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Material Selection of Armature Commutator


Material Selection of Armature Commutator
 
Commutator is a component of starter motor which changes the direction of current in order to allow the starter motor keeps turning.

Copper is a key material of commutator, usually the hardness ranges in HB80-HB120 and the best range should remain between HB95 and HB105. Besides adding silver into the copper can ensure that the commutator have sufficient hardness even at extreme temperature.

When the starter motor is running, the commutator is subjected to a high linear velocity. So when it changes the direction, the spark will constantly ablate the surface of the commutator, causing a dramatic increase in the surface temperature of commutator. But after adding a certain amount of silver into the copper, it can increase the high temperature resistance of the silver-copper alloy as well as the hardness of the commutator, so that the surface of the commutator won’t be distorted easily. And there is a misunderstanding that the more silver it contents in the copper the better it is. In fact, it is not the case. With higher content of the silver, the conductivity of the silver-copper alloy will decline, also it brings negative impact to the commutator itself which makes it not easy to form a useful oxide film.

One other factor that needs to be considered is the softening temperature of the copper. The general softening temperature is 150℃ and it can rise to 250℃ when the copper contains 0.06% of silver. Hence here involves another issue, the matching problem of carbon brush. With higher silver content of the copper, the hardness of the carbon brush should adjust accordingly or the life of the carbon brush will be shorter. So the content of silver in the copper can’t be generalized.

At present, the general material using in the commutator of low-end configuration starter motor in domestic is T2 copper ( US grade C11000/ Japan grade C1100) or 0.08% silver-copper for standard ones.

The case of the commutator is made of glass-reinforced thermosetting plastics or bakelite powder which has high requirements on the heat resistance and mechanical strength of plastic, and also requires the thermal expansion to be as close as possible to the copper of the commutator segment, so the material of the bakelite in the commutator is also critical.

Bakelite is a kind of hard and brittle thermosetting plastic which has high mechanical strength, good insulation, tough wear-resistant, dimensional stability, heat and corrosion resistance.

Generally the bakelite powder using on the commutator of high-end configuration starter motor is 6830 and for standard configuration ones is 6630. As for low-end starter motors, some manufacturers will use fiberglass to replace bakelite powder too.




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