Join 60,000+ competitive exam aspirants
or with email
Your Cart
SubtotalтВ╣0
TotalтВ╣0
Examoogle тАв User тАв info@examoogle.com тАв EE-2024-8821
Chapter 1 of 12 тАв Page 1 of 248ЁЯФТ Protected PDF тАв Watermarked
Join 60,000+ competitive exam aspirants
Brushes of DC machines are made of
Carbon
Soft copper
Hard copper
All of above
Carbon
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.
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.
RcтАЛтИЭA1тАЛ тАФ Contact resistance relation where A is the brush contact area.
EsтАЛ=LdtdiтАЛ тАФ Reactance voltage equation representing the cause of sparking during commutation.
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 (I2R 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.
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.
Self-lubricating properties reduce friction.
Helps in reducing sparking at the commutator.
High contact resistance limits circulating currents.
Higher electrical resistance compared to copper brushes.
Requires more frequent cleaning due to carbon dust accumulation.
DC motors and generators.
Universal motors in home appliances.
Traction motors.
| Feature | Carbon Brushes | Copper Brushes |
|---|---|---|
Material Property | Carbon | Copper |
Contact Resistance | High | Low |
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.
A is correct тАФ Brushes in DC machines are manufactured using carbon because of its high contact resistance and self-lubricating nature, which effectively minimize sparking during commutation.
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.