Induction Motor - Bearing Defect Due to Static Load and Water Ingress
Pulsating noise in an induction motor with bearing 6322 is caused by a beating effect between the motor’s rotational frequency and a multiple of the bearing’s failure frequency. The damage is due to static load and water ingress, and can be mitigated by using sealed bearings and rotating the axle periodically.
Damage to the outer race of bearing 6322 of induction motor with rotational speed 1185 CPM controlled by frequency inverter, with pulsating noise at normal speed.
1. In the vibration spectrum, a frequency close (122Hz) to the frequency 2FL (120 Hz) can be observed; The proximity of two frequencies will result in the beating effect, unlike amplitude modulation where a pair of sidebands are perceived in the spectrum, in this case the pulsations do not generate sidebands.
2. The corresponding origin of the frequency 122 Hz, in this case coincides with a multiple of the failure frequency of the outer track of the bearing 6322 installed on the load side of the motor, so it is recommended to check the integrity of the motor.
3. The difference between the two close periods will be responsible for defining the period of the pulses, in this case (1/122Hz-1/120Hz).
4. After checking the bearing, ball marks were observed on the outer race, the mechanism of damage is evident by two factors: The first is the static load exerted by the rolling element in relation to the weight of the rotor when the machine has a long period of inactivity, and the second penetration of water through the bearings that develops due to pickling effect, explained in other previous cases.
5. An alternative to avoid this effect is to use bearings with a friction seal that protect the ingress of
humidity, as well as the execution of regenerative activities in the engine, changing the position of the axle every given established period.
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