Induction Motor Bearing Failure and the Cause?

If a VFD is involved, then correct grounding of the rotor shaft other than via the bearings is essential to prevent premature bearing failure

Dear Mr.scott165,

You have to furnish more details such as

1. Whether the duty conditions demand motor of vfd application.

2. If VFD application, whether the motor is specially designed motor for vfd application.

3. If vfd motor, whether it is fitted with insulated bearing is fitted or bearing insulation is provided on one side.

4.Whether the earthing is properly done with required size of earthing flat/electrode.

If it is ordinary duty motor,

1. Whether the bearing used is the supply from bearing manufacturer.

2. Whether the selection criteria for bearing is followed for details of clearances C1, C2 etc. axial and radial play.

3. The failure is the original bearing fitted by the motor manufacturer or it is replaced one during maintenance.

4.Whether right type of grease /lubrication and correct quantity is used. 5.Whether the temp. rise is in the permissible limit.

5. Whether vibration analysis is made and whether parameters are with in the limit.

Pl. post the details and CR4 Members may be in a position to suggest ways and means to solve the problem.

DHAYANANDHAN.S

The bearing failed because there was a significant electrical current passing through it for a significant period of time while it was turning, and it is not designed to do this. From the smell the thing has produced, smart money is on an insulation failure somewhere in the rotor.

Two failure modes for ball bearings:

1. Current discharge across the bearing - Any arc erodes a point on the bearing ball or race resulting in roughness and accelerated wear. I once knew a guy who tried using a lathe as a rotary welding fixture, clipped the ground lead to the lathe bed and successfully welded all the ball bearings and several gears in the lathe headstock.

2. Fretting - If the motor is left in a stalled or stopped condition with power applied, the ball bearing ball repeatedly rolls back and forth across a localized point in the bearing race and creates a spot of wear that results in rough bearing operation and accelerated wear. I have seen this occur in an idled turbine generator where mechanical vibration from other equipment mounted on the same deck led to bearing roughness in as little as 30 minutes.

Plain bearings are less susceptible and generally fail from loss of lubrication which is accelerated by heat in the bearing or of the shaft passing through.

Having a welder connected to a piece of machinery with an improper ground connection will do damage to the motor bearings if the motor bearing race ends up as an easier path to ground than the ground system. I used to see this a lot on conveyor systems. The welder would attach his ground clamp to the main structure, then move his welding tip over to the conveyor structure and ASSume the grounding was continuous, but in fact there was paint or rubber vibration isolators between them. So because the motor frame was on one part and the rotor on the other, the bearings and races become the return path for the welding current and the bearings are damaged. If the motor happens to be running when that happens, the damage looks exactly the same as if the motor were on a VFD.