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The armature of a DC motor has $20\Omega$ resistance. It draws a current of 1.5 amp when run by 200V DC supply. The value of back emf induced in it will be
(A) 150V
(B) 170V
(C) 180V
(D) 190V

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Last updated date: 20th Jun 2024
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Answer
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Hint: We should know that in a DC motor, the stator provides a rotating magnetic field that drives the armature to rotate. A simple DC motor uses a stationary set of magnets in the stator, and a coil of wire with a current running through it to generate an electromagnetic field aligned with the centre of the coil. Electrical DC Motors are continuous actuators that convert electrical energy into mechanical energy. The DC motor achieves this by producing a continuous angular rotation that can be used to rotate pumps, fans, compressors, wheels, etc.

Complete step-by step answer:
Let us first define the meaning of the time of Back emf. So, it should be known to us that Counter-electromotive force (counter EMF, CEMF), also known as back electromotive force (back EMF), is the electromotive force or voltage that opposes the change in current which induced it.
We should know that a motor has coils turning inside magnetic fields, and a coil turning inside a magnetic field induces an emf. This emf, known as the back emf, acts against the applied voltage that's causing the motor to spin in the first place, and reduces the current flowing through the coils of the motor.
Back emf = Applied voltage potential drop across armature coil
$=200-iR\,$
$=200-1.5\times 20\,$
$=170V$
Back emf is a measure of energy conversion from electrical to mechanical in an electric motor. If you are able to make it zero no energy conversion to mechanical. Eb is back emf, I armature current and R armature circuit resistance. So, if we want energy conversion back emf is good.

Hence, the correct answer is Option B.

Note: We should know that a DC motor consists of two parts i.e. rotor and stator. Stator consists of field winding while the rotor (also called armature) consists of armature winding. When both armature and field are excited by DC supply, current flows through windings and magnetic flux proportional to the current is produced. The armature, which is the rotating part, is a simple coil. The armature is connected to a DC power source through a pair of commutator rings. When the current flows through the coil an electromagnetic force is induced on it according to the Lorentz law, so the coil will start to rotate.
It should be known to us that the field of a synchronous generator is the winding to which the DC excitation current is applied. The armature is the winding to which the load is connected. In small generators, the field windings are often on the stator, and the armature windings are on the rotor.