A Fluorspar mine based in the Derbyshire Peak District had a major failure on its processing plant. A 350HP motor had failed. A direct replacement motor was not available from any motor stockist. Whilst a new energy efficient motor controlled by an inverter would have been the ideal solution, in this instance, the repair of the original motor was considered to be cost affective option.
We collected the motor from site and upon receipt at our works it was immediately booked into our job tracking system.

Electric Motor Type. The motor was a 40 year old Brook, 350hp, 985 rev/min, D400L frame, Slip Ring, TEFC, weighing 3.5 tonnes.

Assessment. Our engineers carried out all the normal static tests prior to dismantling. Significantly the stator failed the Baker Surge Test. Upon dismantling, the failure was evident as the stator windings had blown at the edge of one of the stator slots. Blast marks from the stator failure were also seen on the bar wound rotor. Static tests on the rotor, including the Baker Surge Test, indicated that the rotor windings were satisfactory and that the blast damage was purely cosmetic.

Stator. The burnt out stator windings were stripped by first removing the “crown” and checking the winding connection data. Subsequently the stator was placed into our Temperature Controlled Pyrolysis Oven (burn off oven) in order that burnt out stator windings could be removed without further damage to the interlamina resistance of the stator core. The winding data was tabulated and checked. After the core had been cleaned, a core test was carried out owing to severity of the blow up and remedial work was performed. Originally wound in a class B insulation system, we rewound the stator in our superior full class H system allowing the windings to withstand a high temperature rise. Flash test and Baker Surge Test were further quality checks. Our next step involved the impregnation and curing of the stator windings by our full class H, VPI system (Vacuum & Pressure Impregnation), giving the benefits of better heat transfer and environmental protection.

350 Hp, 6 pole, half coil, 108 slot winding, in progress

Rotor. The wound rotor windings were cleaned up and the blast marks caused by the stator winding failure were removed. Both bearing journals were found to be on the lowest tolerance limit, therefore they were reclaimed and machined back to an appropriate tolerance. The 1100kg rotor, less the fitments of the coupling, slip rings and cooling fan, was then dynamically balanced to Grade G6.3 (appropriate for this type of motor), after which the rotor was rebalanced with each of the fitments in turn.

Other Repair Work. As the stator and rotor repairs were taking place, the rest of the motor parts were being overhauled comprising of:- Slip rings were machined after electrical tests for shorting and grounds; End shields were cleaned with bearing housings and spigots checked for size; Slip ring shorting mechanism overhauled; manufacture a new terminal support block; Brush gear blast cleaned and overhauled. All internal surfaces were coated with anti-tracking enamel.

Reassembly. With the major component repairs having been completed, the motor was reassembled. New bearings were fitted together with the correct amount of lubricating grease. Particular attention was paid to thoroughly cleaning out the grease pipes of old grease. New carbon brushes were fitted and correctly tensioned. The slip rings were profiled to an acceptable level of run out (0.1mm) Inspection covers and shaft fitments were repositioned.

Test. Prior to full operating voltage being applied to the motor, static electrical tests such as Flash Test, Baker Surge Test, and winding resistance were carried out. At the applied stator voltage of 415v, the open circuit rotor voltage was balanced across the 3 phases at 545v. With the rotor shorted out the motor ran smoothly and accelerated to 985 rpm with a no load current of 145A per phase. We performed a 1 hour run test to check the bearing condition at the temperature plateau. Vibration tests were finally completed. Our QA department verified the test results and signed off the repair.

Completion. After completion of the tests the motor was spray painted, a repair report was compiled and the motor was dispatched via our transport back to the customer where it was refitted to the process plant.

This repair is just one example of how our range of skills, experience and equipment can be brought together to provide our customers with the confidence they need in ensuring a successful outcome to an unplanned breakdown.