Failure of an Inductive Motor on Ball Mill 3

[KingCole76]

We recently had a failure on one of the two 6500kW Inductive motors driving a Ball Mill. Upon investigation it was realized that one of the three busbars connecting the slip ring to the rotor has melted and sheared off. What could have caused this failure?

 

[edison123]

Post photos of failure.

Muthu

http://www.edison.co.in

 

[Tmoose]

Failure after years of good faithful service?

Did the ball mill suffer some catastrophic event or extreme operating conditions in the last 6 months?

Was there a major discrepancy in the electric power any time in the last 6 months?

Operation history will very possibly show some unusual loads and events.

Is the motor completely disassembled?
What was the bearing condition?
Any signs of rotor rub?

As other said, pictures are among the essential information that should be included in the very first post, to help bypass the first dozen replies trying to get the missing most basic baseline information.

 

[waross]

Most likely a stuck brush arcing.
Do you have damage to the brush holder also?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Gr8blu]

Option 1 - accumulation of conductive contaminant (brush dust, iron dust, etc.) within the collector enclosure leads to arcing of brushes. Arc energy either "attacks" the bus, or the current draw (due to the short circuit of the arc) is sufficient to "melt" the bus.
Option 2 - unbalanced voltage on either stator or rotor supply leads to overcurrent on remaining phase connection resulting in excessive heat and eventually bus damage.
Option 3 - mechanical vibration from the process leads to fatigue failure of something (bolted or welded joint, bus itself, one of the supports preventing the bus from contacting a different potential plane) along the ring-to-rotor connection, with resulting open circuit and arc damage to bus.
Option 4 - one of those oddball process events.

Converting energy to motion for more than half a century

 

[Tmoose]

Guess OP solved his/her problem Sept 30.

 

[KingCole76]

Hello Team, thanks for the information shared so far and l appreciate it. We are now going through the Root Cause Analysis processes and your inputs are good for the team. We are yet to dismantle the motor to inspect the internal.

From all the parameters, before the failure everything looked fine but yet the motor failed suddenly

 

[waross]

Contamination on the slip ring?
I had generator failures when some dirty oil was splashed on the field slip rings.
When the brush hit the oil and lost contact, the inductive kick arced through the oil film and eroded the slip ring.
I had the lathe operator first check his tool distance just touching a good spot on the slip ring.
He had to cut 30 thousands of an inch to clean up the bad spots.
Any contamination lifting a brush or a brush stuck in a brush holder will lead to damage.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Tmoose]

Ehen I think of induction motors I don't think of slip rings.

Guess I better get a-Googling.

 

[Gr8blu]

@Tmoose There are two types of induction machine. The traditional "squirrel cage" which has no external power source (voltage and current are "induced" by the magnetic field that bridges the air gap from stator to rotor). There is also the "wound rotor" type of induction machine - which, as the name suggests, actually has coils instead of simple bars. The coils are fed through a mechanical contact (i.e., brushes riding on a set of collector rings). Usually, there is one ring per phase, and may or may not include two additional rings - "neutral" and "ground". The "ground" would be where the shaft grounding brush rides.

Converting energy to motion for more than half a century

 

[waross]

The characteristics of an induction motor are determined to a large extent by the resistance of the rotor winding.
In the motors that you are familiar with, the rotor winding is often a ring of aluminum alloy bars cast into the rotor.
The characteristics such as torque curve and, to an extent, the locked rotor current depend on the resistance of the bars or "squirrel cage" winding.
A wound rotor induction motor has insulated windings brought out to slip rings.
External resistance is connected across the slip rings to reduce the starting current and increase the starting torque.
The last mill I worked on weighed 400 tons and held 100 tons of ore.
The two 6000 HP motors were wound rotor induction motors, started with liquid rheostats.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!