How electric motor fail and what we can do for it ?

Electric motor is used everywhere in industry and in term of technology,it is complex , sometimes making it a challenge to keep it running at peak performance.
Before motor fails we can do many things to suppress eventual failure or heavy damages. There are different reasons given below for failure of motor.

motor failure


1.Power Quality 

  • Transient voltage
  • Voltage Imbalance
  • Harmonic Distortion 

2.Variable Frequency Drive

  • Sigma current
  • Reflections on drive output PWM signals
  • Operational overload

3.Mechanical 

  • Misalignment 
  • Shaft imbalance
  • Shaft looseness

4.Improper Installation Factor 

  • Soft foot 
  • Shaft voltage
Here below we discuss about all factors in brief 

Power Quality

  • Transient 
Transient voltage can come from a number of sources either inside or outside of the plant. Adjacent loads turning on/off, power factor correction capacitor banks or even distant weather can generate transient voltage on distribution systems. These transients, which vary in amplitude and frequency,can cause insulation breakdown in motor windings.
Transient may appear on control cables that don't necessarily cause equipment damage directly,but may disrupt operations.

  • Voltage Imbalance 
Three phase distribution systems often serve single phase loads, An imbalance in impedance or load distribution can contribute to imbalance across all three of the phases.
  Please check voltage of each phase by using multimeter.



  • Harmonic Distortion 
Harmonic are any unwanted additional source of high frequency AC voltages or currents supplying energy to motor windings.
These looses dissipate in the form of heat, which, over time, will deteriorate the insulation capability of the windings.some harmonic distortion of the current is normal on any part of the system serving electronic loads.
To start investigating harmonic distortion, use a power quality analyzer to monitor electrical current levels & temperatures at transformers to be sure that they are not overstressed.   
harmonic

    Variable Frequency Drive

    • Sigma Current
    Sigma currents are essentially stray currents that circulate in a system. The sigma currents are created as a result of the signal frequency, voltage level, capacitance and inductance in conductors.These circulating currents can find their way through protective earth system causing nuisance tripping or in some cases excess heat in windings.
    Sigma current find in motor cable.
    The sum of the three currents would equal to zero. means the return current from the drive would be equal to the current to the drive. sigma current can also be understood as asymmetrical signals in multiple conductors that can capacitively couple currents into the ground conductor.


    • Reflections  on drive output  PWM singals 

    VFD employ a pulse width modulation PWM technique  to control the output voltage and frequency to a motor. Reflections are generated when there is an impedance mismatch between the source and load. Impedance mismatch can occur as a result of improper installation, improper component selection or equipment degradation over time, In a motor drive circuit, the peak of the reflection could be as high as the DC bus voltage level.

    • Operational Overload
    When motor is overloaded it draws excessive heat, over current and insufficient torque.
    Motor component which are bearings, motor windings, and other components may be working fine, but the motor will continue to run hot.  30% motor fail because of overloading.


    Mechanical 

    • Misalignment of a motor
    Misalignment occurs when the motor drive shaft is not in correct alignment with the load,or the component that couples the motor to the load is misaligned. Many believe that a flexible coupling eliminates and compensates for misalignment, but a flexible coupling only protects the coupling from misalignment.
    There are few types of misalignment:
    1. Angular misalignment- shaft centerlines intersect but are not parallel

    2. Parallel misalignment- shaft centerlines are parallel but not concentric
      

    3. Compound misalignment- a combination of parallel and angular misalignment
    • Shaft Imbalance 
    Imbalance is a condition of a rotating part where the center of mass does not lie on the axis of rotation. In other words there is a heavy spot somewhere on the rotor. Although you can never completely eliminates motor imbalance, you can identify when it is out of normal range,and take action to rectify the problem.
    • Shaft looseness
    Looseness occurs when there is excessive clearance between parts,looseness can occur in several places.
    Rotating looseness is caused by excessive clearance between rotating and stationary elements of the machine, such as in a bearing.
    Non rotating looseness happens between two normally stationary parts, such as a foot and foundation,or a bearing housing and a machine.

    Improper Installation Factor 

    • Soft foot
    Soft foot refers to a condition in which the mounting feet or a motor or driven component are not even, or the mounting surface upon which the mounting feet sit is not even.
    • Shaft voltage
    When motor shaft voltage exceed the insulating capability of the bearing grease, flashover currents to the outer bearing will occur, thereby causing pitting and grooving to the bearing races. The first signs of this problem will be noise and overheating as the bearing begin to lose their original shape and metal fragments mix with grease and increase bearing friction.

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