Fault in Rotating Machine

Fault in Rotating Machine

The rotating machine or electric motor consist of many mechanical & electrical parts such as rotor bar, rotor magnet, stator winding, end rings, bearing, shaft, gear box etc. Due to commonly harsh  industrial environment , each part of electric motor is potentially exposed to high risk of unexpected mechanical , electrical or sometimes chemical failures ( depends upon the application where the motor used ) .The reason for common motor failure in industries are as follows :

1.  Motor used after standard lifetime

2. Wrong rated power, voltage & current

3. Wrong selection of motor

4. Wrong motor connections

4. Unbalance or unstable voltage & current source

5. Overload, unbalance or fluctuation load

6. Bearing failure

 

7. Bolt loosening

8. Residual stress during manufacturing

9. Electrical stress from switching inverters or unstable ground

10. Harsh environment (Dust, water leakage on machine, Vibration, high temperature, chemical contamination.)

11. Mismatch from motor to connected load (loose coupling)

12. Temperature increase 

·         Faults occur in Rotating Machines :

                           I.  Open or Short circuit in motor winding : This fault is mainly occur due insulation failure in winding or motor lock during running condition due to bearing jam or overload or voltage drop . To protect motor from this type of fault over load, over current / voltage protection relay or other switchgear is used.

                           II.  Wrong Connection of Winding : This type of fault is rarely occur. To avoid such type of fault check the motor supply terminal connection as per connection diagram.

                          III.   High Contact Resistance or Resistance Unbalance : Resistance unbalance is due to lose lead wire crimping lugs or winding turns is less in stator slot. If there is any break of lead wire then this fault is occur. To solve this fault check lead wire lugs tightness or joint of winding wire to lead wire.

                          IV.    Stator Fault : The stator fault can be broadly classified as the lamination or frame fault i.e. defective core, circulating current or ground, winding insulation damage, displacement of conductor. To avoid this fault proper insulation is used & care should be taken during insert insulation paper in stator slot. In motor the torque is result of force created by current in conductor & surrounding magnetic field.This shows that the  winding insulation must have electrical as well as mechanical properties to withstand mechanical  & electrical stress. The manufacturing process itself damage the stator core or some aging effect. Thermal stress is also one of the reason of stator winding insulation failure. The main source of thermal stress in electrical motor or rotating machines are copper loss , eddy current loss & stray load loss in copper conductor .Additional heating due to core loss & friction loss in machine. Another significance effect on winding insulation aging is partial discharge in large motor. Partial discharges are small electric spark that occur within air bubbles in the winding insulation material due to non-uniform electric field distribution. To protect machine from this fault phase & ground fault relays are used.

                          V.   Broken Rotor Bar Fault : In squirrel cage induction motor rotor used is basically two types : cast or fabricated. Almost all squirrel cage motor bars & end rings are made of alloys of aluminum or copper or pure copper .Cast rotor although more rugged than fabricated type ,can hardly repaired once fault cracked or broken rotor bars develop in them. There are number of reason to broken rotor bar fault & end ring damage . They can be cause due to thermal stress , magnetic , dynamic , environmental , mechanical & residual stress on rotor.  An incipient broken rotor bar condition aggravates itself almost exponentially in time as excessive current flow is expected to be concentrated on adjacent bars instead of the broken rotor bar. Which provides propagated electrical stress to adjacent areas. That causes unbalance line current, torque fluctuation or pulsation. The electric & magnetic asymmetry in induction motor rotor boosts up the left side band of supply frequency .There is also chance to damage the stator winding. To reduce this type of fault care should be taken during rotor bar assembly & end ring fitting.

                           VI.   Uneven Air Gap or Eccentricity Fault : Air gap eccentricity is known as a condition that occurs when there is a non-uniform distance between the rotor & stator in the air gap.When there is eccentricity in the air gap,varing inductance cause unbalance magnetic flux within the air gap that creates fault harmonics in line current. If the distance between the stator bore & rotor is not equal throughout the entire machine, varying magnetic flux within air gap creates imbalance in the current flow. When eccentricity become significant,the resulating radial forces can cause stator to rotor rub & this can result damage of the stator & rotor. Improper mounting ,the non-circularity of the stator core ,a loose or missing bolt , a bent rotor shaft or misalignment , bearing wear & rotor unbalance might be causes of air-gap eccentricity. 

                            VII.   Voltage Unbalance : This fault is occurs when there is unbalance in supply voltage or winding connections are not properly joint, lead wire connecting lugs are loose. Due to variation in voltage there is also variation on rotating machine current & that effect on machine temperature rise & performance. As per standard 10 % voltage variation is acceptable.

                            VIII.  Current Unbalance : Fluctuation load , unbalance in supply wrong rotor used or defective rotor ,terminal connections are not proper or loose ,winding coil turns are different than other winding ,uneven air gap between stator & rotor then this type of fault occurs.

                              IX.   Frequency Variation : Frequency variation is also clause of motor failure. As we know that motor or rotating machine RPM is depend the frequency of machine. If there is drop in frequency during running condition then machine RPM is drop & load is suddenly increase , machine take more current that its rated current .As current is increases there is increase in losses of motor .Result is overheating of machine & if this frequency variation is continuous then there is chance to burn the machine. To reduce such type of fault Variable Frequency Drive is used for stale frequency supply.

                              X.   Bearing Fault : Most of bearing in industries are running under non-ideal load condition alignment, mechanical vibration, overloading of motor, contamination, current flowing through the bearing, corrosion & wrong lubricant or excess lubricant used. To reduce this fault care should be taken during fitting of bearing on shaft of rotation machine.

                             XI.   Vibration in Machine : It present in machine due to improper assembly of motor & prime mover, mounting bolt is not tight , rotor balancing is not proper or bearing assembly on shaft of rotation machine is not fitted properly. If there is heavy vibration then it will damage the rotating machine & prime mover connecting assembly also damage the rotating machine body & other part.

                             XII.   Noise : There are sometimes two type of noise is available in rotating machines. One is electrical noise & other is mechanical noise .Electrical noise is also called as humming noise. It is increase as supply voltage increases & dresses the supply as voltage is decreases. It present due to low quality of stamping used or faulty rotor used. To avoid such noise high quality stamping material to be used. Mechanical noise is due to rotation of rotating machine  cooling fan & Prime mover .To reduce or limit this noise proper design cooling fan is used.