Maintenance Resources

Sunday, March 14, 2010

Motor Failures and its prevention

Not a single machine goes under sudden breakdown. Before to collapse it shows symptoms ( MOANGROANSQUEAKSQUEAL….COLLAPSE!) but it is our experience, knowledge and monitoring efficiency to recognize and track the record of deterioration.

If we collect all data, we can see that 70% of the time motor fails due to mechanical reason and 30% for electrical reason. Bearing failure contributes 35% of mechanical reason. See bellow pie-chart for easy understand.



Why Bearing fails:
1.    Inadequate lubrication – Most of the cases, bearing fails due to lack of lubrication (Even greasing nipples are found blocked due to no use!) See bearing lubrication method video.

2.    Heat As it is the main cause of winding failure, discussed commonly in bellow.
3.    Faster Worn out due to wrong selection of lining in case of sleeve bearing
4.    Shaft induced current – Motor frequently feeds by PWM drive with IGBT inverters. It’s frequent switching causes parasitic capacitance between stator and rotor. This induced a low frequency current in shaft which finds its path through bearing to ground. They have two types.
Conductive current: Conduction current may flow through the motor bearing if the shaft happens to be shorted to the frame by means of bearing ball contact. It’s low in amplitude and flows continuously. It increases bearing temperature and eats life of lubrication.
Discharge current: may flow through the bearings when the voltage across the bearing lubricant film exceeds its film breakdown voltage. A voltage buildup occurs as the capacitive coupled currents act to charge up the rotor. It is high in amplitude, can causes brittle in bearing material which causes material chip off in load zone if bearing races are altered through re-hardening (Re use of bearing which is available with low cost  non authentic merchants )
It is considerable for frame size more that NEMA 400

5.    Vibration –
Loose foundation, aging of structure, Wrong alignment of coupling, bent shaft and Lack of periodic maintenance.

Over heat Contributes in both bearing and Winding Failure.
1.    WRONG MOTOR: It may be too small or have the wrong starting torque characteristics for the load. This may be the result of poor initial selection or changes in the load requirements.
2.    POOR COOLING: Accumulated dirt or poor motor location may prevent the free flow of cooling air around the motor. In other cases, the motor may draw heated air from another source. Internal dirt or damage can prevent proper air flow through all sections of the motor. Dirt on the frame may prevent transfer of internal heat to the cooler ambient air.
3.    OVERLOADED DRIVEN MACHINE: Excess loads or jams in the driven machine force the motor to supply higher torque, draw more current and overheat.




4.    Over lubrication – It is more dangerous. When heated, grease changes it forms and deposited on stator winding. It further welcomes dust and dart and catches heat and reducing insulation level.

Protection:

1.    Use correct grade of grease for specified application.
2.    Wherever possible, remove old grease before lubricate
3.    Use proper method and quantity of greasing.
4.    Keep motor clean from dust and dirt
5.    Monitor heat and current (it gives excellent picture of gradual deterioration )
6.    If unable to afford the failure than afford thermal imager (Cost aprox. 2 Lakhs INR at entry level ). Clue : Calculate TDC (Total downtime cost includes spare cost + product loss cost + maintenance crew cost engaged in breakdown) to find ROI……...you will find that we can’t afford the failure
7.    Over load relay (Monitor specifically. It is general practice to set it at higher value at the time of machine setup and standard is 120 % of full load current. But though in many assembly application motors are operating with very less load (Over sizing of motor at design) I think it is preferable to set the value at 90% of full load. It will give time to arrest other faults to catch earlier.
8.    Single phase preventer – Overload relay is a bi-metallic strip and need some time to actuate. (Consider a motor is running 25% of it’s rated load. Then for single phasing condition there is chance that the current of other phase will not exceed overload current setting.) it gives extra measures of protection to winding.

2 comments:

  1. Nice article in a good Blog, keep shared us.

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