Question

Draw a labelled circuit diagram of a simple electric motor and explain its working. In what way these simple electric motors are different from commercial motors?

Answer 1

Principle of a simple electric motor is when a rectangular coil carrying current is placed in a magnetic field, a torque acts on the coil which rotates it continuously. When the coil rotates, the shaft attached to it also rotates and thus it is able to do mechanical work. The main parts of electric motor are the magnets, armature, split ring commutators and brushes. The armature consists of a rectangular coil made of insulated copper wire wound on a soft iron core. The coil is mounted on an axle and is placed between the cylindrical concave poles of a magnet. The commutator is used to reverse the direction of flow of current. It is a copper ring split into two parts P and Q. The split rings are insulated from each other and mounted on the axle of the motor. The two ends of the coil are soldered to these rings. They rotate along with the coil. Commutator rings are connected to a battery. The wires from the battery are not connected to the rings but to the brushes which are in contact with the rings. The brushes are two small strips of carbon, which press slightly against the two split rings, and the split rings rotate between the brushes. The carbon brushes are connected to a D.C. source.
Working of the Electric motor:
When the coil is powered, a magnetic field is generated around the armature. The left side of the armature is pushed away from the left magnet and drawn towards the right, causing rotation. When the coil turns through 90 degrees, the brushes lose contact with the commutator and the current stops flowing through the coil. However, the coil keeps turning because of its own momentum. Now when the coil turns through 180 degrees, the sides get interchanged. As a result the commutator ring P is now in contact with brush Y and commutator ring Q is in contact with brush X. Therefore, the current continues to flow in the same direction.
Difference between simple electric motors and commercial electric motors:
In commercial electric motors, field coils are used for producing magnetic fields and not a permanent magnet as in simple electric motors. These coils become magnetized when current is passed through them. Field coils give a stronger, more easily shaped magnetic field than permanent magnets. More the number of turns in the field coil, more the magnetic field. Therefore, the suitable field coil sizes are used for various commercial purposes.