What are the reasons for vibration in the motor?

What are the reasons for vibration in the motor?  
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1 Answer

Answer :

Vibration could be because of mechanical faults and electrical faults.
1. Mechanical faults.
a. Wrong alignment of the motor on foundation.
b. Wrong installation.
c. Improper fitting of bearing and cooling fans.
d. Periodic impulse loads such as reciprocating compressors.
e. Pulley of heavy weight which cause bending of motor shaft resulting in non uniform air gap.
f. Damage of bearing or bad bearing.
g. Bad coupling.
h. If the axial alignment of the motor and the driven machine is incorrect and rotor is allowed to contact its axial stops, high axial vibrations may occur, together with high bearing temperature high and even bearing failure.
i. Machine base and foundation problem.
j. Under sized bearing. 
2. Electrical faults.
a. Air gas dissymetry.
b. Broken rotor bars.
c. Slackened stator core.
d. Slackened rotor core.
e. Interturn short in the rotor winding in the two-pole machine.
f. Unbalance in rotor winding.
g. Unbalance power supply voltages.
If the vibration is because of electrical fault, de-energise the machine and watch the vibration as it runs down.
The possible vibration frequencies observed are
a. Twice the power supply frequency – it indicates that the vibration is developed
by unbalanced power supply voltages, unbalanced air gap, unbalance in rotor winding, slackened stator core etc.
b. Multiple of power frequency – the stator and rotor slots co-ordinate to develop radial lines of force to deform and pulsate the cores.
c. Twice the slip frequency – magnetic unbalance due to unbalance air gaps, slackened rotor core, interturn short in the rotor-winding etc. of two-pole machine.
d. Beat (Humming) – in case of two-pole machine the beat is developed when the vibration of twice as much as power frequency developed between the stator and rotor is superimposed on the vibration of twice the slip frequency developed due to irregular air gap. 

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