Question

The current drawn by a 220V D.C. series motor of armature resistance 0.50 and back emf 200V is:

A. 40A
B. 44A
C.400A
D.440 A

Answer 1

Hint:- A DC motor is a device which converts electrical energy to mechanical energy, the full form of D.C. motor is Direct Current Motor; there are a variety of D.C. motors, like shunt D.C., series D.C., Permanent magnet D.C., etc.

Complete step-by-step solution :
Given Data:
The terminal voltage is V=220 V
The armature resistance is R =0.5 Ω
The back emf is E=200V.
We know that the relation for back emf is given by:
$E = V - IR$
Here, E is the back emf, V is the terminal voltage, I is the armature current, and R is the armature resistance.
We will now substitute V=220 V, R =0.5 Ω, E=200 V to find I's value.
$\Rightarrow 200\;{\rm{V}} = 220\;{\rm{V}} - I \times 0.5\;\Omega$
$\Rightarrow 200\;{\rm{V}} = 220\;{\rm{V}} - I \times 0.5\;\Omega$
$\Rightarrow I = \dfrac{{20\;{\rm{V}}}}{{0.5\;\Omega }}$
$\Rightarrow I = 40\;{\rm{A}}$
Therefore, the armature current is 40 A, and the correct option is (A).

Note:- The carbon brushes given in the D.C. motor are because when it runs at a very high speed, then its sparks can occur damaging the motor; hence carbon brushes reduce the sparks caused by electric current. We can store D.C. in a battery, but A.C. cannot be stored in a battery. Still, A.C. voltages can be readily transformed lower voltage levels to higher voltage levels and vice versa, and it isn't easy to carry out voltage ups and downs in D.C. This transformation of voltage is required as the current has to travel a long way, and only high voltage is efficient in that case; otherwise, there is a loss of current; hence A.C. is preferred over D.C.