We carried out this DC motor repair as part of our standard regular maintenance service. This 100kw Leroy Somer DC motor was collected from a steel processing company which needed a thorough overhaul and rebuild.

The motor was fully dismantled and it was clear that although it was still running, it needed a good overhaul to place it into good working condition once more.

Once fully degreased and stoved, the field and interpole windings were Baker tested with step voltage, surge and hi-pot to ensure the winding insulation was adequate with regard to earth protection and to ensure that there was no insulation failure between turns of wire within each coil.

The armature coils were also individually tested and re-insulated and the commutator machined. When machining the commutator, this is done as the brushes slowly wear grooves into the copper segments which can cause problems with the commutation when new brushes are fitted such as sparking and overheating. The aim is to remove as little copper as possible when turning the commutator down as this can only be machined a finite number of times before it is too small to be serviceable.

Each separate bar on the commutator is connected to a number of copper coils in the armature & in between each segment is a mica insulator. The mica is harder than the copper and has to be undercut, where a small blade is run down the slot to cut down the mica below the level of the softer copper segments. It also removes any possible shorts between the segments which may be present after the machining process.

If this is not done, as the copper wears the mica will sit proud and the brushes will begin to bounce over the commutator, eventually breaking or causing sparking and serious electrical issues.

The carbon brushes are housed in the brushgear which usually comprises of 4 separate arms but it can be more on larger machines. Each brush box contains a spring to maintain constant pressure on the brush while it creates a circuit on the commutator. This motor’s brushgear was cleaned, degreased and tested to ensure the insulation was again fully serviceable.

After this, our standard sequence of mechanical checks were carried out to ensure bearing journals and housings had not worn, along with drive shaft diameter and keyway. The armature was then dynamically balanced, the windings re-insulated & the bearings replaced with high quality SKF bearings. The last item was the force ventilation fan blower which was fully overhauled with a new motor fitted and the impeller balanced.

Once assembled, the motor was fitted with a new set of carbon brushes and run throughout its speed range in both directions ensuring the brushes were correctly bedded in to the new commutator profile. A good example of our outstanding DC motor repair service.