Question

What is the range of the brush friction coefficients for medium category?

(a) 0.40 and above

(b) 0.22 to 0.40

(c) 0.11 to 0.22

(d) 0.08 to 0.11

This question was addressed to me by my college director while I was bunking the class.

This intriguing question comes from Brushes and Commutator Assembly topic in chapter DC Machines of DC Machines

Answer 1

Right answer is (b) 0.22 to 0.40

The explanation is: Brush friction is influenced by many variables including brush temperature, spring force, current, atmospheric conditions, mechanical conditions, ring or commutator materials, surface films, speed and other factors. Brush friction is of medium category when, coefficient of friction lies in between 0.22 to 0.44.