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.

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.   

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.