to the running amp of a motor when it is running free from any connected load. This is normally established and recorded in the Electrical Machine Card of the Motor when new. All motor after rewinding must be tested for this free running amp before approval for acceptance of the rewinding job. Any higher free running amp indicates unsatisfactory state of the motor. No Load Amp This is the motor operating amp after some repair job or installation but before loading the system to operation. The assembly must be checked to freely rotate by hand for satisfactory mechanical work before switching on the motor. By comparison this No Load Amp with the established value, the repair and re-assembly work is deemed satisfactory. On this note, it may be good to check the RPM of the motor against the driven fan for slippage of beltings. Normal Running Amp At the mill commissioning stage, it is standard to establish this normal running amp of a motor operating under its normal rated capacity for future maintenance reference. I always deem this as the “Blood Pressure” of a motor. This is probably the most important monitoring parameter for a motor. This normal running amp of a motor could be checked when mill is processing. Generally, for critical machine, amp meters are installed for realtime monitoring. For others, the wireman could use a tong tester to carry out periodic routine check of this normal running amp. Any abnormal readings shall be investigated to avoid breakdown of the motor. Overload Relay Protection This is the protection in place to cut off the prolonged overload that could lead to burnt motor. Any tripping for more than 2 times must be reported to the Foreman/Wireman for further checking. Regular test for this Relay is recommended. With all the above said, it is highly recommended that your mill to set up a proper preventive maintenance program to monitor all the big and critical motors to minimize your burnt motors and a systematic certification of all rewound motor after repair. The above presented is based on my personal knowledge and experience. I honestly hope that you would find them useful and helpful to you in your work. If you need further clarification, contact me.
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Tiang Hock Lee
Honorary
Honorary
Apr 11, 2022
Thank you for your comments YC.
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Tiang Hock Lee
Honorary
Honorary
Jan 07, 2022
Hi YC, thanks for the info.
0
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Tiang Hock Lee

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