Correct option is A
Brush friction or contact losses in wound rotor induction motor:
Generally, in a slip ring or wound rotor induction motor the three terminals of the star-connected rotor windings are brought out and are connected to external resistors through the slip rings and brush arrangement mounted on the shaft.
Brushes sliding on the slip rings are used for making the connection of rotor windings with the external circuit.
Brush lifting and short-circuiting gear are used for operating the brushes.
As brushes are pressed against slip rings. And slip ring is a rotating part and a brush is a stationary part, so there will some loss due to friction in the form of heat which is called brush friction loss.
This brush friction loss is not present in the squirrel case induction motor as there no requirement for slip rings and brushes arrangement. So, these losses are present in wound rotor induction motors only.
Important points:
To reduce brush friction loss carbon brushes are used instead of copper brushes, as carbon is a self-lubricant.
Brush friction loss is directly proportional to the square of the speed.
