Brushes of DC machines are made of

Question

Examoogle · Accepted Answer

Carbon

Explanation: Brushes in DC machines are typically made of carbon because it possesses a high negative temperature coefficient of resistance and a self-lubricating property. These characteristics minimize friction and wear on the commutator surface while facilitating efficient current collection.

Working Principle: The brush-commutator contact relies on the high contact resistance of carbon to aid in the commutation process. By increasing the resistance of the short-circuited coil during reversal, carbon brushes reduce the current sparking ($I^2R$ sparking) as the coil moves from one brush position to another. The material also provides a polishing effect on the copper commutator, maintaining a smooth, conductive film.

Key Formulas: $R_c \propto \frac{1}{A}$ — Contact resistance relation where $A$ is the brush contact area.; $E_s = L \frac{di}{dt}$ — Reactance voltage equation representing the cause of sparking during commutation.

Key Points: Carbon is chosen for its negative temperature coefficient of resistance.; Carbon provides self-lubrication, reducing mechanical wear on the copper commutator.; The high contact resistance of carbon brushes limits the short-circuit current during commutation.; Carbon brushes are soft enough to avoid scoring the commutator but hard enough for longevity.

Advantages: Self-lubricating properties reduce friction.; Helps in reducing sparking at the commutator.; High contact resistance limits circulating currents.

Disadvantages: Higher electrical resistance compared to copper brushes.; Requires more frequent cleaning due to carbon dust accumulation.

Applications: DC motors and generators.; Universal motors in home appliances.; Traction motors.

Additional Info: Copper brushes are only used in low-voltage, high-current machines where minimal voltage drop is critical.. Option B and C (Copper) cause severe sparking and rapid commutator wear in standard DC machines.

Answer: A is correct — Brushes in DC machines are manufactured using carbon because of its high contact resistance and self-lubricating nature, which effectively minimize sparking during commutation.

Exam Tip: Always remember that carbon is preferred over copper for brushes in DC machines specifically to utilize its high contact resistance to assist in the commutation process, not to minimize electrical loss.

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