Electric motor blows up

Mine type

All coal mines

Incident

IP55 250kW horizontally mounted induction motor blew up causing the terminal box cover to be thrown more than 5m. The casting around the 4 cover securing bolts was broken and bolts left threaded in base casting. 

Equipment

1000V Pope 250kW IP55 flange mounted induction motor was 1 of 3 used to power a conveyor drive head installation.

Hazard

The motor terminal box cover lid was blown off with enough force to strike the coal rib line and still land over 5m away from the motor installation. The cover weighed in excess of 5kg and would have had serious consequences if it had struck a person.

Cause

There was no fault found with the motor electrical insulation readings when measured using both 1kV and 5kV meggars. Both end shields were mounted upside down, the grease nipple was located on the bottom and grease drain was filled with coal fines and facing vertically up. There was no indication that this bearing had been greased since the motor was installed and commissioned. The fault is believed to have been initiated when the drive end roller bearing seized and the temperature rise reached the grease fire point which falls within the range 250–300ºC. The resultant ignition of the vaporised grease caused an internal explosion that was vented through the motor terminal box when the cover was blown clear.

Comments and recommendations

All electric motors fitted with serviceable bearings are to be checked to ensure they all:

• are serviced correctly
• have grease nipples located above the bearing level
• have grease drain lines positioned below the bearing level.

Authorised by Peter Minahan - Chief Inspector of Mines

Contact: minesafetyandhealth@dnrm.qld.gov.au

Issued by Queensland Department of Mines and Energy