A 40 Year old 350 Hp Slip Ring Motor given a new lease of life.

A fluorspar mine based in the peak district had a major failure on his main processing plant. A 350 Hp slip ring motor had failed. Motors of this type and size are not usually found on the shelf at motor stockists, he knew a repair was his only option. We received the call and sent our 7.5 tonne wagon to collect the motor.

The motor weighing 3.5 tonnes, was lifted off our wagon and into our fitting shop where it was booked in to our job tracking system. The motor was a 40 year old Brook motor, 350Hp, 6pole, slip ring with class ‘B’ insulation.

Our assessing engineers started to locate the faults on the motor and assessed the condition. As usual all static electrical tests were carried out. The stator failed the Baker surge test and once the motor was stripped down the failure was quite evident. The windings had blown at the edge of one of the stator slots. The blast marks from the stator winding failure were evident on the bar wound rotor, we needed to check that this had not damaged the rotor windings. Static electrical tests including a Baker surge test were carried out on the rotor windings and thankfully the damage was proven to be cosmetic.

The stator was lifted to our winding department where the stator ‘crown’ was removed and the winding configuration data was established. The D400L frame stator windings were burnt out in our temperature controlled pyrolysis oven (a burn off oven). The burnt copper windings were then removed from the stator core. Once the core had been cleaned up the 108 slots were insulated with class ‘H’ insulation slot liners ready for the coils.

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

The winding configuration for this motor was half coil, 108 slot, 6 pole, 18 coils, 1-17 pitch with 9 wires in parallel. The coils were manufactured from grade 2 polyester enameled copper wire and were formed on our automatic coil winding machine. The coils were fitted to the stator and fully phase separated, the half coils were separated and closed with class ‘H’ insulation and full core length, class ‘H’ slot wedges were fitted. The windings were connected and the connections were secured. The newly rewound stator was VPI (vacuum pressure impregnated) and cured. After curing the stator was cleaned up ready for assembly.

The wound rotor was cleaned up and the blast marks caused by the winding failure were removed. Both bearing journals were checked for size and we found these to be down on the lower tolerance level for the bearings fitted. The journals were reclaimed and machined back to tolerance. We balance all rotors as a matter of cause and this one was no different. The cooling fan, pulley and the slip rings were fitted and the 1,100 Kg rotor assembly was dual plane dynamically balanced to G6.3 tolerance.

As the major repairs were taking place, the rest of the motor and parts were being overhauled. The slip rings were electrically tested for shorts and grounds then skimmed. The end-shields were cleaned and the bearing housings were checked for size. The rotor short circuiting mechanism was overhauled, a new terminal support block was manufactured as the existing one was in a poor state. The brush gear was cleaned and overhauled. A standard practice of our repair is to spray paint all the motor internal surfaces with an anti-tracking paint, this provides an extra layer of protection against earth shorts.

All the repairs had now been completed and were in a position to start assembling the motor. New bearings were supplied and fitted to the rotor, the rotor was fitted into the stator using our rotor lifting tool and careful operation of our 10 tonne overhead crane. The end-shields were fitted and the grease-plates secured. The slip rings were refitted to the shaft and the connections remade. The short circuiting device was refitted, the brush gear and new brushes were fitted and tensioned. Finally the pulley and all covers were fitted and the motor was lifted to our final test area.

A final static electrical test was carried out, the stator winding resistance was measured and all windings were 28.1 mΩ +/- 1.6 %.

At 415 volts, delta connected the motor ran smoothly and accelerated to 985rpm. At no load the motor was drawing 146 amps on each phase. We performed a 1 hour run test on the motor and checked the bearing condition at temperature plateau. The motor vibration was checked and passed.

After completing the tests the motor was spray painted, a repair report was compiled and the motor was dispatched via our transport back to the processing plant where it was refitted.

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