question_answer 11)

Draw a labelled diagram of an electric motor. Explain its principle and working. What is the function of a split-ring in an electric motor?

Answer:

An electric motor is a device for converting electric energy into mechanical energy. Thus, an electric motor is the reverse of an electric generator. There are two types of electric motors : (i) AC motor and (ii) DC motor. We shall here be describing DC motor. The principle of a DC motor is very much different from that of an AC motor. It is important to remember that all the electric appliances like fan, air-conditioner, coolers, washing machines, mixers and blenders work on AC (house-hold power supply) and as such have AC motors installed in them. Principle : When a coil carrying current is placed in a magnetic field, it experiences a torque. As a result of this torque, the coil begins to rotate. Construction : It consists of the following five parts: 1. Armature : The armature abcd consists of a large number of turns of insulated copper wire wound over a soft iron core. 2. Field Magnet : The magnetic field B is supplied by a permanent magnet NS. 3. Split-ring or Commutator : These are two halve of the same metallic ring. The ends of the armature coil are connected to these halves which also rotate with the armature. 4. Brushes or Sliding Contacts : These are two flexible metal plates or carbon rods B1  and B2 which are so fixed that they constantly touch the revolving rings. 5. Battery : The battery consists of a few cells and is connected across the brushes. The brushes pass the current to the rings from where it is carried to the armature. Working : The working of a DC motor will be clear from the following discussion. (a) Let us suppose that the battery sends current to the armature in the direction shown in figure. Applying Fleming's Left Hand Rule (Motor Rule), we find that arm ad experiences a force which is acting outwards and perpendicular to it and arm bc experiences a force which is acting inwards and perpendicular to it. These two forces form a couple whose moment (i.e., torque) makes the armature rotate in the anticlockwise direction. (b) After the armature has completed half a revolution (i.e., has turned through 180°), the direction of current in the arms ad and bc is reversed. Now arm bc experiences an outward force and arm ad experiences an inward force, as shown in figure. The armature thus continues to rotate about its axis in the same, i.e., anticlockwise direction.   Split-ring in an electric motor takes the current from the battery and passes it on to the coil through the brushes after reversing its direction after every half revolution. The reversal of current in the coil reverses the direction of forces acting on the sides of the loop.