DC MOTOR REPAIR and OVERHAUL

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

Once 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 because 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 several copper coils in the armature, and 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 maybe 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 brush gear, which usually comprises four separate arms – sometimes 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 brush gear was cleaned, degreased and tested to ensure the insulation was fully serviceable.

After this, Webb Elec’s standard sequence of mechanical checks was carried out to ensure bearing journals and housings had not worn, along with driveshaft diameter and keyway. The armature was then dynamically balanced, the windings re-insulated, and 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 into the new commutator profile.

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