Chuck Yung
EASA Senior Technical Support Specialist
Brush Seating
When seating brushes, safety regulations (OSHA in the U.S.) require “qualified personnel” to perform the work if operating at 50 volts DC or above. Even though our industry has been seating brushes for over a century, our recommendation is as follows:
Raise the armature voltage to just under 50V DC, and wear eye protection when seating brushes. If the armature RPM is too slow to facilitate seating of the brushes, carefully field-weaken to increase the speed without raising the armature voltage. The brush seater should be held in light contact against the commutator and positioned “upstream” of the brushes being seated.
Commutator Reprofiling
There are times when a field-expedient procedure to resurface the commutator is needed. The usual method is to use abrasive commutator stones (
Figure 1) to improve the condition of a commutator surface. The problem with this is that while stoning the commutator may improve the surface finish, any eccentricity remains. It is not possible to hand stone an eccentric commutator and make it round.
The second issue is that hand stoning a commutator may introduce an uneven surface -- axial waviness.
Any work scope should begin with a safety meeting involving the customer to establish an agreed upon work plan that meets their safety requirements.
The best practice is to remove the machine from service so that it can be properly machined. For work that must be done in place, portable machines can be used to machine the commutator in place by driving it with an external source.
Pro tip: When the motor in question is a hoist motor, manually controlling the brake, starting with the snatch block at the highest position, is an easy way to rotate the commutator.
Use a lathe compound and fabricate framework that can be bolted to the end bracket or the motor base (Figure 2). Align it so that the lathe compound can be moved axially. Clamp a commutator stone (Figure 3a) in the compound, crank it in to touch the commutator surface and rotate the shaft. Maintain pressure against the commutator while cranking the compound back and forth to resurface the commutator as it rotates.
Depending on the condition of the commutator, use progressively finer (smoother) commutator stones to resurface the commutator.
When it is absolutely necessary to perform this work online – “hot” – meaning the motor is energized, the caution regarding qualified personnel is essential to safety (Figure 3b). The use of a non-conductive tool and abrasive is required. Either a Dynabrade (Figure 4) with Scotchbrite belt or a machine-tool mounted commutator stone is recommended.
Caution: When resurfacing the slip rings of a synchronous machine under power (hot), the tooling must be positioned downstream of the brushes to avoid abrasive material passing under the brushes and accelerating the rate of brush wear.
Specialized in-situ machining is a niche that few service centers offer, and it can be a profitable service.
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