Two dc machines A and B, using the same conductor material, have armature circuit resistances of 0.4 Ω and 1.2 Ω respectively.
Of the two machines,
A. A is bigger than B for the same current rating
B. A is bigger than B for the same voltage rating
C. B is bigger than A for the same current rating
D. both are of the same size for the same current rating
Answer: B
Share your understanding of this question with the correct explanation.
The correct answer is B. A is bigger than B for the same voltage rating.
In a DC machine, the size of the machine is primarily determined by the current rating and the power output. However, the armature circuit resistance also plays a role in determining the size of the machine.
A lower armature circuit resistance results in lower voltage drop and power loss within the armature winding. Therefore, for the same voltage rating, a machine with a lower armature circuit resistance (like machine A with 0.4 Ω) can be designed with larger conductors, resulting in a physically bigger machine compared to a machine with a higher armature circuit resistance (like machine B with 1.2 Ω).
So, in this case, machine A is bigger than machine B for the same voltage rating.