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The armature of a dc motor has $20 \Omega$ resistance. It draws a current of 1.5 A when run by a $220 \mathrm{V}$ dc supply. The value of the back emf induced in it is
A. 150 V
B. 170 V
C. $180 \mathrm{V}$
D. 190 V

Last updated date: 17th Apr 2024
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Hint: We know that DC motors take electrical power through direct current, and convert this energy into mechanical rotation. DC motors use magnetic fields that occur from the electrical currents generated, which powers the movement of a rotor fixed within the output shaft. DC motors are suitable for many applications including conveyors, turntables and others for which adjustable speed and constant or low-speed torque are required. They also work well in dynamic braking and reversing applications, which are common in many industrial machines.

Complete step by step answer
We know that the counter voltage induced across the armature winding is called the back EMF. The counter EMF induced in the DC motor self regulates the armature current and the torque and the speed automatically regulated through armature current. The back EMF depends, of course, on the speed of the motor the change in magnetic flux that generates it increases with motor speed so that as the motor begins to turn, the back EMF grows until the motor has reached its maximum speed, at which point the back EMF stays at its maximum value.
It is known that the armature winding is defined as the conductors which are housed and protected within the armature slots are connected properly. These windings are arranged in the armature slots. The helpful emf will be encouraged in this winding which is received across the brushes.
Back emf = Applied voltage potential drop across armature coil $=200-\mathrm{iR}$
$=200-1.5 \times 20$
$=170 \mathrm{V}$

Hence, we can say that the correct option is option B.

Note It is known that voltage drop is the decrease of electrical potential along the path of a current flowing in an electrical circuit. Voltage drops in the internal resistance of the source, across conductors, across contacts, and across connectors are undesirable because some of the energy supplied is dissipated. Voltage drop is not caused by poor connections, bad contacts, insulation problems, or damaged conductors; those are causes of voltage loss. On DC and most AC motors the purpose of the commutator is to ensure that the current flowing through the rotor windings is always in the same direction, and the proper coil on the rotor is energized in respect to the field coils. The armature is mounted on bearings and is free to rotate. It is mounted in the magnetic field produced by permanent magnets or current passing through coils of wire, these being termed the field coils. When a current passes through the armature coil, forces act on the coil leading to rotation.