Question

What is the formula for brush contact loss of DC machines?

(a) brush contact loss = armature current + brush contact voltage

(b) brush contact loss = armature current – brush contact voltage

(c) brush contact loss = armature current * brush contact voltage

(d) brush contact loss = (armature current + brush contact voltage) / 2

The question was posed to me by my school principal while I was bunking the class.

This is a very interesting question from Design of Commutator and Brushes topic in chapter Thermal Design Aspects of Electrical Machines & D.C of Design of Electrical Machines

Answer 1

Right option is (c) brush contact loss = armature current * brush contact voltage

The best explanation: For calculation of the brush contact loss, first the armature current is obtained. Then the voltage through the brush contacts is calculated and the product gives the brush contact loss.